Synthesis Of Colloidal Nanocrystals Research Articles

Ligand‐Controlled Formation and Photoluminescence Properties of CH3NH3PbBr3 Nanocubes and Nanowires

AbstractLight‐emitting halide perovskites are currently emerging as promising solution‐processed materials for optoelectronic applications, and correlations between their physical properties and morphologies are important drivers to guide material and device optimization. There is still a lack of precise control of the morphology in colloidal perovskite materials, and in the related studies of their shape‐dependent optical properties. Capitalizing on our previous works on the ligand‐assisted reprecipitation synthesis of colloidal perovskite nanocrystals, we have addressed the role of key experimental parameters in determining the degree of supersaturation that drives the crystallization of CH3NH3PbBr3 precursors. By adjusting the amount of ligands, we fabricated single crystalline CH3NH3PbBr3 nanocubes (size: ≈500 nm) and nanowires (length: 2–4.5 μm, width: ≈100 nm), and conducted steady‐state, time‐resolved, excitation power and temperature‐dependent photoluminescence measurements to investigate their shape‐dependent photoluminescence properties.

In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films for Improved Light-Emitting Devices.

Hybrid organic-inorganic halide perovskite semiconductors are attractive candidates for optoelectronic applications, such as photovoltaics, light-emitting diodes, and lasers. Perovskite nanocrystals are of particular interest, where electrons and holes can be confined spatially, promoting radiative recombination. However, nanocrystalline films based on traditional colloidal nanocrystal synthesis strategies suffer from the use of long insulating ligands, low colloidal nanocrystal concentration, and significant aggregation during film formation. Here, we demonstrate a facile method for preparing perovskite nanocrystal films in situ and that the electroluminescence of light-emitting devices can be enhanced up to 40-fold through this nanocrystal film formation strategy. Briefly, the method involves the use of bulky organoammonium halides as additives to confine crystal growth of perovskites during film formation, achieving CH3NH3PbI3 and CH3NH3PbBr3 perovskite nanocrystals with an average crystal size of 5.4 ± 0.8 nm and 6.4 ± 1.3 nm, respectively, as confirmed through transmission electron microscopy measurements. Additive-confined perovskite nanocrystals show significantly improved photoluminescence quantum yield and decay lifetime. Finally, we demonstrate highly efficient CH3NH3PbI3 red/near-infrared LEDs and CH3NH3PbBr3 green LEDs based on this strategy, achieving an external quantum efficiency of 7.9% and 7.0%, respectively, which represent a 40-fold and 23-fold improvement over control devices fabricated without the additives.

Effects of Copper Precursor Reactivity on the Shape and Phase of Copper Sulfide Nanocrystals

Controlling the crystal structure and shape and/or faceting of colloidal nanocrystals during growth through easily variable reaction parameters is highly desirable. The choice of precursors is one such parameter that can significantly impact achievable shapes and phases. Here, we examine how different Cu precursors in the synthesis of colloidal copper sulfide nanocrystals affect the resulting shape and crystal phase. An observed decreasing aspect ratio in one-dimensional nanorods (eventually transitioning to two-dimensional nanodisks) is consistent with the expected effects of decreasing Cu precursor reactivity. Nanorods are predominantly chalcocite at the early stages of growth, but a phase transition to djurleite occurs and is accompanied by a change in tip faceting upon further growth. In contrast, nanodisks appear in the djurleite phase early on and remain so upon continued growth. Localized surface plasmon resonance in various shapes of nanocrystals achieved is enhanced with chemical oxidation, and t...

Low Curie-transition temperature and superparamagnetic nature of Fe3O4 nanoparticles prepared by colloidal nanocrystal synthesis

Colloidal size, narrow size-distributed magnetite (Fe3O4) nanospheres of 12 nm diameter were synthesized by colloidal nanocrystal synthesis protocols. X-ray diffraction and transmission electron microscopy studies reveal that the as-synthesized magnetite particles were single grain, spherical shaped and well crysallined in cubic spinel structure. Lattice vibrational studies confirms the existence of metal-oxide nanospheres and organic functional group (oleic acid) present on the particles surface. The nanospheres exhibits slightly enhanced energy band gap compared to counterpart bulk. The sample shows space-charge type polarization under low electric field frequencies (0.1–3 MHz) in the high temperature range (305–790 K), with Curie temperature at 713 K. Hence the dielectric constant (ε') reduces with enhance of electric field frequency. Dielectric loss (ε″) also reduces with enhance of frequency and the loss is 0.015 upto 650 K under 3 MHz. Hence it may be suitable for low loss device applications. AC electrical conductivity (σac) enhances with frequency and polaron hopping is slower than the site relaxation. Temperature dependent impedance spectra analysis reveals that grain contribution is predominant than grain boundary contribution with Debye-type relaxation. The nanospheres exhibits typical superparamagnetic behaviour with reduced saturation magnetization (Ms) due to disordered spins on the nanospheres' surface. Langevin function fit gives 10.5 nm magnetic domain diameter in the nanospheres.

Facile Synthesis of Pd@Pt3–4L Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction

AbstractThe presence of a capping agent or stabilizer in the synthesis of colloidal metal nanocrystals will compromise their performance if employed as electrocatalysts. Herein we demonstrate the synthesis of Pd@Pt3–4L core–shell octahedral nanocrystals with greatly enhanced activity toward the oxygen reduction reaction by eliminating the use of any capping agent or stabilizer. This was achieved by employing Pd octahedral seeds with well‐defined {1 1 1} facets and by dispersing them on a carbon black support prior to Pt deposition. Upon optimization of the reaction conditions, Pt ultrathin shells could be conformally deposited on the Pd octahedral seeds in a layer‐by‐layer fashion without involving self‐nucleation or island growth for the Pt atoms. The as‐obtained octahedral Pd@Pt3–4L/C catalyst exhibited a specific activity 50 % greater than that of a reference sample prepared in the presence of a polymer stabilizer such as poly(vinyl pyrrolidone). The polymer‐free catalyst also showed 5‐fold enhancement in specific activity if benchmarked against a commercial Pt/C catalyst.

Palladium Supported on Mesoporous Alumina Catalyst for Selective Hydrogenation

Colloidal metal nanoparticles are of great interest because of their use as catalysts, photocatalyst, adsorbent and sensors and their application in optical, electronic and magnetic devices. Capping agents are widely used in the synthesis of colloidal nano crystals for stabilizing the nanoparticles. The well dispersed Palladium nanoparticles (PdNP) supported on mesoporous γ-Al2O3 were prepared by using capping agents namely PVA, PVP, PEG and MEG using tetrahydrofuran as solvent. The catalytic properties of Pd-nanoparticle on mesoporous γ-Al2O3 were studied for hydrogenation model reaction. The alumina supported palladium nanoparticles were characterized by using instrumental techniques namely BET surface area, HR-TEM and XRD. HR-TEM studies shows that the average particle size of Pd metal in the fresh and spent catalyst was in the range from 5 to 10 nm. The catalyst could be efficiently recycled for three times without losing catalytic activity and selectivity.

Reduction rate as a quantitative knob for achieving deterministic synthesis of colloidal metal nanocrystals.

Despite the incredible developments made to the synthesis of colloidal metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.

High-throughput and tunable synthesis of colloidal CsPbX3 perovskite nanocrystals in a heterogeneous system by microwave irradiation.

By simple microwave irradiation, a high-throughput, single-step, rapid but controllable synthesis of stable colloidal CsPbX3 perovskite NCs with tunable properties and morphologies (nanocubes, nanorods, nanowires, quadrate nanoplates and hexagonal nanoplates) was achieved. It was employed in a heterogeneous solid-liquid reaction system within minutes without any tedious pre-preparation.

Low-temperature colloidal synthesis of CuBiS2 nanocrystals for optoelectronic devices

A new facile colloidal synthesis of CuBiS2 nanocrystals has been developed and the prototype solar cell has presented a decent performance.

Synthesis and characterization of colloidal nanocrystals of ternary chalcogenide compounds

Colloidal nanocrystals of CuInS2 and CuInSe2 were synthesized in an apolar noncoordinating medium using 1-dodecanethiol as a ligand. A semiconductor shell of ZnS was formed for CuInS2 nanocrystals obtained by the injection method. The obtained samples were characterized by absorption spectroscopy and photoluminescence.

Continuous synthesis of CuInS2 quantum dots

The impact of reactor type on synthesis parameters and disperse properties.

Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance.

We report the colloidal synthesis of ∼5.5 nm inverse spinel-type oxide Ga2FeO4 (GFO) nanocrystals (NCs) with control over the gallium and iron content. As recently theoretically predicted, some classes of spinel-type oxide materials can be intrinsically doped by means of structural disorder and/or change in stoichiometry. Here we show that, indeed, while stoichiometric Ga2FeO4 NCs are intrinsic small bandgap semiconductors, off-stoichiometric GFO NCs, produced under either Fe-rich or Ga-rich conditions, behave as degenerately doped semiconductors. As a consequence of the generation of free carriers, both Fe-rich and Ga-rich GFO NCs exhibit a localized surface plasmon resonance in the near-infrared at ∼1000 nm, as confirmed by our pump–probe absorption measurements. Noteworthy, the photoelectrochemical characterization of our GFO NCs reveal that the majority carriers are holes in Fe-rich samples, and electrons in Ga-rich ones, highlighting the bipolar nature of this material. The behavior of such off-stoichiometric NCs was explained by our density functional theory calculations as follows: the substitution of Ga3+ by Fe2+ ions, occurring in Fe-rich conditions, can generate free holes (p-type doping), while the replacement of Fe2+ by Ga3+ cations, taking place in Ga-rich samples, produces free electrons (n-type doping). These findings underscore the potential relevance of spinel-type oxides as p-type transparent conductive oxides and as plasmonic semiconductors.

Seed-Mediated Growth of Colloidal Metal Nanocrystals.

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Microwave-Assisted Synthesis of Colloidal Ag Nanocrystals and Colorimetric Detection of Mercury (I and II) Ions in Aqueous Solution

Microwave-Assisted Synthesis of Colloidal Ag Nanocrystals and Colorimetric Detection of Mercury (I and II) Ions in Aqueous Solution

Colloidal Chemistry to Advance Studies in Artificial Photosynthesis.

This article presents an overview of our research in the field of colloidal nanocrystal synthesis and their implementation into water splitting and CO2 reduction electrochemical cells. We discuss our approaches to tailor-made novel material platforms to advance our knowledge in energy storage in chemical bonds, namely artificial photosynthesis. Herein, we focus on complex metal oxides as light absorbers to drive water splitting, nanocrystal hybrids and metals as electrocatalysts for carbon dioxide conversion. Our approach to solve the synthetic challenges so to achieve very precise control on size, shape and composition of such materials is highlighted.

Predicting kinetic nanocrystal shapes through multi-scale theory and simulation: Polyvinylpyrrolidone-mediated growth of Ag nanocrystals.

In the shape-controlled synthesis of colloidal Ag nanocrystals, structure-directing agents, particularly polyvinylpyrrolidone (PVP), are known to be a key additive in making nanostructures with well-defined shapes. Although many Ag nanocrystals have been successfully synthesized using PVP, the mechanism by which PVP actuates shape control remains elusive. Here, we present a multi-scale theoretical framework for kinetic Wulff shape predictions that accounts for the chemical environment, which we used to probe the kinetic influence of the adsorbed PVP film. Within this framework, we use umbrella-sampling molecular dynamics simulations to calculate the potential of mean force and diffusion coefficient profiles of Ag atom deposition onto Ag(100) and Ag(111) in ethylene glycol solution with surface-adsorbed PVP. We use these profiles to calculate the mean-first passage times and implement extensive Brownian dynamics simulations, which allows the kinetic effects to be quantitatively evaluated. Our results show that PVP films can regulate the flux of Ag atoms to be greater towards Ag(111) than Ag(100). PVP's preferential binding towards Ag(100) over Ag(111) gives PVP its flux-regulating capabilities through the lower free-energy barrier of Ag atoms to cross the lower-density PVP film on Ag(111) and enhanced Ag trapping by the extended PVP film on Ag(111). Under kinetic control, {100}-faceted nanocrystals will be formed when the Ag flux is greater towards Ag(111). The predicted kinetic Wulff shapes are in agreement with the analogous experimental system.

Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite Nanocrystals with Bright Blue Emission.

We developed a colloidal synthesis of CsPbBr3 perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue emission which differs from the original green emission (∼510 nm) of CsPbBr3 nanocubes as reported previously. Shapes of the obtained CsPbBr3 NCs can be systematically engineered into single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets via tuning the amounts of oleic acid (OA) and oleylamine (OM). They exhibit sharp excitonic PL emissions at 453, 472, 449, and 452 nm, respectively. The large blue shift relative to the emission of CsPbBr3 bulk crystal can be ascribed to the strong quantum confinement effects of these various nanoshapes. PL decay lifetimes are measured, ranging from several to tens of nanoseconds, which infers the higher ratio of exciton radiative recombination to the nonradiative trappers in the obtained CsPbBr3 NCs. These shape-controlled CsPbBr3 perovskite NCs with the bright blue emission will be widely used in optoelectronic applications, especially in blue LEDs which still lag behind compared to the better developed red and green LEDs.

One-Pot Synthesis of Monodisperse Colloidal Copper-Doped CdSe Nanocrystals Mediated by Ligand–Copper Interactions

We report a one-pot synthesis of high-quality colloidal copper-doped cadmium selenide nanocrystals (Cu+:CdSe NCs) by injection of a mixture of copper iodide (CuI) and trioctylphosphine (TOP) into solutions containing preformed CdSe NCs. This method allows NC doping to be separated from nucleation and growth, thereby simultaneously achieving large size tunability, narrow size dispersion, and exclusively copper-based photoluminescence (PL). The copper doping level is affected by both the reaction time and the relative concentrations of the cadmium precursor, CuI, and TOP. A correlation is demonstrated between the copper dopant concentration and the intensities of the characteristic near-IR PL and midgap absorption bands, both associated with metal-to-ligand (conduction band) charge-transfer (MLCBCT) excitation of Cu+ dopants. Mechanistic studies reveal that Cu2–xSe NCs are easily formed as kinetic intermediates under reaction conditions involving substantial copper and that these NCs then act as a copper so...

Seed‐Mediated Growth of Colloidal Metal Nanocrystals: Scaling up the Production through Geometric and Stoichiometric Analyses

AbstractDespite the remarkable progress in shape‐controlled synthesis of noble‐metal nanocrystals, there still exists a major gap between academic studies and industrial applications. This gap can be largely attributed to the inability to produce nanocrystals in large quantities while retaining uniformity and good control over both size and shape. Here we provide a new perspective on increasing the volume of production during a synthesis of colloidal nanocrystals via seed‐mediated growth. We demonstrate that the yield of Pd octahedra and Pd@Pt3−4L (L: atomic layers) core–shell octahedra with different sizes can be increased from ca. 2 mg to over 500 mg without compromising the product quality. Specifically, we apply geometric and stoichiometric analyses to calculate the minimum amounts of reagents needed for the complete evolution of Pd cubic seeds into Pd octahedra, and subsequently for the formation of Pd@Pt3−4L core–shell octahedra through conformal Pt coating. The simple analyses allow us to obtain uniform products by avoiding homogeneous nucleation while minimizing the waste of reagents.

Facile, Economic and Size-Tunable Synthesis of Metal Arsenide Nanocrystals

Synthesis of colloidal nanocrystals (NC) of important arsenide nanomaterials (e.g., InAs, Cd3As2) has been limited by the lack of convenient arsenic precursors. Here we address this constraint by identifying a convenient and commercially available As precursor, tris-dimethylaminoarsine (As(NMe2)3), which can be used to prepare high quality InAs NCs with controlled size distributions. Our approach employs a reaction between InCl3 and As(NMe2)3 using diisobutylaluminum hydride (DIBAL-H) to convert As(NMe2)3 in situ into reactive intermediates AsHx(NMe2)3–x, where x = 1,2,3. NC size can be varied by changing DIBAL-H concentration and growth temperature, with colloidal solutions of InAs showing size dependent absorption and emission features tunable across wavelengths of 750 to 1450 nm. We also show that this approach works well for the colloidal synthesis of Cd3As2 NCs. By circumventing the preparation of notoriously unstable and dangerous arsenic precursors (e.g., AsH3 and As(SiMe3)3), this work improves th...