Cover Picture: Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals (Adv. Funct. Mater. 11/2006)

Hollow nanocrystals have been synthesized through a mechanism analogous to the Kirkendall Effect. When a cobalt nanocrystal reacts with sulfur in solution, the outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms through the sulfide shell. The dominating outward diffusion of cobalt cations produces vacancies that can condense into a single void in the center of the nanocrystal at high temperatures. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds and is described in the Full Paper by A. P. Alivisatos and co‐workers on p. 1389. Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at > 120 °C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature‐dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell‐size statistics and transmission electron microscopy images of as‐grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.     

Size‐ and Shape‐Control of Crystalline Zinc Oxide Nanoparticles: A New Organometallic Synthetic Method

A novel organometallic synthetic method has been developed for the preparation of crystalline ZnO nanoparticles of controlled size and shape. Isotropic nanoparticles with a mean size between 3 and 6 nm and nanorods with a mean diameter of 3–4 nm and length up to 120 nm have been obtained in this way. This synthetic method takes advantage of the exothermic reaction of the precursor Zn(c‐C₆H₁₁)₂ ( 1 ) toward moisture and air and involves the presence of long‐alkyl‐chain amines as stabilizing ligands. The influence of the different experimental parameters (concentration, solvent, nature of the ligand, time, and temperature) on the size and shape of the ZnO nanoparticles has been studied, together with the mechanism of their formation, by NMR spectroscopy, transmission electron microscopy, and X‐ray diffraction techniques. The nanoparticles prepared in this way can be dissolved in most of the common organic solvents, forming colloidal solutions. The surface state of the nanoparticles as well as the possibility of forming luminescent solutions from which regular monolayers can be deposited are also reported.     

Synthesis of Nickel Sulfide Submicrometer‐Sized Hollow Spheres Using a γ‐Irradiation Route

Nickel sulfide (NiS) hollow spheres have been successfully synthesized by γ‐irradiation, at room temperature, of an aqueous PMMA–CS₂–ethanol solution that contains NiSO₄·6H₂O. Electron microscopy results show that the diameter of the NiS hollow spheres and the thickness of the sphere shells are about 500 nm and 20 nm, respectively. The room‐temperature UV‐vis absorption spectrum of the NiS hollow spheres gives a peak centered at around 233 nm (5.56 eV) with a remarkable blue‐shift relative to that of bulk NiS (2.1 eV). This remarkable blue‐shift may be attributed to the small dimensions of the materials. A possible growth mechanism of NiS hollow spheres by γ‐irradiation method is also presented. The successful preparation of NiS hollow spheres on a large scale under mild conditions could be of interest for both applications and fundamental studies.     

Synthesis of NaYF4 Nanocrystals with Predictable Phase and Shape

In this Full Paper, a water/alcohol/oleic acid system was developed to prepare NaYF₄ nanocrystals with predictable size, shape and phase. The structural and kinetic factors that govern the phase and shape evolution of NaYF₄ nanocrystals have been carefully studied, and the influence of NaF to Y³⁺ ratio, reaction time and temperature on the phase and shape evolution of the as‐prepared NaYF₄ samples was systematically investigated and discussed. It was found that the NaF to Y³⁺ ratio was responsible for the shape evolution while temperature and reaction time was the key for the phase control of the NaYF₄ nanocrystals. This study would be suggestive for the precisely controlled growth of inorganic nanocrystals, especially for those usually crystallizing in diverse crystal structures.     

Influence of Engineered Peptides Cadmium Sulfide Nanocrystals

Cadmium sulfide (CdS) nanocrystals continue to generate scientific and technological interest, owing to their valuable optical, electronic, and chemical properties. Aqueous nanocrystal syntheses rely on functional capping agents to control nanocrystal form and function. We present a series of linear and dendritic engineered peptides, rich in cysteines and aspartic acids, as CdS capping agents. The chemical composition and peptide geometry were found to significantly influence CdS nanocrystal size, optical properties, and aggregation behavior.     

Synthesis of Nearly Monodisperse Iron Oxide and Oxyhydroxide Nanocrystals

Uniform magnetite, hematite, and goethite nanocrystals were prepared through an attractive method based on an oleic acid/alcohol/water system. By adjusting the synthetic parameters (base concentration, alcohol content, categories of alcohols, etc.), the controlled synthesis of uniform magnetite, hematite, and goethite nanocrystals can be easily achieved. Detailed investigations on the effect of the experimental parameters on the morphology of the final products and the phase transitions among the magnetite, hematite, and goethite phases were carried out. Finally, a method of doping other metal ions into magnetite was developed and the magnetic properties of magnetite doped with different metal elements were studied.     

PbSe/PbS and PbSe/PbSexS1–x Core/Shell Nanocrystals

The synthesis of PbSe/PbS and PbSe/PbSe_xS_1–x core/shell nanocrystals (NCs) with luminescence quantum efficiencies of 45–55 % is reported. PbSe/PbS NCs are prepared via a two‐stage process, while the PbSe/PbSe_xS_1–x NCs are formed in a single‐stage procedure. The core/shell NCs exhibit an energy tuning of the exciton transitions, with respect to that of the core NC, that is dependent on the core diameter, shell thickness, and composition.     

Fabrication of Hollow Inorganic Microspheres by Chemically Induced Self‐Transformation

Two contrasting approaches, involving either polymer‐mediated or fluoride‐mediated self‐transformation of amorphous solid particles, are described as general routes to the fabrication of hollow inorganic microspheres. Firstly, calcium carbonate and strontium tungstate hollow microspheres are fabricated in high yield using sodium poly(4‐styrenesulfonate) as a stabilizing agent for the formation and subsequent transformation of amorphous primary particles. Transformation occurs with retention of the bulk morphology by localized Ostwald ripening, in which preferential dissolution of the particle interior is coupled to the deposition of a porous external shell of loosely packed nanocrystals. Secondly, the fabrication process is extended to relatively stable amorphous microspheres, such as TiO₂ and SnO₂, by increasing the surface reactivity of the solid precursor particles. For this, fluoride ions, in the form of NH₄F and SnF₂, are used to produce well‐defined hollow spheroids of nanocrystalline TiO₂ and SnO₂, respectively. Our results suggest that the chemical self‐transformation of precursor objects under morphologically invariant conditions could be of general applicability in the preparation of a wide range of nanoparticle‐based hollow architectures for technological and biomedical applications.     

Reversible Cluster Formation of Colloidal Nanospheres by Interparticle Photodimerization

Crosslinked spherical nanoparticles based on trimethoxysilane monomers have been prepared by polycondensation in aqueous emulsion. These particles have been labeled chemically at their surface region with two different types of organic dye molecules (cinnamate, coumarin), which both are well known for their ability to undergo a reversible photodimerization if irradiated with light of a suitable wavelength. Upon irradiation of dilute solutions of these nanoparticles with UV light, the photodimerization of labels belonging to different colloidal nanoparticles caused the formation of large colloidal clusters consisting of chemically bound individual nanospheres. This process has been quantitatively investigated using light scattering and atomic force microscopy. Importantly, utilizing the reversibility of the photoreaction, the clusters could be broken up by irradiation of the sample with UV light of shorter wavelengths than the light used for their formation     

One‐Step Synthesis of Highly Ordered Mesoporous Silica Monoliths with Metal Oxide Nanocrystals in their Channels

A simple, one‐step synthetic route to prepare ordered mesoporous silica monoliths with controllable quantities of metal oxide nanocrystals in their channels is presented. The method is based on the assisted assembly effect for mesostructure‐directing of the metal complexes formed by the interaction of metal ions with the –O– groups of copolymers. Highly ordered hexagonal silica monoliths, loaded with various metal oxide nanocrystals, including those of Cr₂O₃, MnO, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, CdO, SnO₂, and In₂O₃, can be obtained by this one‐step pathway. In the NiO/SiO₂ nanocomposite, nickel oxide nanorods with face‐centered cubic lattices are formed at low doping ratios, and they can be transformed into nanowires by increasing the quantities of the precursors. In the Fe₂O₃/SiO₂ nanocomposites, a one‐dimensional assembly of iron oxide nanoparticles is observed. In the In₂O₃/SiO₂ nanocomposites, single crystal nanowires with high aspect ratios are obtained. For the other metal oxide nanocomposites, including Cr₂O₃, MnO, Co₃O₄, CuO, ZnO, CdO, and SnO, only crystalline nanorods are obtained. N₂ sorption results of the metal oxide/SiO₂ mesostructured nanocomposites reveal that nanocrystals inside the pores do not severely decrease the pore volume or the Brunauer–Emmett–Teller (BET) surface area of the mesoporous silica host. The bandgaps of SnO₂ and In₂O₃ nanocrystals, calculated from UV‐vis spectra, are much larger than the corresponding bulk materials, implying the quantum confinement effect in the small particles. Co₃O₄/SiO₂ mesostructured nanocomposites catalyze the complete combustion of CH₄. These studies provide a new and simple method for templating synthesis of metal oxide nanostructures.     

Some New Developments in the Synthesis,Functionalization, and Utilization of Monodisperse Colloidal Spheres

This article provides an overview of some recent developments related to the synthesis and functionalization of monodisperse colloidal spheres, a class of colloidal materials that has found widespread use in applications such as the fabrication of photonic crystals, optical sensing, and drug delivery. Traditionally, the choice of materials has been limited to polystyrene and silica. We and other groups have recently expanded the scope of materials by developing a number of methods for producing monodisperse colloidal spheres from various semiconductors and metals. This article is confined to our own work; it covers three different synthetic strategies: the bottom–up approach, the top–down approach, and template‐directed synthesis. The colloidal spheres may have a solid, hollow, or core–shell structure, and the chemical compositions can include Se, Bi, Pb, In, Sn, Cd, Pt, Ag₂Se, CdSe, PbS, or TiO₂. As an example to illustrate the attractive features of these colloidal spheres, we demonstrate the fabrication of Ag₂Se‐based photonic crystals whose stop bands can be thermally switched between two spectral positions.     

Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes

HgTe nanocrystals are demonstrated to increase the photon‐harvesting efficiency of hybrid solar cells over a broad spectral region between 350 and 1500 nm. Devices combining two solar cell concepts, a solid‐state nanocrystal‐sensitized solar cell and a nanocrystal/polymer‐blend solar cell, are described. These devices give incident photon to current efficiencies up to 10 % at around 550 nm monochromatic irradiation and short‐circuit current densities of 2 mA cm^–2 under simulated AM1.5 (100 mW cm^–2) illumination (AM: air mass).     

One‐Pot Synthesis and Hierarchical Assembly of Hollow Cu2O Microspheres with Nanocrystals‐Composed Porous Multishell and Their Gas‐Sensing Properties

Hierarchical assembly of hollow microstructures is of great scientific and practical value and remains a great challenge. This paper presents a facile and one‐pot synthesis of Cu₂O microspheres with multilayered and porous shells, which were organized by nanocrystals. The time‐dependent experiments revealed a two‐step organization process, in which hollow microspheres of Cu₂(OH)₃NO₃ were formed first due to the Ostwald ripening and then reduced by glutamic acid, the resultant Cu₂O nanocrystals were deposited on the hollow intermediate microspheres and organized into finally multishell structures. The special microstructures actually recorded the evolution process of materials morphologies and microstructures in space and time scales, implying an intermediate‐templating route, which is important for understanding and fabricating complex architectures. The Cu₂O microspheres obtained were used to fabricate a gas sensor, which showed much higher sensitivity than solid Cu₂O microspheres.     

Synthesis of Hexagonal‐Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up‐Conversion Fluorescence

IR‐to‐visible up‐conversion fluorescent nanocrystals of hexagonal‐phase NaYF₄:20 %Yb,2 %Er and NaYF₄:20 %Yb,2 %Tm have been synthesized by decomposition of multiprecursors of CF₃COONa, (CF₃COO)₃Y, (CF₃COO)₃Yb, and (CF₃COO)₃Er/(CF₃COO)₃Tm in oleylamine at 330 °C. The average particle size is 10.5 ± 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 ± 1.3 nm (from dynamic‐light‐scattering measurements). The up‐conversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic‐phase NaYF₄:Yb,Er nanocrystals, including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal‐phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential applications in optical nanodevices and bioprobes.     

A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size‐Selected Pure Cobalt Dots to Cobalt–Cobalt Oxide Core–Shell Systems

Starting with Co‐salt‐loaded inverse micelles, which form if the diblock copolymer polystyrene‐block‐poly(2‐vinylpyridine) is dissolved in a selective solvent like toluene and CoCl₂ is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in‐situ X‐ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2p_3/2 line pointing to the formation of Co₃O₄. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures pequation/tex2gif-inf-5.gif, a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (pequation/tex2gif-inf-7.gif × time), thus giving the opportunity to control the ferromagnetic–antiferromagnetic composition of an exchange‐biased magnetic system.     

Synthesis of MnO2 Nanoparticles Confined in Ordered Mesoporous Carbon Using a Sonochemical Method

A sonochemical method has been successfully used in order to incorporate MnO₂ nanoparticles inside the pore channels of CMK‐3 ordered mesoporous carbon. Modification of the intrachannel surfaces of CMK‐3 to make them hydrophilic enables KMnO₄ to readily penetrate the pore channels. At the same time, the modification changes the surface reactivity, enabling the formation of MnO₂ nanoparticles inside the pores of CMK‐3 by the sonochemical reduction of metal ions. The resultant structures were characterized by X‐ray diffraction (XRD), nitrogen adsorption, and transmission electron microscopy (TEM). CMK‐3 with 20 wt.‐% loading of MnO₂ inside CMK‐3 delivered an improved discharge performance of 223 mA h g^–1 at a relatively high rate of 1 A g^–1. Almost no decrease in specific capacity is observed for the second cycle, and a discharge capacity of more than 165 mA h g^–1 is retained after 100 cycles. This is attributed to the nanometer‐sized MnO₂ formed inside CMK‐3 and the high surface area of the mesopores (3.1 nm) in which the MnO₂ nanoparticles are formed.     

Efficient Infrared Electroluminescent Devices Using Solution‐Processed Colloidal Quantum Dots

We report efficient electroluminescence in the near‐infrared from PbS–MEH‐PPV (poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene)) large‐area, solution‐cast nanocomposite devices. We employ multivariate optimization of the structural and materials components that govern the radiative, energy‐transfer, and bipolar‐injection efficiencies into the devices. As a result, we report an external electroluminescence quantum efficiency of 0.27 %, which corresponds to an internal electroluminescence quantum efficiency of 1.9 %. The very best devices exhibit internal‐radiative‐efficiency‐limited performance and not transport‐ or capture‐limited performance, indicating that further gains in efficiency may be achieved if the internal radiative efficiency of the nanocrystal–polymer composite can be further increased without compromising transfer and device bipolar‐injection efficiency.     

Preparation of Hollow Silica Nanospheres by Surface‐Initiated Atom Transfer Radical Polymerization on Polymer Latex Templates

Poly(vinylbenzyl chloride), (PVBC) latex particles of about 100 nm in size are prepared by emulsion polymerization. Silyl functional groups are introduced onto the PVBC‐nanoparticle templates via surface‐initiated atom transfer radical polymerization of 3‐(trimethoxysilyl)propyl methacrylate. The silyl groups are then converted into a silica shell, approximately 20 nm thick, via a reaction with tetraethoxysilane in ethanolic ammonia. Hollow silica nanospheres are finally generated by thermal decomposition of the PVBC template cores. Field‐emission scanning electron microscopy and field‐emission transmission electron microscopy are used to characterize the intermediate products and the hollow nanospheres. Fourier‐transform infrared spectroscopy results indicate that the polymer cores are completely decomposed.     

Synthesis of Hexagonal Yb3+,Er3+‐Doped NaYF4 Nanocrystals at Low Temperature

Nanocrystals of NaYF₄ doped with Yb³⁺ and Er³⁺ are synthesized in oleylamine using Y₂(CO₃)₃, Yb₂(CO₃)₃, Er₂(CO₃)₃, Na₂CO₃, and NH₄F as precursors. In contrast to other starting materials normally used for such syntheses, these precursors react even at room temperature to form hexagonal‐phase (β‐phase) NaYF₄:Er,Yb nanoparticles. Cubic‐phase (α‐phase) NaYF₄:Yb,Er particles are formed only at elevated temperatures (>250 °C). The formation of the cubic phase at high temperatures can be suppressed by replacing pure oleylamine with oleic acid/oleylamine mixtures. Under optimized reaction conditions, particles with an average particle size of about 7 nm are generated in 84% yield. Heat treatment (30 min, 280 °C) of the particles significantly increases the luminescence efficiency. A transparent solution of the heat‐treated, nanometer‐sized phosphor in toluene shows intense visible light emission upon excitation in the near infrared.     

Kinetically Controlled Synthesis of Hexagonally Close‐Packed Cobalt Nanorods with High Magnetic Coercivity

High‐quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of Co^II in 1,2‐butanediol using a rapid, simple, and solid‐template‐free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close‐packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.