Self-assembled materials for catalysis

The purpose of this review is to highlight developments in self-assembled nanostructured materials (i.e., mesoporous and nanoparticle-based materials) and their catalytic applications. Since there are many available reviews of metal-based nanoparticles as catalysts, this review will mainly focus on self-assembled oxide-based catalytic materials. The content includes: (1) design and synthetic strategies for self-assembled mesoporous catalysts, (2) polyoxometalate (POM)-based nanocatalysts, (3) dendrimer-based nanocatalysts, and (4) shaped nanomaterials and catalytic applications. We show that controlled assembly of molecules, crystalline seeds, and nano building blocks into organized mesoscopic structures or controlled morphologies is an effective approach for tailoring porosities of heterogeneous catalysts and controlling their catalytic activities. This article is published with open access at Springerlink.com     

Synthesis and Ex situ doping of ZnTe and ZnSe nanostructures with extreme aspect ratios

We report synthesis windows for growth of millimeter-long ZnTe nanoribbons and ZnSe nanowires using vapor transport. By tuning the local conditions at the growth substrate, high aspect ratio nanostructures can be synthesized. A Cu-ion immersion doping method was applied, producing strongly p-type conduction in ZnTe and ionic conduction in ZnSe. These extreme aspect ratio wide-bandgap semiconductors have great potential for high density nanostructured optoelectronic circuits.      

Size-dependent joule heating of gold nanoparticles using capacitively coupled radiofrequency fields

Capacitively coupled shortwave radiofrequency fields (13.56 MHz) resistively heat low concentrations (∼1 ppm) of gold nanoparticles with a thermal power dissipation of ∼380 kW/g of gold. Smaller diameter gold nanoparticles (< 50 nm) heat at nearly twice the rate of larger diameter gold nanoparticles (≥50 nm), which is attributed to the higher resistivity of smaller gold nanostructures. A Joule heating model has been developed to explain this phenomenon and provides critical insights into the rational design and engineering of nanoscale materials for noninvasive thermal therapy of cancer. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. These two authors made an equal contribution to the work.     

Synthesis and characterization of bionanoparticle—Silica composites and mesoporous silica with large pores

A sol-gel process has been developed to incorporate bionanoparticles, such as turnip yellow mosaic virus, cowpea mosaic virus, tobacco mosaic virus, and ferritin into silica, while maintaining the integrity and morphology of the particles. The structures of the resulting materials were characterized by transmission electron microscopy, small angle X-ray scattering, and N₂ adsorption-desorption analysis. The results show that the shape and surface morphology of the bionanoparticles are largely preserved after being embedded into silica. After removal of the bionanoparticles by calcination, mesoporous silica with monodisperse pores, having the shape and surface morphology of the bionanoparticles replicated inside the silica, was produced,. This study is expected to lead to both functional composite materials and mesoporous silica with structurally well-defined large pores. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Role of catalysts in the surface synthesis of single-walled carbon nanotubes

We demonstrate the role of catalysts in the surface growth of single-walled carbon nanotubes (SWNTs) by reviewing recent progress in the surface synthesis of SWNTs. Three effects of catalysts on surface synthesis are studied: type of catalyst, the relationship between the size of catalyst particles and carbon feeding rates, and interactions between catalysts and substrates. Understanding of the role of catalysts will contribute to our ability to control the synthesis of SWNTs on various substrates and facilitate the fabrication of nanotube-based devices.      

Exploring the transferability of large supramolecular assemblies to the vacuum-solid interface

We present an interplay of high-resolution scanning tunneling microscopy imaging and the corresponding theoretical calculations based on elastic scattering quantum chemistry techniques of the adsorption of a gold-functionalized rosette assembly and its building blocks on a Au(111) surface with the goal of exploring how to fabricate functional 3-D molecular nanostructures on surfaces. The supramolecular rosette assembly stabilized by multiple hydrogen bonds has been sublimed onto the Au(111) surface under ultra-high vacuum conditions; the resulting surface nanostructures are distinctly different from those formed by the individual molecular building blocks of the rosette assembly, suggesting that the assembly itself can be transferred intact to the surface by in situ thermal sublimation. This unanticipated result will open up new perspectives for growth of complex 3-D supramolecular nanostructures at the vacuum-solid interface. This article is published with open access at Springerlink.com     

Nanocrystals: Solution-based synthesis and applications as nanocatalysts

Nanocrystals are emerging as key materials due to their novel shape- and size-dependent chemical and physical properties that differ drastically from their bulk counterparts. The main challenges in this field remain rationally controlled synthesis and large scale production. This article reviews recent progress in our laboratory related to solution-based synthesis of various nanostructures, including zero-dimensional (0-D) nanocrystals, 1-D nanowires and nanorods, hollow structures, and superlattice materials. On the other hand, the essential goal for nanoresearchers is to achieve industrial applications of nanostructured materials. In the past decades, these fascinating materials have been widely used in many promising fields such as nanofabrication, nanodevices, nanobiology, and nanocatalysis. Herein, we focus on their applications as nanocatalysts and try to illustrate the main problems and future directions in this area based on our recent endeavors in catalytic applications of nanocrystals. This article is published with open access at Springerlink.com     

Self-organized growth of complex nanotube patterns on crystal surfaces

The organization of carbon nanotubes into well-defined straight or curved geometries and arrays on surfaces is a critical prerequisite for their integration into nanocircuits and a variety of functional nanosystems. We review the recent development of a new approach to carbon nanotube organization based on self-organized growth directed by well-defined crystal surfaces, or “nanotube epitaxy”. We identify three different modes of surface-directed growth, namely by atomic rows, atomic steps, and nanofacets. Particular emphasis is given here to the combinations of such surface-directed growth with external forces—like those exerted by an electric field or gas flow—for the creation of well-defined complex geometries, including crossbar architectures, serpentines, and coils.      

Bending-induced conductance increase in individual semiconductor nanowires and nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts. Significant conductance increases of up to 113% were observed on bending individual ZnO nanowires or CdS nanobelts. This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements, such as using different manipulating tips (silicon, glass or tungsten) to bend the nanowires or nanobelts, and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect. This article is published with open access at Springerlink.com     

Thermoelectric properties of p-type PbSe nanowires

The thermoelectric properties of individual solution-phase synthesized p-type PbSe nanowires have been examined. The nanowires showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at room temperature.      

Single-walled carbon nanotube growth with non-iron-group “catalysts” by chemical vapor deposition

Various materials have been found to “catalyze” carbon nanotube growth in chemical vapor deposition (CVD) when they become nano-sized particles. These involve not only metals, such as Pd, Pt, Au, Ag, and Cu, but also semiconductors, such as Si, Ge, and SiC. Alumina and diamond nanoparticles also produce carbon nanotubes. These “catalysts”, which are better called “seeds”, can be categorized into two types: one type forms a eutectic liquid or highly-mobile alloy with carbon, and carbon atoms precipitate from the eutectic alloy; the other type remains as a solid phase and form a carbon surface layer during CVD growth. In this paper, we review recent studies of SWCNT growth with these non-iron-group materials and highlight the mechanisms involved.      

Holed nanostructures formed by aluminum droplets on a GaAs substrate

We have studied the morphology evolution of holed nanostructures formed by aluminum droplet epitaxy on a GaAs surface. Unique outer rings with concentric inner holed rings were observed. Further, an empirical equation to describe the size distribution of the outer rings in the holed nanostructures has been established. The contour line generated by the equation provides physical insights into quantum ring formation by droplets of group III materials on III–V substrates.      

Tuning reaction processes for the synthesis of micron and nanometer sized, single crystalline lamellae of copper 7,7,8,8-tetracyano-p-quinodimethane (Phase II) with large area

Two simple methods have been demonstrated to obtain large area, single crystalline lamellae of copper-7,7,8,8-tetracyanoquinodimethane (CuTCNQ). The formation of the lamellae was a result of fine tuning of the processes during the synthesis processes of CuTCNQ phase II. This facile synthesis of large area single crystalline lamellae suggests bright prospects for the study and understanding of the electrical switching of CuTCNQ by using single crystals of its phase II, and future applications of the material in memory and switching devices. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Synthesis of WS2 and MoS2 fullerene-like nanoparticles from solid precursors

Inorganic fullerene-like WS₂ and MoS₂ nanoparticles have been synthesized using exclusively solid precursors, by reaction of the corresponding metal oxide nanopowder, sulfur and a hydrogen-releasing agent (NaBH₄ or LiAlH₄), achieved either by conventional furnace heating up to ∼900 °C or by photothermal ablation at far higher temperatures driven by highly concentrated white light. In contrast to the established syntheses that require toxic and hazardous gases, working solely with solid precursors permits relatively safer reactor conditions conducive to industrial scale-up.      

Viruses and virus-like protein assemblies—Chemically programmable nanoscale building blocks

Supramolecular proteins are generated using a limited set of twenty amino acids, but have distinctive functionalities which arise from the sequential arrangement of amino acids configured to exquisite three-dimensional structures. Viruses, virus-like particles, ferritins, enzyme complexes, cellular micro-compartments, and other supramolecular protein assemblies exemplify these systems, with their precise arrangements of tens to hundreds of molecules into highly organized scaffolds for nucleic acid packaging, metal storage, catalysis or sequestering reactions at the nanometer scale. These versatile protein systems, dubbed as bionanoparticles (BNPs), have attracted materials scientists to seek new opportunities with these pre-fabricated templates in a wide range of nanotechnology-related applications. Here, we focus on some of the key modification strategies that have been utilized, ranging from basic protein conjugation techniques to more novel strategies, to expand the functionalities of these multimeric protein assemblies. Ultimately, in combination with molecular cloning and sophisticated chemistries, these BNPs are being incorporated into many applications ranging from functional materials to novel biomedical drug designs.      

Nanopumping molecules via a carbon nanotube

We demonstrate the feasibility of using a carbon nanotube to nanopump molecules. Molecular dynamics simulations show that the transport and ejection of a C₂₀ molecule via a single-walled carbon nanotube (SWNT) can be achieved by a sustained mechanical actuation driven by two oscillating tips. The optimal condition for nanopumping is found when the tip oscillation frequency and magnitude correlate to form quasi steady-state mechanical wave propagation in the SWNT, so that the energy transfer process is optimal leading to maximal molecular translational motion and minimal rotational motion. Our finding provides a potentially useful mechanism for using an SWNT as a vehicle to deliver large drug molecules.      

Au-Ag alloy nanoporous nanotubes

Metallic nanostructures with hollow interiors or tailored porosity represent a special class of attractive materials with intriguing chemicophysical properties. This paper presents the fabrication of a new type of metallic nanoporous nanotube structure based on a facile and effective combination of nanocrystal growth and surface modification. By controlling the individual steps involved in this process, such as nanowire growth, surface modification, thermal diffusion, and dealloying, one-dimensional (1-D) metallic nanostructures can be prepared with tailored structural features and pre-designed functionalities. These tubular and porous nanostructures show distinct optical properties, such as tunable absorption in the near-infrared region, and enhanced capability for electrochemiluminescence signal amplification, which make them particularly desirable as novel 1-D nanocarriers for biomedical, drug delivery and sensing applications. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Thin films of ruthenium phthalocyanine complexes

Four new ruthenium phthalocyanine complexes bearing axial ligands with thioacetate groups that facilitate thin film formation on gold surfaces are presented. Scanning tunnelling microscopy (STM) images and surface coverage data obtained by solution inductively coupled plasma mass spectrometry (ICP-MS) experiments show that peripheral and axial ligand substituents on the complexes have a significant effect on their surface coverage. A laser ablation ICP-MS technique that provides information about thin films across macro-sized areas is described here for the first time. Using the technique, the maximum surface coverage of a ruthenium phthalocyanine complex was found to occur within one minute of gold substrate immersion in the complex-containing solution.      

Synthesis of high magnetic moment CoFe nanoparticles via interfacial diffusion in core/shell structured Co/Fe nanoparticles

We report the synthesis of high magnetic moment CoFe nanoparticles via the diffusion of Co and Fe in core/shell structured Co/Fe nanoparticles. In an organic solution, Co nanoparticles were coated with a layer of Fe to form a Co/Fe core/shell structure. Further raising the solution temperature led to inter-diffusion of Co and Fe and formation of CoFe alloy nanoparticles. These nanoparticles have high saturation magnetization of up to 192 emu/g CoFe and can be further stabilized by thermal annealing at 600 °C. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. These two authors made an equal contribution to the work.     

Isolation of single-walled carbon nanotube enantiomers by density differentiation

Current methods of synthesizing single-walled carbon nanotubes (SWNTs) result in racemic mixtures that have impeded the study of left- and right-handed SWNTs. Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation. Enantiomer separation is enabled by the chiral surfactant sodium cholate, which discriminates between left- and right-handed SWNTs and thus induces subtle differences in their buoyant densities. This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm. In addition, circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.