Christopher B. Murray

Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface

The spontaneous organization of multicomponent micrometre-sized colloids or nanocrystals into superlattices is of scientific importance for understanding the assembly process on the nanometre scale and is of great interest for bottom-up fabrication of functional devices. In particular, co-assembly of two types of nanocrystal into binary nanocrystal superlattices (BNSLs) has recently attracted significant attention, as this provides a low-cost, programmable way to design metamaterials with precisely controlled properties that arise from the organization and interactions of the constituent nanocrystal components. Although challenging, the ability to grow and manipulate large-scale BNSLs is critical for extensive exploration of this new class of material. Here we report a general method of growing centimetre-scale, uniform membranes of BNSLs that can readily be transferred to arbitrary substrates. Our method is based on the liquid–air interfacial assembly of multicomponent nanocrystals and circumvents the limitations associated with the current assembly strategies, allowing integration of BNSLs on any substrate for the fabrication of nanocrystal-based devices. We demonstrate the construction of magnetoresistive devices by incorporating large-area (1.5u2009mmu2009×u20092.5u2009mm) BNSL membranes; their magnetotransport measurements clearly show that device magnetoresistance is dependent on the structure (stoichiometry) of the BNSLs. The ability to transfer BNSLs also allows the construction of free-standing membranes and other complex architectures that have not been accessible previously.

Control of Metal Nanocrystal Size Reveals Metal-Support Interface Role for Ceria Catalysts

A Measure of Metal-Oxide Interfaces The rate of a catalytic reaction can sometimes be enhanced by using a different metal oxide as the support for adsorbed metal nanoparticles. Such enhancement is often attributed to more active sites at the metal-oxide interface, but it can be difficult to quantify this effect. Cargnello et al. (p. 771, published online 18 July) synthesized monodisperse nanoparticles of nickel, platinum, and palladium and dispersed them on high-surface-area ceria or alumina supports. High-resolution transmission electron microscopy enabled a detailed analysis of interfacial site structure, which showed that the rate of CO oxidation on ceria was indeed enhanced greatly at interface sites. Comparing nanocrystals of different sizes on different oxides shows that ceria-metal interface sites enhance carbon monoxide oxidation. Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts. We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals. We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support.

Prospects of Nanoscience with Nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Todays strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Quasicrystalline order in self-assembled binary nanoparticle superlattices

The discovery of quasicrystals in 1984 changed our view of ordered solids as periodic structures and introduced new long-range-ordered phases lacking any translational symmetry. Quasicrystals permit symmetry operations forbidden in classical crystallography, for example five-, eight-, ten- and 12-fold rotations, yet have sharp diffraction peaks. Intermetallic compounds have been observed to form both metastable and energetically stabilized quasicrystals; quasicrystalline order has also been reported for the tantalum telluride phase with an approximate Ta1.6Te composition. Later, quasicrystals were discovered in soft matter, namely supramolecular structures of organic dendrimers and tri-block copolymers, and micrometre-sized colloidal spheres have been arranged into quasicrystalline arrays by using intense laser beams that create quasi-periodic optical standing-wave patterns. Here we show that colloidal inorganic nanoparticles can self-assemble into binary aperiodic superlattices. We observe formation of assemblies with dodecagonal quasicrystalline order in different binary nanoparticle systems: 13.4-nm Fe2O3 and 5-nm Au nanocrystals, 12.6-nm Fe3O4 and 4.7-nm Au nanocrystals, and 9-nm PbS and 3-nm Pd nanocrystals. Such compositional flexibility indicates that the formation of quasicrystalline nanoparticle assemblies does not require a unique combination of interparticle interactions, but is a general sphere-packing phenomenon governed by the entropy and simple interparticle potentials. We also find that dodecagonal quasicrystalline superlattices can form low-defect interfaces with ordinary crystalline binary superlattices, using fragments of (33.42) Archimedean tiling as the ‘wetting layer’ between the periodic and aperiodic phases.

Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation. This general method optimizes the synthesis with initial heating rates up to 100 °C/minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms, and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these nanocrystals (NCs) into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC’s size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.

Platinum nanocrystals selectively shaped using facet-specific peptide sequences

The properties of a nanocrystal are heavily influenced by its shape. Shape control of a colloidal nanocrystal is believed to be a kinetic process, with high-energy facets growing faster then vanishing, leading to nanocrystals enclosed by low-energy facets. Identifying a surfactant that can specifically bind to a particular crystal facet is critical, but has proved challenging to date. Biomolecules have exquisite specific molecular recognition properties that can be explored for the precise engineering of nanostructured materials. Here, we report the use of facet-specific peptide sequences as regulating agents for the predictable synthesis of platinum nanocrystals with selectively exposed crystal surfaces and particular shapes. The formation of platinum nanocubes and nanotetrahedrons are demonstrated with Pt-{100} and Pt-{111} binding peptides, respectively. Our studies unambiguously demonstrate the abilities of facet-selective binding peptides in determining nanocrystal shape, representing a critical step forward in the use of biomolecules for programmable synthesis of nanostructures.

The State of Nanoparticle-Based Nanoscience and Biotechnology: Progress, Promises, and Challenges

Colloidal nanoparticles (NPs) have become versatile building blocks in a wide variety of fields. Here, we discuss the state-of-the-art, current hot topics, and future directions based on the following aspects: narrow size-distribution NPs can exhibit protein-like properties; monodispersity of NPs is not always required; assembled NPs can exhibit collective behavior; NPs can be assembled one by one; there is more to be connected with NPs; NPs can be designed to be smart; surface-modified NPs can directly reach the cytosols of living cells.

Size‐ and Shape‐Selective Synthesis of Metal Nanocrystals and Nanowires Using CO as a Reducing Agent

ties unobtainable simply by tuning the size of the spheres. The synthesis of metal NCs typically employs the reduction or decomposition of metal precursors in the presence of ligands, which prevent aggregation and improve the colloidal stability of the NCs. Among the wide spectrum of reducing agents that have been used, gases such as hydrogen under pressure have proven effective in delicately manipulating the growth kinetics and thus tailoring the size and morphology of the metal NCs. [15, 16] Despite these efforts, a one-pot synthesis of highly monodisperse metal NCs at ambient pressure using gaseous reducing agents generated at point-of-use is still an important advance. Herein we report the size- and shape-selective formation of metal nanostructures including Pt nanocubes, Pd spherical NCs, and Au nanowires (NWs) using carbon monoxide (CO, generated at point-of-use) as a reducing agent. We also discuss the implications of our observation on several recent reports of the preparation of Pt NCs utilizing metal carbonyls. In catalysis, it is well-known that particle shape (the facets exposed) can be as important as the particle surface area in activity and selectivity. For example, Pt(100) exhibits higher electrocatalytic activity than Pt(111) for the oxygen reduction reaction in H2SO4 electrolyte. [17, 18] Pt(100) also shows different selectivity from Pt(111) towards hydrogenation reactions. [19] Thus Pt nanocubes with well-defined {100} facets provide a model system for understanding microscopic surface phenomena in many catalytic processes. We report the synthesis of Pt nanocubes employing CO (generated by dehydration of formic acid; Supporting Information, Fig

Highly active Pt3Pb and core-shell Pt3Pb-Pt electrocatalysts for formic acid oxidation.

Formic acid is a promising chemical fuel for fuel cell applications. However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of direct formic acid fuel cells has been impeded by the low activity of existing electrocatalysts at desirable operating voltage. We report the first synthesis of Pt(3)Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electrocatalysts. The activity can be further improved by manipulating the Pt(3)Pb-Pt core-shell structure. Combined experimental and theoretical studies suggest that the high activity from Pt(3)Pb and the Pt-Pb core-shell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps. Therefore, the Pt(3)Pb and Pt-Pb core-shell nanocrystals can improve the performance of direct formic acid fuel cells at desired operating voltage to enable their practical application.

Synthesis and Oxygen Storage Capacity of Two‐Dimensional Ceria Nanocrystals

Shape-controlled synthesis of inorganic nanomaterials has received great attention due to their unique shapedependent properties and their various applications in catalysis, electronics, magnetics, optics, and biomedicine. Among these nanomaterials, ultrathin twodimensional (2D) anisotropic nanomaterials are especially attractive due to their high surface-to-volume ratio and potential quantum size effects. A variety of approaches have been developed to prepare such nanomaterials. Typical methods include vapor deposition, templated synthesis, electrochemical deposition, sol–gel processing, and solvothermal/hydrothermal treatments. Solution-phase chemical synthesis has proven particularly effective in controlling the size and morphology of the nanomaterials. Ceria has been widely used in catalysis, optics, sensors, and solid oxide fuel cells. Due to its high oxygen storage capacity (OSC), which originates from easy conversion between CeO2 and CeO2 x, ceria has found its primary utilization in catalysis as an oxygen carrier. Ceria nanomaterials with various morphologies, mainly polyhedra, have been reported. Recently, 1D ceria nanostructures, such as nanowires, have also been reported. However, with the exception of one report on the preparation of nanosheets, well-controlled 2D ceria nanomaterials have not been explored and the comparison of the OSC properties between 3D and 2D structures has not been possible. On the other hand, the different properties of the (100), (110), and (111) ceria facets has been debated. There is no consensus on whether crystallographic orientation or particle size affects reactivities. Therefore, high-quality ceria nanocrystals selectively exposing different low Miller-index surfaces, are crucial to enabling experiments that resolve the controversy. Here we report a simple, robust solution-phase synthesis of ultrathin ceria nanoplates in the presence of mineralizers. The morphology of nanoplates can be easily controlled by changing reaction parameters, such as precursor ratio, reaction time, etc. In addition, we also prepare ceria nanomaterials in various 3D morphologies by hydrothermal and combustion methods. The OSC of our 2D ceria materials have been tested and compared to the OSC of their 3D counterparts. In brief, the synthesis of ceria nanoplates involves the thermal decomposition of cerium acetate at 320–330 8C in the presence of oleic acid and oleylamine as stabilizers and employs sodium diphosphate or sodium oleate as mineralizers. Transmission electron microscopy (TEM) images of ceria nanoplates are shown in Figure 1. Square ceria nanoplates (S-nanoplates, Figure 1a) with an edge length of 11.9 nm (s= 7%), are synthesized with sodium diphosphate as the mineralizer while elongated ceria nanoplates (Lnanoplates, Figure 1e) with a length of 151.6 nm (s= 9%) and a width of 14.3 nm (s= 12%), are produced with sodium oleate as the mineralizer. The nanoplates in both samples have a thickness of about 2 nm. As shown in Figure 1c and g, the stacks of nanoplates confirm that the sample consists of 2D plates rather than 3D cubes or rods. S-nanoplates readily form the demonstrated stacking arrays as seen in drop-cast TEM samples. L-nanoplates only form stacks by a selfassembly at a liquid–liquid (e.g. hexane–ethylene glycol) interface. The S-nanoplates also self-assemble to a ceria nanosheet at a hexane–acetonitrile interface, as shown in Figure 3a. High-resolution TEM (HRTEM) images of both nanoplates (Figures 1d,h and S1c in the Supporting Information) reveal an interplanar distance of 0.27 nm, consistent with the (200) lattice spacing of the ceria crystal. The fast Fourier transform (FFT) patterns confirm the {100} textures of ceria nanoplates. Plates (e.g. square plates) could be enclosed by either six (100) facets or a combination of two (100) facets and four (110) facets. As illustrated in Figure S1, our HRTEM images and simulations of HRTEM images suggest that our ceria nanoplates are enclosed by six (100) [*] D. Y. Wang, Y. J. Kang, Prof. C. B. Murray Department of Chemistry University of Pennsylvania, Philadelphia, PA 19104 (USA) E-mail: cbmurray@sas.upenn.edu