Faceted Nanoparticles Research Articles

Finding the Limits of Magnetic Hyperthermia on Core-Shell Nanoparticles Fabricated by Physical Vapor Methods

Magnetic nanoparticles can generate heat when exposed to an alternating magnetic field. Their heating efficacy is governed by their magnetic properties that are in turn determined by their composition, size and morphology. Thus far, iron oxides (e.g., magnetite, Fe3O4) have been the most popular materials in use, though recently bimagnetic core-shell structures are gaining ground. Herein we present a study on the effect of particle morphology on heating efficiency. More specifically, we use zero waste impact methods for the synthesis of metal/metal oxide Fe/Fe3O4 nanoparticles in both spherical and cubic shapes, which present an interesting venue for understanding how spin coupling across interfaces and also finite size effects may influence the magnetic response. We show that these particles can generate sufficient heat (hundreds of watts per gram) to drive hyperthermia applications, whereas faceted nanoparticles demonstrate superior heating capabilities than spherical nanoparticles of similar size.

Macroscopic materials assembled from nanoparticle superlattices.

Nanoparticle assembly has been proposed as an ideal means to program the hierarchical organization of a material by using a selection of nanoscale components to build the entire material from the bottom up. Multiscale structural control is highly desirable because chemical composition, nanoscale ordering, microstructure and macroscopic form all affect physical properties1,2. However, the chemical interactions that typically dictate nanoparticle ordering3-5 do not inherently provide any means to manipulate structure at larger length scales6-9. Nanoparticle-based materials development therefore requires processing strategies to tailor micro- and macrostructure without sacrificing their self-assembled nanoscale arrangements. Here we demonstrate methods to rapidly assemble gram-scale quantities of faceted nanoparticle superlattice crystallites that can be further shaped into macroscopic objects in a manner analogous to the sintering of bulk solids. The key advance of this method is that the chemical interactions that govern nanoparticle assembly remain active during the subsequent processing steps, which enables the local nanoscale ordering of the particles to be preserved as the macroscopic materials are formed. The nano- and microstructure of the bulk solids can be tuned as a function of the size, chemical makeup and crystallographic symmetry of the superlattice crystallites, and the micro- and macrostructures can be controlled via subsequent processing steps. This work therefore provides a versatile method to simultaneously control structural organization across the molecular to macroscopic length scales.

Structural Evolution of the Surface and Interface in Bimetallic High-Index Faceted Heterogeneous Nanoparticles.

Bimetallic high-index faceted heterostructured nanoparticles represent a new class of high-performance nanocatalysts. In this work, we investigated the structural evolution of PtAu tetrahexahedral heterostructured nanoparticles enclosed by {210} facets using molecular dynamics simulations. The surface and interface were specifically addressed. The results show that the PtAu nanoparticle exhibits a heterogeneous melting pattern, leading to solid-liquid coexistence over a wide temperature range. In terms of particle shape evolution, the critical transformation temperature for the surface structure of the PtAu heterostructured nanoparticle is much lower than the melting point of each domain. In comparison, the interface could be basically retained even when the Au domain completely melts. These results extend our fundamental understanding of the thermally driven structural evolution of the surface and interface in bimetallic high-index faceted heterostructured nanoparticles and provide insight into the design and application of metallic nanoparticles with multifunctional performance.

Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope (AFM-SECM).

Scanning electrochemical microscopy (SECM) is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. Atomic force microscopy (AFM) is a versatile tool to characterize micro- and nanostructure in terms of topography and mechanical properties. However, conventional SECM or AFM provides limited laterally resolved information on electrical or electrochemical properties at nanoscale. For instance, the activity of a nanomaterial surface at crystal facet levels is difficult to resolve by conventional electrochemistry methods. This paper reports the application of a combination of AFM and SECM, namely, AFM-SECM, to probe nanoscale surface electrochemical activity while acquiring high-resolution topographical data. Such measurements are critical to understanding the relationship between nanostructure and reaction activity, which is relevant to a wide range of applications in material science, life science and chemical processes. The versatility of the combined AFM-SECM is demonstrated by mapping topographical and electrochemical properties of faceted nanoparticles (NPs) and nanobubbles (NBs), respectively. Compared to previously reported SECM imaging of nanostructures, this AFM-SECM enables quantitative assessment of local surface activity or reactivity with higher resolution of surface mapping.

Experimental and Theoretical Studies on Sustainable Synthesis of Gold Sol Displaying Dichroic Effect.

Gold nanoparticles depending on their shape and mixtures of multiple shapes can exhibit peculiar optical properties, including the dichroic effect typical of the Lycurgus cup, which has puzzled scientists for a long time. Such optical properties have been recently exploited in several fields such as paint technology, sensors, dichroic polarizers, display (LCD) devices, laser applications, solar cells and photothermal therapy among others. In this article, we have demonstrated a simple room temperature one-pot synthesis of gold sol displaying a dichroic effect using a slow reduction protocol involving only trisodium citrate as a reducing agent. We found that the dichroic gold sol can be easily formed at room temperature by reducing gold salt by trisodium citrate below a certain critical concentration. The sol displayed an orangish-brown color in scattered/reflected light and violet/blue/indigo/purple/red/pink in transmitted light, depending on the experimental conditions. With minor changes such as the introduction of a third molecule or replacing a small amount of water in the reaction mixture with ethanol, the color of the gold sol under transmitted light changed and a variety of shades of red, pink, cobalt blue, violet, magenta and purple were obtained. The main advantage of the proposed method lies in its simplicity, which involves the identification of the right ratio of the reactants, and simple mixing of reactants at room temperature with no other requirements. TEM micrographs displayed the formation of two main types of particles viz. single crystal gold nanoplates and polycrystalline faceted polyhedron nanoparticles. The mechanism of growth of the nanoplates and faceted polyhedron particles have been described by an enhanced diffusion limited aggregation numerical scheme, where it was assumed that both trisodium citrate and the gold ions in solution undergo a stochastic Brownian motion, and that the evolution of the entire system is regulated by a principle of energy minimization. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct.

Oxygen adsorption on high-index faceted Pt nanoparticles.

High-index faceted Pt nanoparticles with excellent electrocatalytic performances are promising to efficiently accelerate the oxygen reduction reactions in fuel cells. By adopting the hybrid grand canonical Monte Carlo reactive molecular dynamics (GCMC/RMD) simulations, we examined the oxygen adsorption on three 24-facet nanoparticles respectively enclosed by {310}, {311}, and {331} high-index facets. The site-dependent adsorption energies on each open-structure surface are calculated. Meanwhile, the adsorption ratios under various pressures and temperatures are presented. It is revealed that the adsorption capacity of these high-index faceted nanoparticles is considerably higher than that of the ones terminated by low-index facets. Moreover, oxygen adsorption exerts a significant impact on their thermodynamic behaviors.

Dependence of inherent selective atomic layer deposition of FeOx on Pt nanoparticles on the coreactant and temperature

Selective growth of metal oxides on metal via atomic layer deposition (ALD) has attracted great interest due to their potential applications in the semiconductor industry, as well as energy and environment fields. In this work, the influence of an oxidizing coreactant and the reaction temperature on the selective growth of FeOx on the facets of Pt nanoparticles and low coordination edge sites are studied via in situ Fourier transform infrared spectroscopy (FTIR) and first-principles calculations combined with microkinetic methods. It is found that selective deposition on the low coordination edge sites of Pt nanoparticles is realized when using O3 as the coreactant at low temperature (150 °C), while the ALD reaction takes place only above 250 °C without selectivity when using O2 as the coreactant. Based on density functional theory calculations, the edge-selective growth of FeOx on Pt is attributed to energy barrier differences for ALD reactions taking place at the Pt(111), Pt(100), and edge sites. Moreover, reaction rate analysis indicates that the selectivity of FeOx ALD on Pt nanoparticles is temperature dependent and that a high temperature suppresses the selectivity between different sites, which is also confirmed by the FTIR results.

The role of defects in solid state dewetting of ultrathin Ag film on Si(557)

We have studied the solid state dewetting of ten monolayers thick Ag film deposited on periodically patterned Si(557) surface. The annealing of the system in the ultra-high vacuum at the temperatures between 300 and 400 °C resulted in full agglomeration of the film and formation of faceted single crystalline Ag nanoparticles exhibiting bimodal size distribution. We demonstrated that some particles contain screw dislocations producing a step on the upper particle facet. We related the bimodality in particles distribution with the ability of dislocation-containing Ag particles to evolve by Ostwald ripening mechanism.

Microstructure evolution of thin nickel films with embedded chromium oxide nanoparticles

We developed a fabrication method of thin metal matrix composite films reinforced with ceramic nanoparticles. To this end, we deposited a 20 nm Ni / ultrathin Cr-oxide layer / 20 nm Ni tri-layer on a sapphire substrate, and annealed the sample at the temperature of 700°C. We found that after annealing for 10 min the Cr-oxide layer agglomerates into single- or bi-crystalline faceted Cr2O3 nanoparticles, with the orientation relationship of (111¯)Ni∥(0006)Cr2O3and [112]Ni∥[12¯10]Cr2O3 to the Ni matrix. We encountered a surprising vertical spread of the centers of mass of the Cr-oxide particles with respect to the initial position of the Cr-oxide layer. We developed a kinetic model of the evolution of particles shape and of their vertical drift, which enabled an estimate of the effective diffusion coefficient of the Ni-Cr2O3 interface (Di=7.4·10−19m2s). Though much slower than grain boundary diffusion in Ni, this interface diffusion enables fast transformation of ultra-thin Cr-oxide layer into an array of isolated Cr2O3 nanoparticles embedded in the Ni film. The insight into the kinetics of the system obtained in the present work paves the way for the design of particle reinforcement of thin metal films with a precise control over the spatial positions of the particles.

Microstructure Evolution of Thin Nickel Films with Embedded Chromium Oxide Nanoparticles

We developed a fabrication method of thin metal matrix composite films reinforced with ceramic nanoparticles. To this end, we deposited a 20 nm Ni / ultrathin Cr-oxide layer / 20 nm Ni tri-layer on a sapphire substrate, and annealed the sample at the temperature of 700 °C. We found that after annealing for 10 min the Cr-oxide layer agglomerates into single- or bi-crystalline faceted Cr2O3 nanoparticles, with the orientation relationship of (111)Ni Ι Ι (000) Cr203 and [112] NiΙ Ι [1210] Cr2O3 to the Ni matrix. We encounter a surprising vertical spread of the centers of mass of the Cr-oxide particles with respect to the initial position of the Cr-oxide layer. We developed a kinetic model of the evolution of particles shape and of their vertical drift, which enabled an estimate of the effective diffusivity of the Ni-Cr2O3 interface ( Di = 7.4 . 10 -19 m2/s). Though much slower than grain boundary diffusion in Ni, this interface diffusivity enables fast transformation of ultra-thin Cr-oxide layer into an array of isolated Cr2O3 nanoparticles embedded in the Ni film. The insight into the kinetics of the system obtained in the present work paves the way for the design of particle reinforcement of thin metal films with a precise control over the spatial positions of the particles.

Giant shape- and size-dependent compressive strength of molybdenum nano- and microparticles

The ability to process metallic samples at the sub-micrometer scale raised the strength limits of pure metals to the Giga Pascal (GPa) range. Here, we fabricated Mo nanoparticles with a giant compressive strength surpassing the previous strength records of metallic materials. Round and faceted particles were produced by manipulating the annealing atmosphere during two-stage solid-state dewetting of Mo thin films deposited on sapphire. The round particle underwent a huge elastic deformation before yielding abruptly. Using finite element analysis, we found that the resolved shear stress on a {112}〈110〉 slip system beneath the punch reaches an enormous value of 20±1 GPa at yield, regardless of particle size. The faceted nanoparticles, contrarily, followed a “smaller is stronger” rule, with uniaxial compressive strength of up to 46 GPa for the smallest nanoparticles. Molecular dynamics simulations indicated that the size effect diminishes with increasing roundness of the particle edges. This work demonstrates how shape and size of particles can be manipulated to achieve giant strength.

Exsolution of Catalytically Active Iridium Nanoparticles from Strontium Titanate.

The search for new functional materials that combine high stability and efficiency with reasonable cost and ease of synthesis is critical for their use in renewable energy applications. Specifically in catalysis, nanoparticles, with their high surface-to-volume ratio, can overcome the cost implications associated with otherwise having to use large amounts of noble metals. However, commercialized materials, that is, catalytic nanoparticles deposited on oxide supports, often suffer from loss of activity because of coarsening and carbon deposition during operation. Exsolution has proven to be an interesting strategy to overcome such issues. Here, the controlled emergence, or exsolution, of faceted iridium nanoparticles from a doped SrTiO3 perovskite is reported and their growth preliminary probed by in situ electron microscopy. Upon reduction of SrIr0.005Ti0.995O3, the generated nanoparticles show embedding into the oxide support, therefore preventing agglomeration and subsequent catalyst degradation. The advantages of this approach are the extremely low noble metal amount employed (∼0.5% weight) and the catalytic activity reported during CO oxidation tests, where the performance of the exsolved SrIr0.005Ti0.995O3 is compared to the activity of a commercial catalyst with 1% loading (1% Ir/Al2O3). The high activity obtained with such low doping shows the possibility of scaling up this new catalyst, reducing the high cost associated with iridium-based materials.

Investigation on the photocatalytic and sonophotocatalytic activities of {002} facets of ZnO nanoparticles synthesized through template/surfactant-free hydrothermal method at different temperatures and time durations

Template/surfactant-free ZnO nanoparticles were synthesized by hydrothermal process at 120 °C, 150 °C, 180 °C and 210 °C for 3-h duration and were annealed at 400 °C for 3 h. Since the ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h showed improved photocatalytic activities, the ZnO nanoparticles were further synthesized at 150 °C for 6-h, 9-h, and 12-h durations and were annealed at 400 °C for 3 h. All the synthesized ZnO nanoparticles were characterized for their structural, optical, and morphological properties. X-ray diffraction analysis confirmed that the ZnO nanoparticles belong to the hexagonal wurtzite system. Transmission electron microscopy and high-resolution transmission electron microscopy analyses revealed that the hydrothermally synthesized ZnO nanoparticles at 150 °C for 3 h and annealed at 400 °C for 3 h acquired spherical and hexagonal morphologies with size ~ 50 nm. Variation in the hydrothermal temperatures and time durations with annealing at 400 °C for 3 h acquired nearly hexagonal, spheroidal, and hexagonal morphology of the ZnO nanoparticles. Optical band gap of synthesized ZnO nanoparticles was slightly influenced by the different synthesis parameters. The photocurrent measurements revealed that the ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h possess relatively enhanced of photocurrent of about 3.58 μA than that of the other synthesized samples. The ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h showed ~ 78% and 36% of photodegradation efficiency against Rhodamine B dye and 4-chlorophenol, respectively, in a time period of 60 min. The sonophotocatalytic activity process enhanced degradation efficiency to 99% in 60 min against Rhodamine B and 80% in 60 min against 4-chlorophenol.

Probing the facet-dependent intermediate in the visible-light degradation of RhB by carbon-coated anatase TiO2 nanoparticles

The Facet-dependent photocatalytic process related to the degradation of organic pollutants by semiconductor catalysts is of great significance for understanding the photocatalytic mechanism of nanomaterials and improving their catalytic performance. Herein, the titanium dioxide catalysts with {001} or {101} dominant exposed facets coated with carbon (TiO2(001)/C and TiO2(101)/C) were synthesized by a hydrothermal method, and the photo-degradation research of rhodamine B (RhB) was analyzed by using UV–visible absorption spectroscopy, fluorescence spectroscopy and time-resolved fluorescence spectroscopy. The results demonstrated that the degradation of RhB was the competition for both N-deethylation and the destruction of aromatic ring reactions. The N-deethylation of RhB molecules dominated the reaction in the TiO2(001)/C system during the initial irradiation period. Whereas, both N-deethylation and cleavage of the aromatic ring occurred simultaneously in TiO2(101)/C/RhB system. The results of the fluorescence spectra and fluorescence lifetime analysis also suggested that the degradation rate of the intermediate product (Rh-110) is faster for TiO2(001)/C than TiO2(101)/C. We proposed a possible explanation that the carbonaceous species on the surface of TiO2 can absorb visible light to assist the degradation of RhB. The difference in surface properties and energy band structures lead to the different degradation processes and facet dependent features. Our findings presented the intermediate evidences for revealing the relation between the photocatalytic activity and exposed facets of TiO2 nanoparticles.

Theoretical study of CO2 hydrogenation on Cu surfaces.

CO2 reduction has attracted extensive attentions for its wide applications in chemical engineering and green chemistry. As one of major commercial catalysts, Cu have been widely studied considering its low price and high catalytic efficiency. However, previous studies were mostly focused on the Cu(111) surface, while other surfaces were rarely studied. In this work, we employed the density functional theory calculations to fully investigate the adsorption of all intermediates and products of CO2 hydrogenation on three low-index surfaces as Cu(111), Cu(100), and Cu(110), which have been reported as the main facets of Cu nanoparticles under reaction conditions. Besides, the reaction pathways were also discussed. Our results indicated CO2 hydrogenation is preferred to adopt formate pathways on the Cu surfaces, while the COOH pathway is least favorable. Moreover, Cu(100) and Cu(110) surfaces have the comparable (even better) catalytic activities compared with Cu(111) surface. This study provides the fundamental data for the adsorption and reaction of CO2 hydrogenation, which will be helpful for the design of Cu-based nanocatalysts.

Giant Size- and Shape-Dependent Uniaxial Compressive Strength of Molybdenum Nano- and Microparticles

The ability to process metallic samples at the sub-micrometer scale raised the strength limits of pure metals to the Giga Pascal (GPa) range. Here, we fabricated Mo nanoparticles with a giant compressive strength surpassing the previous strength records of metallic materials. Round and faceted particles were produced by manipulating the annealing atmosphere during two-stage solid-state dewetting of Mo thin films deposited on sapphire. The round particle underwent a huge elastic deformation before yielding abruptly. Using finite elements analysis, we found that the resolved elastic shear stress on a {112} slip system beneath the punch reaches an enormous value of 20±1 GPa regardless of particle size. The faceted nanoparticles, contrarily, followed a smaller is stronger rule, with uniaxial compressive strength of up to 46 GPa for the smallest nanoparticles. Molecular dynamics simulations indicated that the size effect diminishes with increasing roundness of the particle edges. This work demonstrates how shape and size of particles can be manipulated to achieve giant strength.

A One‐Pot Route to Faceted FePt‐Fe3O4 Dumbbells: Probing Morphology–Catalytic Activity Effects in O2 Reduction Catalysis

AbstractThe design and synthesis of faceted nanoparticles with a controlled composition is of enormous importance to modern catalyst engineering. Faceted FePt‐Fe3O4 dumbbell nanoparticles are prepared by a simple, one‐pot technique that avoids the need for expensive additives or preformed seeds. The faceted product consists of an FePt octopod and a cubic Fe3O4 lobe, of mean diameter 13.6 and 14.9 nm, respectively. The mass normalized activity for electrocatalytic oxygen reduction shows that this new structure types outperforms related catalysts in alkaline media. This work illustrates the power of morphology control and tailoring crystal facet abundance at the nanoparticle surface for enhancing catalytic performance.

Polyhedral Water Droplets: Shape Transitions and Mechanism.

While classical liquid droplets are rounded, transitions have recently been discovered which render polyhedral water-suspended droplets of several oils. Yet, the mechanism of these transitions and the role of the droplets' interfacial curvature in inducing these transitions remain controversial. In particular, one of the two mechanisms suggested mandates a convex interface, in a view from the oil side. Here we show that oil-suspended water droplets can spontaneously assume polyhedral shapes, in spite of their concave interface. These results strongly support the alternative mechanism, where the faceting in both oil and water droplets is driven by the elasticity of a crystalline monolayer, known to self-assemble at the oil-water interface, independent of its curvature. The faceting transitions in the water droplets allow the fundamental elastic properties of two-dimensional matter to be probed, enable new strategies in faceted nanoparticle and nanoshell synthesis, and provide insight into the molecular mechanisms of morphogenesis.

Tailored Hydrothermal Synthesis of Specific Facet-Dominated TiO2 Nanocrystals from Lepidocrocite-Type Layered Titanate Nanosheets: Systematical Investigation and Enhanced Photocatalytic Performance

A series of samples including leaf-like and rod-like rutile TiO2 nanoparticles with various facets exposed on the surface, parallelepiped-shaped anatase nanoparticles with [111] vertical facet exposed on the surface, irregular anatase nanoparticles, microsized six-point star-like anatase aggregates, and almond-like brookite aggregates had been hydrothermally synthesized from lepidocrocite-type layered titanate nanosheets. A systematical investigation was established to uncover the phase transition and morphological evolution from nanosheets to TiO2 polymorphs, and a phase diagram was determined by adjusting the synthesis parameters of the pH value and temperature. Two kinds of mechanisms composed of the dissolution-deposition process following Ostwald's ripening mechanism and the in situ topochemical conversion process following Ostwald's step rule had been proposed based on the time-dependent hydrothermal experiments. Briefly, the formation of the single-crystalline rutile phase appeared only at high temperatures with very low pH values, and similarly, the brookite phase strictly formed at high temperatures with a very high pH value. Nevertheless, the anatase phase could moderately appear in a wide range of temperatures and pH values. In addition, the single-crystalline rutile adopted a leaf-like morphology at low temperatures with high pH values and a rod-like morphology at high temperatures with low pH values, while the morphological evolution of anatase particles proceeded from irregular to parallelepiped-shaped and finally to six-point star-like morphology, and the crystal size was reduced from 1000 to 5 nm with decreasing pH values. Meanwhile, with the prolongation of the hydrothermal time, the layered titanate nanosheets first dissolved into the amorphous state and further converted into small anatase nanoparticles and finally to rutile or anatase nanoparticles based on the dissolution-deposition process, or the {010}-faceted layered titanate structure first converted into the [111]-vertical faceted anatase nanosheets by the topochemical transformation reaction and then split into the [111]-vertical faceted anatase nanoparticles. More importantly, the mesoporous [111]-vertical faceted anatase nanoparticles exhibited enhanced photocatalytic performance compared to that of Degussa P25, which was ascribed to its superior electronic band structure and effective charge separation. The systematical investigation in this work would be significant for consummating the preparation of the TiO2 polymorphs from layered titanate nanosheets and provided some reference values and guide schemes for the preparation of TiO2 nanoparticles with outstanding photocatalytic performance.

Morphology Evolution of Hcp Cobalt Nanoparticles Induced by Ru Promoter

AbstractThe modification of the equilibrium morphology to expose active facets of cobalt nanoparticles is essential for tailoring and controlling its catalytic performance. Periodic density functional theory (DFT) was applied to systematically investigate the structure and stability of exposed facets of hcp Co nanoparticles induced by Ru promoter in order to clarify the facet‐dependent equilibrium morphology of hcp Co nanoparticles stabilized by Ru promoter. The adsorption and migration of Ru atom can give rise to the change of the surface energy of exposed facets, thus affecting the equilibrium morphology of the hcp Co nanoparticles. Selectively exposed the high Miller index facets, such as Co(10‐12), Co(11‐20) and Co(11‐21) surfaces, can be tuned by adding Ru promoter. Our theoretical calculation results show that Co(11‐21) becomes the predominantly exposed facet accompanied with the decrease of exposed Co(10‐10) and Co(10‐11) with the increase of Ru loading. Further experimental studies suggest that the area of selectively exposed the Co(10‐11) surface for uniform Co nanoplates loaded with 3.0 wt % Ru will be reduced, which is in agreement with the theoretical predicted result. This work is helpful to design the desired Co‐based FTS catalysts rationally with well‐controlled surface structure.