Size- and shape-dependent phase transformations in wurtzite ZnS nanostructures

This paper describes the equilibrium morphologies of zinc sulfide nanoparticles in the wurtzite phase as a function of size, determined using ab initio Density Functional Theory (DFT) simulations and a shape-dependent thermodynamic model predicting the Gibbs free energy of a nanoparticle. We investigate the relative stabilities of a variety of nanoparticle shapes based on the wurtzite structure and show how the aspect ratio of wurtzite nanorods moderates the size-dependent phase transformation to the zinc blende phase. We find that while wurtzite nanoparticles are thermodynamically unstable with respect to the low energy rhombic dodecahedron morphology in the zinc blende phase at all sizes, shape- and size-dependent phase transformations occur when other zinc blende morphologies are present. Despite popular synthesis of zinc sulphide nanoparticles in the wurtzite phase, an in-depth thermodynamic study relating to the relative stability of wurtzite shapes and comparison with the zinc blende phase does not exist. Therefore this is the first thermodynamic study describing how shape can determine the solid phase of zinc sulfide nanostructures, which will be of critical importance to experimental applications of nanostructured zinc sulfide, where phase and shape determines properties.     

On the formation mechanism of the "pancake" decahedron gold nanoparticle

We have studied the thermodynamic and kinetic growth mechanisms behind the formation of the "pancake" decahedron (D(h)) gold nanoparticle using computer simulation. Free energy calculations showed that the full pancake morphology is thermodynamically unstable across all the nanoparticle size ranges studied. However, from observations of growth simulations we discovered that a kinetic transport mechanism plays a significant contributing role in the formation process through a transfer of adatoms from the top and bottom (111) D(h) faces to the side (100) faces. More specifically we observed how diffusing adatoms on the (111) face are at times "pulled" off this face and into the (111)-(100) edge of the D(h), forcing a row of (100) side atoms into a (1x5) hexagonal reconstruction. Subsequently, this row of atoms was observed to buckle and then deconstruct forcing adatoms out onto the (100) side face completing the transfer. This transport mechanism is shown to be the main kinetic driving force behind the growth of the thermodynamically unstable pancake D(h) nanoparticle. The observed mechanism has implications for the nonequilibrium morphologies of nanoparticles involving a (100)-(111) surface boundary, especially for systems with surface reconstructions which increase the density of the surface.     

Experimental and computational growth morphology of two polymorphs of a yellow isoxazolone dye

We report on the crystal structures and the experimentally found and the computationally predicted growth morphologies of two polymorphs of a yellow isoxazolone dye. The stable polymorph I has a blocklike habit, and the metastable polymorph II grows as fine needles, nucleating only by heterogeneous or contact nucleation. The habits of both polymorphs depend on the supersaturation during growth. The experimental observations are compared with predictions of the attachment energy model and kinetic Monte Carlo lattice simulations in which the growth is modeled as an "atomistic process", governed by surface energetics. These Monte Carlo simulations correctly predict the shape and the dependence on supersaturation of the crystal morphology for both polymorphs: for polymorph I, a strong dependence on supersaturation is found from the simulations. For polymorph II, the order of morphological importance is reproduced correctly, as well as the needlelike morphology.     

Computational modeling of nanorod growth

In this computational study, we used molecular dynamics and the embedded atom method to successfully reproduce the growth of gold nanorod morphologies from starting spherical seeds in the presence of model surfactants. The surfactant model was developed through extensive systematic attempts aimed at inducing nonisotropic nanoparticle growth in strictly isotropic computational growth environments. The aim of this study was to identify key properties of the surfactants which were most important for the successful anisotropic growth of nanorods. The observed surface and collective dynamics of surfactants shed light on the likely growth phenomena of real nanoprods. These phenomena include the initial thermodynamically driven selective adsorption, segregation, and orientation of the surfactant groups on specific crystallographic surfaces of spherical nanoparticle seeds and the kinetic elongation of unstable surfaces due to growth inhibiting surfactants on those surfaces. Interestingly, the model not only reproduced the growth of nearly all known nanorod morphologies when starting from an initial fcc or fivefold seed but also reproduced the experimentally observed failure of nanorod growth when starting from spherical nanoparticles such as the I(h) morphology or morphologies containing a single twinning plane. Nanorod morphologies observed in this work included fivefold nanorods, fcc crystalline nanorods in the [100] direction and [112] directions and the more exotic "dumbell-like" nanorods. Non-nanorod morphologies observed included the I(h) and the nanoprism morphology. Some of the key properties of the most successful surfactants seemed to be suggestive of the important but little understood role played by silver ions in the growth process of real nanorods.     

各向异性金纳米粒子的制备及其在催化中的应用

尽管有关金纳米粒子催化的研究工作很多,但其中大多数都是采用传统的浸渍法将金盐负载到载体上、共沉淀或沉积-沉淀法制得负载的纳米粒子,但这些方法并未吸收最新的纳米技术。最近,金催化剂的研究者开发了在胶态悬浮液中制取金属纳米粒子,然后进行固载,从而使得单金属和双金属催化剂的催化活性和形貌控制取得较大进展。另一方面,最近十年出现了金纳米粒子合成的高级控制技术,得到了许多各向异性的金纳米粒子,且很容易制得新的形貌,可以控制纳米粒子的表面原子配位数和光学特性(可调的等离子体带),这些都与催化密切相关。这些形貌包括纳米棒、纳米星、纳米花、树枝状纳米结构或多面体纳米粒子等。除了高度关注各向异性金纳米粒子的最新开发的制备方法和性质,本综述也清楚地总结了这些纳米粒子独特的催化性能,以及通过提供更高催化性能的金催化剂、控制暴露的活性位,以及热、电和光催化的鲁棒性和可调性,从而给多相催化领域带来令人惊奇的潜在变革。     

On morphologies of gold nanoparticles grown from molecular dynamics simulation

The authors use a newly fitted gold embedded atom method potential to simulate the initial nucleation, coalescence, and kinetic growth process of vapor synthesized gold nanoparticles. Overall the population statistics obtained in this work seemed to mirror closely recent experimental HREM observations by Koga and Sugawara [Surf. Sci. 529, 23 (2003)] of inert gas synthesized nanoparticles, in the types of nanoparticles produced and qualitatively in their observance ratio. Our results strongly indicated that early stage coalescence (sintering) events and lower temperatures are the mainly responsible for the occurrence of the Dh and fcc based morphologies, while "ideal" atom by atom growth conditions produced the Ih morphology almost exclusively. These results provide a possible explanation as to why the Dh to Ih occurrence ratio increases as a function of nanoparticle size as observed by Koga and Sugawara.     

纳米金淀积的多孔硅靶增强样品的激光解吸/电离质谱信号

硅片类型和多孔硅结构的多样性影响了多孔硅表面的激光解吸/离子化质谱(DIOS)(无辅助基质的激光解吸/电离飞行时间质谱(LDI-TOF-MS))数据的重复性和靶的耐储时间。本工作通过在多孔硅的表面淀积金纳米颗粒并将其作为目标靶来增强软物质分子如聚乙二醇和多肽的激光解吸/电离质谱信号。纳米金的淀积钝化了多孔硅表面的Si-H活性基团,增加了靶的耐储时间。用场发射扫描电镜表征了多孔硅淀积金纳米颗粒前后的形貌,用X射线能量色散光谱法分析金的百分含量,结果表明其含量随沉积时间的延长而增加。激光解吸/电离质谱信号的增强可能是由多孔硅及其支持的金纳米颗粒的光学和物理性质引起的,该类型的样品靶在激光解吸/电离飞行时间质谱的应用上结合了多孔硅和金纳米颗粒的双重优势。     

Synthesis of gold nanoparticles by microemulsion assisted photoreduction method

A new, clean, cost-effective and rapid method for the synthesis of stable spherical gold nanoparticles (AuNPs) is developed. This novel technique combines microemulsion as one of soft-nanotechnology techniques of wet chemistry, with photo-physics of UV-radiation in a unique versatile method to design and obtain controlled nanostructures for multifunctional materials. Based on a phase diagram in ternary water/Brij 30/n-heptane system pristine, and thiol functionalized, gold nanoparticles were obtained by a microemulsion assisted photoreduction technique, allowing increased flexibility during the synthesis and selection of materials. The spherical nanoparticles obtained by this route show a homogeneous size distribution, with an average diameter of 11 nm, for pristine gold nanoparticles and of 12 nm, for functionalized species. The evolution of the system at the nanoscale has been studied using, in tandem, UV-VIS and DLS measurements. The structure, size and shape of the final nanoparticles obtained have been evaluated by adequate instrumental techniques: FTIR, XRD and TEM image analysis. Kinetic studies have also been performed in order to follow the evolution of nanospecies during irradiation procedure.     

Facet selectivity in gold binding peptides: exploiting interfacial water structure

Peptide sequences that can discriminate between gold facets under aqueous conditions offer a promising route to control the growth and organisation of biomimetically-synthesised gold nanoparticles. Knowledge of the interplay between sequence, conformations and interfacial properties is essential for predictable manipulation of these biointerfaces, but the structural connections between a given peptide sequence and its binding affinity remain unclear, impeding practical advances in the field. These structural insights, at atomic-scale resolution, are not easily accessed with experimental approaches, but can be delivered via molecular simulation. A current unmet challenge lies in forging links between predicted adsorption free energies derived from enhanced sampling simulations with the conformational ensemble of the peptide and the water structure at the surface. To meet this challenge, here we use an in situ combination of Replica Exchange with Solute Tempering with Metadynamics simulations to predict the adsorption free energy of a gold-binding peptide sequence, AuBP1, at the aqueous Au(111), Au(100)(1 × 1) and Au(100)(5 × 1) interfaces. We find adsorption to the Au(111) surface is stronger than to Au(100), irrespective of the reconstruction status of the latter. Our predicted free energies agree with experiment, and correlate with trends in interfacial water structuring. For gold, surface hydration is predicted as a chief determining factor in peptide–surface recognition. Our findings can be used to suggest how shaped seed-nanocrystals of Au, in partnership with AuBP1, could be used to control AuNP nanoparticle morphology.     

Synthesis of uniform gold nanoparticles using non-pathogenic bio-control agent: evolution of morphology from nano-spheres to triangular nanoprisms

Green synthesis of gold nanospheres with uniform diameter and triangular nanoprisms with optically flat surface was carried out using a non-pathogenic bio-control agent Trichoderma asperellum for reduction of HAuCl(4). Kinetics of the reaction was monitored by UV-Vis absorption spectroscopy. No additional capping/complexing agent was used for stabilizing the gold nanoparticles. Evolution of morphology from pseudospherical nanoparticles to triangular nanoprisms was studied by transmission electron microscopy (TEM). It revealed that three or more pseudospheres fused to form nanoprisms of different shapes and sizes. Slow rate of reduction of HAuCl(4) by constituents of cell-free fungal extract was instrumental in producing such exotic morphologies. Isolation of gold nanotriangles from the reacting masses was achieved by differential centrifugation.     

Plasmonic properties of single multispiked gold nanostars: correlating modeling with experiments

Gold nanostars, possessing multiple sharp spikes, have emerged as promising plasmonic particles in the field of ultrasensitive sensing. We have developed a water-based method for high-yield synthesis of size-tunable anisotropic gold nanoparticles with a varying number of spiky surface protrusions, and performed systematic experimental and theoretical analyses of the optical properties of the single gold nanostars by characterizing them simultaneously with scanning electron microscopy and dark-field scattering spectroscopy. The morphologies and corresponding scattering spectra of the individual gold nanostars have been compared with electromagnetic simulations of the plasmonic resonances utilizing the finite-difference time-domain (FDTD) method. The study provides a correlation between the experimental and calculated scattering spectra and charge distributions of the different plasmon modes in the individual gold nanostars with varying numbers and relative orientations of surface protrusions. Our results provide guidelines for choosing gold nanostars with a proper number of spikes and appropriate dimensions of the core and arms for particular plasmonic applications as well as for further developing preparation methods of multispiked metal nanoparticles.     

A General Route to Hollow Mesoporous Rare‐Earth Silicate Nanospheres as a Catalyst Support

Hollow mesoporous structures have recently aroused intense research interest owing to their unique structural features. Herein, an effective and precisely controlled synthesis of hollow rare‐earth silicate spheres with mesoporous shells is reported for the first time, produced by a simple hydrothermal method, using silica spheres as the silica precursors. The as‐prepared hollow rare‐earth silicate spheres have large specific surface area, high pore volume, and controllable structure parameters. The results demonstrate that the selection of the chelating reagent plays critical roles in forming the hollow mesoporous structures. In addition, a simple and low‐energy‐consuming approach to synthesize highly stable and dispersive gold nanoparticle–yttrium silicate (AuNPs/YSiO) hollow nanocomposites has also been developed. The reduction of 4‐nitrophenol with AuNPs/YSiO hollow nanocomposites as the catalyst has clearly demonstrated that the hollow rare‐earth silicate spheres are good carriers for Au nanoparticles. This strategy can be extended as a general approach to prepare multifunctional yolk–shell structures with diverse compositions and morphologies simply by replacing silica spheres with silica‐coated nanocomposites.     

Gold and silver nanoparticles: a class of chromophores with colors tunable in the range from 400 to 750 nm

This paper presents several solution-phase methods for the large-scale synthesis of metal nanoparticles with controllable compositions (e.g., spherical nanoparticles of gold/silver alloys), morphologies (e.g., nanospheres, triangular nanoplates, circular nanodisks, and nanocubes of silver), and structures (e.g., solid vs. hollow colloids). Spectral measurements indicated that the positions of surface plasmon resonance (SPR) bands for these nanoparticles could be tuned by varying all these parameters. The number of SPR peaks was found to increase as the symmetry of the nanoparticles decreased. In addition to their use as chromophores with strong extinction coefficients, these nanoparticles could serve as a platform to probe binding events of chemical and biochemical species on their surfaces. Gold nanoshells with hollow interiors were, in particular, shown to exhibit a much higher sensitivity to environmental changes than gold solid colloids with roughly the same size.     

Effect of Gold Nanosphere Surface Chemistry on Protein Adsorption and Cell Uptake In Vitro

Gold nanoparticles exhibit unique spectral properties that make them ideal for biosensing, imaging, drug delivery, and other therapeutic applications. Interaction of gold nanoparticles within biological environments is dependent on surface characteristics, which may rely on particular capping agents. In this study, gold nanospheres (GNS) synthesized with different capping agents??specifically citric acid (CA) and tannic acid (TA)??were compared for serum protein adsorption and cellular uptake into a lung epithelial cell line (A549). Both GNS samples exhibited noticeable protein adsorption based on surface charge data after exposure to serum proteins. Light scattering measurements revealed that GNS-CA-protein composites were smaller and less dense compared to GNS-TA-protein composites. The cell uptake characteristics of these nanoparticles were also different. GNS-CA formed large clusters and elicited high uptake, while GNS-TA were taken up as discrete particles, possibly through nonendosomal mechanisms. These results indicate that the capping agents used for GNS synthesis result in unique biological interactions.     

Microscale golden Candock leaves self-aggregated on a polymer surface: Raman scattering enhancement and superhydrophobicity

Gold nanoparticles (AuNP's) prepared through a controllable synthesis and aggregation process are attractive for their unique properties that arise from their surface plasmon resonances (SPRs). However, aggregation-controlled AuNP's on amorphous surfaces have not been well explored. In this study, we present a simple in situ synthesis method for preparing AuNP's in which the AuNP's self-aggregate into microscale Candock-leaf-like structures on a polyelectrolyte film (PEF) surface. In this approach, the PEF plays an important role in adsorbing and storing AuCl(4)(-) as well as in controlling the release speed of AuCl(4)(-) in the preparation process. The mechanism for forming these Candock-leaf-like structures has been illustrated by both the growth process of gold nanoparticles and the Ostwald ripenning of the aggregations. AuNP's with a unique structure exhibited significantly enhanced surface Raman scattering and strong superhydrophobicity.     

Characterization of gas-expanded liquid-deposited gold nanoparticle films on substrates of varying surface energy

Dodecanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO(2)-expanded hexane onto substrates of different surface energy. The different surface energies were achieved by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). Following the deposition of AuNP films, the films were characterized to determine the effect of substrate surface energy on nanoparticle film deposition and growth. Interestingly, the critical surface tension of a given substrate does not directly describe nanoparticle film morphology. However, the results in this study indicate a shift between layer-by-layer and island film growth based on the critical surface tension of the capping ligand. Additionally, the fraction of surface area covered by the AuNP film decreases as the oleophobic nature of the surfaces increases. On the basis of this information, the potential exists to engineer nanoparticle films with desired morphologies and characteristics.     

Solid Phase Supported “Click”-Chemistry Approach for the Preparation of Water Soluble Gold Nanoparticle Dimers

We investigated the potential of the Cu(I) catalyzed azide-alkyne cycloaddition between water soluble azide and alkyne functionalized gold nanoparticles in terms of dimer formation via a solid phase support. Alkyne and azide lipoic acid derivatives are prepared and utilized as stabilizing ligands for 15?nm gold colloids. For the solid phase supported click reaction first citrate stabilized gold nanoparticles are immobilized on amine terminated silicon wafers. In the following step the citrate ligands of the upper free accessible nanoparticle surface are exchanged against a mixture of the alkyne derivative of lipoic acid and lipoic acid. Upon addition of lipoic acid/lipoic acid azide derivative stabilized 15?nm gold nanoparticles and the Cu(I) catalyst solution covalent interparticle coupling between immobilized and gold nanoparticles added is achieved. The formed structures are analyzed by scanning electron microscopy directly on the solid support. It is demonstrated that the yield of dimeric structures on the solid phase support increases with increased molar ratio of the catalyst, thus indicating that dimers are indeed formed by covalent bond formation. Upon treatment with ultrasound the formed structures could be released and detected with transmission electron microscopy measurements.     

Use of capillary electrophoresis for characterisation of vinyl‐terminated Au nanoprisms and nanooctahedra

It is described a simple, rapid and efficient methodology to characterise and separate gold nanoprisms and nanooctahedra by capillary electrophoresis. This technique is suitable to distinguish between morphologies and it can be used as a powerful separation tool after a customised synthesis of both structures. This synthesis was carried out by amending two parameters, temperature and pH, and a sharp decrease was found in nanotriangles when temperature was increased from 70 up to 95°C. However, when the synthesis was performed at a given temperature, an increase in pH did not promote an important change in isolation of any structure until pH = 9.5, critical in the final morphology of the nanoparticle. Gold nanoprisms and nanooctahedra were successfully separated by capillary electrophoresis according to differences in charge‐to‐mass ratio of the morphologies. Final particle morphology was confirmed by transmission electron microscopy analysis. Under optimal working conditions, a mixture containing both shapes of gold nanoparticles was initially injected and two major peaks were obtained for each structure. Capillary electrophoresis allowed to study pH and temperature influence on both morphologies. It was inferred that the ratio between triangles and octahedra decreased to a great extent when increasing both temperature and pH.     

Chemiluminescent reactions induced by gold nanoparticles

The reaction of gold nanoparticles with a potassium periodate-sodium hydroxide-carbonate system undergoes chemiluminescence with three emission bands at 380-390, 430-450, and 490-500 nm, respectively. It was found that the light intensity increased linearly with the concentration of the gold nanoparticles, and the CL intensity increased dramatically when the citrate ions on the nanoparticle surface were replaced by SCN(-). The shape, size, and oxidation state of gold nanoparticles after the chemiluminescent reaction were characterized by UV-visible absorption spectrometry, transmission electron microscopy (TEM), and X-ray photoelectron spectrometry (XPS). Gold nanoparticles are supposed to function as a nanosized platform for the observed chemiluminescent reactions. A chemiluminescent mechanism has been proposed in which the interaction between free CO(3)(*-) and O(2)(*-) radicals generated by a KIO(4)-NaOH-Na(2)CO(3) system and gold nanoparticles results in the formation of emissive intermediate gold(I) complexes, carbon dioxide dimers, and singlet oxygen molecular pairs on the surface of the gold nanoparticles. This work is not only of great importance for gaining a better understanding of the unique optical and surface properties and chemical reactivity of nanoparticles but also of great potential for developing new biosensing and immunolabeling technologies.     

Dendritic structures of poly(ethylene glycol) on silicon nitride and gold surfaces

A hydrophilic silicon nitride surface was grafted with poly(ethylene glycol) monomethyl ether (average formula weight of 5000 Da) in a one-step protocol. The domains of stable dendritic structures of self-assembled monolayer islands on a silicon nitride surface were observed with atomic force microscopy. The moduli of elasticity of these dendritic structures in air and in KCl aqueous solution were compared. The value of the Young's modulus of these structures is reduced by more than 3 orders of magnitude, from approximately 12 GPa measured in air to approximately 5 MPa in KCl solution. This dramatic reduction in elasticity was attributed to the swelling of the dendritic structures in aqueous solution, which was verified by the increased film thickness. These dendritic structures were not stable in the aqueous environment and could be removed by soaking in water for 22 h because of the hydrolysis of the silicate bonds. This fact was confirmed by the reduction of the C1s signal in the X-ray photoelectron spectroscopy experiments. These morphologies are not unique to silicon nitride substrate; similar features were also observed for thiolated poly(ethylene glycol) monomethyl ether molecules absorbed on a gold surface.