Shape-selective Synthesis Research Articles

A facile route for shape-selective synthesis of silica nanostructures using poly-l-lysine as template

Abstract A facile method for the shape-selective synthesis of silica nanostructures using a reverse-microemulsion-mediated template (RMMT) technique is reported. In this method, positive poly- l -lysine (PLL) is selected as template due to its configuration diversity. By adjusting pH and concentration, PLL demonstrates various secondary structures containing random coil, α -helix and β -sheet, which result in the formation of silica nanorods, silica nanospheres and silica nanotubes in the reverse-microemulsion system, respectively. Thus, the shape-selective synthesis of silica nanostructures might be achieved by using PLL as structural template in the reverse-microemulsion system.

Binding of Polyvinylpyrrolidone to Ag Surfaces: Insight into a Structure-Directing Agent from Dispersion-Corrected Density Functional Theory

We use dispersion-corrected density functional theory (DFT) to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanostructures by probing the interaction of its 2-pyrrolidone (2P) ring with Ag(100) and Ag(111). We employ two different semiempirical methods for including van der Waals (vdW) interactions in DFT calculations: DFT+vdWsurf and DFT-D2. We find that DFT-D2, in its original parametrization, overestimates the Ag metal dispersion interaction and causes an unphysical herringbone-like reconstruction of Ag(100). This can be remedied in DFT-D2 by using modified vdW parameters for Ag that account for many-body screening effects. The results obtained using DFT-D2 with the modified parameters agree well with experiment and with DFT+vdWsurf results. We find that 2P binds more strongly to Ag(100) than Ag(111), consistent with experiment. We analyze the origins of the surface-sensitive binding and find that vdW attraction is stronger on Ag(111), but the direct chemical bonding of 2P is stronger on Ag(100). We also study the influence of strain on binding energies and find that tension tends to lower the vdW interaction with the surfaces, while increasing the direct chemical-bonding interaction, consistent with the d-band center model. Overall, our work indicates that strain has little impact on the structure-directing capabilities of PVP, which is consistent with the fact that strained, 5-fold twinned Ag nanowires have extensive {100} facets and relative small {111} facets.

Shape-selective synthesis and optical performance of ceria nanocrystal/graphene hybrid composites

The morphology of CeO2 nanomaterials determines their properties and synthesis of well dispersed CeO2 nanocrystals with desired morphology is thus crucial for their applications. In this paper, we report the successful synthesis of CeO2 nanocrystals with different morphologies anchored on graphene hybrid composites by a hydrothermal method. Nanorods/graphene (NRs-G), nanoparticles/graphene (NPs-G) and nanocubes/graphene (NCs-G) have been selectively synthesized by optimizing experimental conditions. These nanocrystals show different exposed crystal planes and the size or morphology of the loaded CeO2 can be controlled by the alkali concentration and reactive temperature, as well as the adding of GO or not. In addition, significant changes in the photoluminescence (PL) spectra intensity and position (energy) have been observed in the synthesized samples, which depend on the morphology and size of the loaded CeO2. These interesting phenomena can be easily rationalized by the difference of oxygen storage capacity (OSC) of samples.

Hierarchically order porous lotus shaped nano-structured MnO2 through MnCO3: chelate mediated growth and shape dependent improved catalytic activity

Design of hierarchical nanostructures towards a specific morphology is an important research area due to their shape dependent properties. Here, 3D hierarchically assembled lotus shaped porous MnO2 is synthesized using a simple aqueous solution based chelating agent (citric acid) mediated growth of MnCO3 followed by calcination at 350 °C. MnCO3 in other shapes, such as rods, spheres and nano-aggregates, is also synthesized just by varying the chelating agents. It is observed that the geometry and strength of the chelating ligands has a crucial role in the controlled shape selective synthesis and based on this a probable chelating agent driven formation mechanism is discussed. The synthesized porous MnO2 shapes exhibit excellent shape dependent catalytic oxidation of α-pinene to verbenone using molecular oxygen as the oxidant. The lotus shaped porous MnO2 shows superior activity, with 94% conversion of α-pinene and 87% selectivity of verbenone, to that of other MnO2 shapes. The activity is reasonably high compared to heterogeneous as well as homogeneous catalysts reported in the literature and bulk MnO2 with respect to both their conversion and selectivity. The synthesized lotus shaped MnO2 also showed good catalytic activity towards oxidation of allylic compounds to corresponding ene–ones using molecular oxygen as oxidant and is reusable.

Facile synthesis of Cu and Cu@Cu–Ni nanocubes and nanowires in hydrophobic solution in the presence of nickel and chloride ions

A highly shape selective synthesis of Cu and Cu@Cu-Ni nanocubes and nanowires has been developed by modulating the coordination chemistry of transition metal ions with a trioctylphosphine (TOP)-Cl(-) ligand pair in oleylamine under mild organic solvent conditions. The as-prepared nanocubes have a face-centered cubic (fcc) phase and are covered by six {100} facets, whereas the as-prepared nanowires have a multi-twinned structure and grow along the [110] direction. Both the Ni(2+) and Cl(-) ions, along with TOP, play vital roles in determining the final morphology of the as-prepared nanocrystals (NCs). TOP can be used to selectively generate single-crystal seeds at the initial stage, which then grow into nanocubes in the presence of Cl(-) ions, while the absence of TOP leads to the formation of multi-twined crystal seeds that finally develop into nanowires. Moreover, Ni can be incorporated to form a Cu-Ni alloy shell over a Cu core at higher temperatures in a one-pot process, which makes diamagnetic Cu NCs magnetically responsive and has a significant influence on their optical properties.

PH- and surfactant-mediated tunable morphology and upconversion of rare-earth doped fluoride microcrystals

A novel and facile citrate-assisted hydrothermal approach has been developed for the shape-selective synthesis of fluoride microcrystals, including NaYF4 and YF3, by tuning the pH of the mother liquid. A series of controlled experiments demonstrate that three important factors, including the pH values of the mother solution and the amount of trisodium citrate in conjunction with the intrinsic character of rare-earth ions, are responsible for varied distinctive morphologies such as the truncated octahedrons, prismatic microtubes with unusual zigzag top ends, prismatic short rods with regular hexagonal concave centers, and thin plates. The possible formation mechanisms for products with various morphologies have been explored, and the mechanism based on selective adsorption and the cation exchange are proposed. The upconversion spectra of Yb/Er-doped fluoride samples with different shapes have been investigated. The shape effect on luminescence is attributed to the different crystal facet components of an anisotropic orientational microcrystal in which the crystal planes composed of the a-axis and c-axis have the difference in ion-space distance and crystallization quality.

A facile approach for shape selective synthesis of rhodium nanostructures and conductivity studies

Shape selective synthesis of Rh nanostructures has been demonstrated with the help of a single step chemical vapor deposition, where different shapes like cubes at 500 °C, pyramids and hexagons at 500 °C, 700 °C and 900 °C respectively have been obtained as a function of temperature. Scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and four probe conductivity measurements were used to study the morphology, crystallinity and phase purity of the structures. The conductivity values for as synthesised nanostructures have been obtained in the range of 2-7 kS/cm2. On the basis of the experimental results, from TGA and XPS studies possible mechanistic pathway for the evolution of Rh nanostructures was discussed.

Highly Shape‐Selective Synthesis of Monodispersed Fivefold Twinned Platinum Nanodecahedrons and Nanoicosahedrons

Highly Shape‐Selective Synthesis of Monodispersed Fivefold Twinned Platinum Nanodecahedrons and Nanoicosahedrons

Shape-selective synthesis of 2,6-diisopropylnaphthalene on H-mordenite catalysts

To finally dispel any doubts on the shape-selective formation of 2,6-diisopropylnaphthalene (2,6-DIPN) over H-MOR zeolites, naphthalene alkylation was carried out over high-silica H-MOR catalysts with propylene or isopropanol as an alkylating agent and with or without cyclohexane as a solvent. Isomeric composition of DIPN’s, determined by one-dimensional GC analysis, was additionally confirmed with advanced two-dimensional GC×GC. Our results proved beyond any doubt shape-selective formation of 2,6-DIPN over these H-MOR catalysts from naphthalene and propylene and without cyclohexane as a solvent. The DIPN mixture contained 60–64% 2,6-DIPN, and the ratio of 2,6-DIPN/2,7-DIPN was in the range 2.5–2.8. We also showed that shape-selective formation of 2,6-DIPN over H-MOR catalyst was depressed by using isopropanol instead of propylene and in the presence of cyclohexane.

A Simple Method for Controlling the Morphology of CuO Nanostructures by Droplet

Different morphology CuO nanostructures, including platelets, flower-like were simply synthesized at 350 °C controlled by droplet on hydrophobic substrate. This is a simple method which does not require any template, catalyst, or surfactant but can control the morphology of CuO from platelets to flowerlike. The morphologies are strongly dependent on the volume of droplet. Scanning electron microscopy (SEM), Optical microscope and X-ray diffraction (XRD) were used to observe the morphology, crystallinity, and chemical composition of the CuO structures. Growth mechanisms for shape selective CuO synthesis were proposed based on these results.

Adsorption of Polyvinylpyrrolidone on Ag Surfaces: Insight into a Structure-Directing Agent

We use density functional theory to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanostructures. At the segment level, PVP binds more strongly to Ag(100) than Ag(111) because of a surface-sensitive balance between direct binding and van der Waals attraction. At the chain level, correlated segment binding leads to a strong preference for PVP bind to Ag(100). Our study underscores differences between small-molecule and polymeric structure-directing agents.

Shape-selective synthesis and facet-dependent electrocatalytic activity of CoPt3 nanocrystals

Testing the facet-dependent electrocatalytic activity of noble metal alloy nanocrystals has attracted increasing interest towards improving the performance of direct methanol fuel cells. Herein, CoPt3 nanocrystals with high shape- and facet-selectivity are synthesized via a non-injection strategy. The (111)-facet-dominant nanoflowers are more active for catalytic oxidation of methanol than the (100)-facet-dominant nanocubes and nanoparticles with irregular facets.

Synthesis and piezoelectric response of cubic and spherical LiNbO3 nanocrystals

Methods have been developed for the shape-selective synthesis of ferroelectric LiNbO3 nanoparticles. Decomposition of the single-source precursor, LiNb(O-Et)6, in the absence of surfactants, can reproducibly lead to either cube- or sphere-like nanoparticles. X-Ray diffraction shows that the LiNbO3 nanoparticles are rhombohedral (R3c). Sample properties were examined by piezoresponse force microscopy (PFM) and Raman where both sets of nanoparticles exhibit ferroelectricity. The longitudinal piezoelectric coefficients, d33, varied with shape where the largest value was exhibited in the nanocubes (17 pm V−1 for the cubes versus 12 pm V−1 for spheres).

Surfactant-assisted hydrothermal synthesis of Bi2O3 nano/microstructures with tunable size

A novel and simple citrate-assisted solution approach has been developed for the shape-selective synthesis of Bi2O3 nanostructures with controllable bandgaps and morphologies at a relatively low temperature of 40 °C. Different distinctive morphologies, including nanorods, nanoplates, plate-built cylinders, nanoplates with holes, and nanorings, are created due to the selective adsorption of the citrate molecules on certain faces during crystal growth. The bandgaps and aspect ratios of the Bi2O3 nanostructures are easily tuned by modifying the product morphologies by adjusting the amount of trisodium citrate. More novel and complex Bi2O3 nanostructures with controllable morphologies and sizes can be manufactured with our method by optimizing the experimental parameters. The distinctive nanostructures presented here extend the family of Bi2O3 nanostructures, and they also provide new opportunities for exploring the potential applications of Bi2O3 in a number of fields including photocatalysis, gas sensors, and photoelectrochemistry.

Shape-tailoring and catalytic function of anisotropic gold nanostructures

We report a facile, one-pot, shape-selective synthesis of gold nanoparticles in high yield by the reaction of an aqueous potassium tetrachloroaurate(III) solution with a commercially available detergent. We prove that a commercial detergent can act as a reducing as well as stabilizing agent for the synthesis of differently shaped gold nanoparticles in an aqueous solution at an ambient condition. It is noteworthy that the gold nanoparticles with different shapes can be prepared by simply changing the reaction conditions. It is considered that a slow reduction of the gold ions along with shape-directed effects of the components of the detergent plays a vital function in the formation of the gold nanostructures. Further, the as-prepared gold nanoparticles showed the catalytic activity for the reduction reaction of 4-nitrophenol in the presence of sodium borohydride at room temperature.

Shape-Selective Synthesis and Facet-Dependent Enhanced Electrocatalytic Activity and Durability of Monodisperse Sub-10 nm Pt−Pd Tetrahedrons and Cubes

Monodisperse single-crystalline sub-10 nm Pt-Pd nanotetrahedrons (NTs) and nanocubes (NCs) were synthesized with high shape selectivity via one-pot hydrothermal routes with small ions as efficient facet-selective agents. These alloy nanocrystals showed facet-dependent enhanced electrocatalytic activity and durability for methanol electrooxidations with commercial Pt/C catalyst as a reference. The (100)-facet-enclosed Pt-Pd NCs demonstrated a higher activity, whereas the (111)-facet-enclosed Pt-Pd NTs exhibited a better durability.

Shape-selective synthesis of methylamines over the RRO zeolite Al-RUB-41

Aluminum was incorporated into the layered silicate RUB-39, which is transformed by calcination into RUB-41. This new zeolite with RRO topology contains 8- and 10-ring pores, and the acid sites in the aluminated material catalyze the synthesis of methylamines, in particular mono- and dimethylamine, by amination of methanol. Owing to the shape-selective catalytic properties of (H)Al-RUB-41, low selectivity to the thermodynamically favored trimethylamine product is obtained in comparison with results on RUB-39 or non-shape-selective materials. Both activity and selectivity are highest for RUB-41 catalysts with a high Si to Al ratio. Silylation reduces the number of unselective sites and results in a further suppression of trimethylamine formation. The introduction of acidity in the intact RUB-41 structure is supported by Al-MAS NMR and NH 3-TPD data. Additional characterization by XRD and SEM is provided.

EDTA-mediated shape-selective synthesis of Bi2WO6 hierarchical self-assemblies with high visible-light-driven photocatalytic activities

Bi2WO6 with multilayered disc-like and 3D hierarchical nest-like architectures self-assembled from 2D nanosheets were successfully synthesized by a hydrothermal method in EDTA-mediated processes. It was found that the morphology modulation of the as-obtained products could be easily realized simply by changing the amount of EDTA introduced into the reaction system. The stepwise self-assembly formation mechanisms for two different hierarchical architectures were proposed on the basis of a series of time-dependent experiments. The photocatalytic activities of the different Bi2WO6 samples for the degradation of Rhodamine B (RhB) were also evaluated. Due to the unique morphology, porous structure and large surface area, Bi2WO6 nest-like architectures exhibited better activity performance than Bi2WO6 multilayered microdiscs. The present work not only presents an efficient way for the selectively controllable preparation of Bi2WO6 hierarchical structures viaself-assembly from 2D nanosheets, but also provides a step forward in the design of photocatalysts with controllable morphology and enhanced photocatalytic activities.

Shape-Dependent Catalytic Properties of Pt Nanoparticles

Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al(2)O(3). Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis.

Highly Shape-Selective Synthesis, Silica Coating, Self-Assembly, and Magnetic Hydrogen Sensing of Hematite Nanoparticles

The open forced hydrolysis method and controllable silica growth based on bound water to polyvinylpyrrolidone molecules have been developed for the highly shape (including rhombohedra, semispheres, and rods) selective synthesis, self-assembly, and uniform silica coating (in the unprecedented range of 5-200 nm) of hematite nanoparticles. The open system realizes the direct short-range self-assembly of hematite semispheres in their growth process. The bound water method has been extended to coat gold nanoparticles with tunable silica shell and directly assemble the cores into one-dimensional, dimer, and trimer nanostructures during the coating process. The silica coating increases the particle stability and monodispersity even as hematite is modified into ferromagnetic Fe(3)O(4). The hematite@silica core-shell spheres are assembled into long-range ordered structures with considerable photonic bandgap for the first time due to their high monodispersity. By exploiting the hematite antiferromagnetism caused by the superexchange interaction via intervening oxygen ions that are sensitive to hydrogen, a novel hydrogen sensing based on magnetization variations is achieved in the hematite assemblies. Weakening the antiferromagnetism by reducing the hematite size and/or covering the hematite surface by silica coating suppresses the sensitivity to hydrogen, showing that the antiferromagnetic spin variations on the hematite surface are responsible for the gas sensing.