Morphology control of α-Fe2O3 towards super electrochemistry performance

α-Fe₂O₃ with various morphologies including spindle, rod, tube, disk, and ring were synthesized through controlling the H₂PO₄⁻ etching process. The concentrations of H₂PO₄⁻ plays an important role in controlling the morphology change of the samples. Selected adsorption of H₂PO₄⁻ ions resulted in anisotropic growth. In addition, the etching of H₂PO₄⁻ occurred in the center of rods which resulted in tubal α-Fe₂O₃. Nanodiscs were created once the etching process occurred on the wall of the tube. The electrochemical test shows that disklike samples revealed excellent specific capacitance, rate capacity and cycling stability because of relative higher surface area and pore structure. For the CO catalytic oxidation properties, spindle samples exhibited super catalytic activity.

α-Fe₂O₃ with various morphologies including spindle, rod, tube, disk, and ring were synthesized through controlling the H₂PO₄⁻ etching process.

Shape-controlled continuous synthesis of metal nanostructures

A segmented flow-based microreactor is used for the continuous production of faceted nanocrystals. Flow segmentation is proposed as a versatile tool to manipulate the reduction kinetics and control the growth of faceted nanostructures; tuning the size and shape. Switching the gas from oxygen to carbon monoxide permits the adjustment in nanostructure growth from 1D (nanorods) to 2D (nanosheets). CO is a key factor in the formation of Pd nanosheets and Pt nanocubes; operating as a second phase, a reductant, and a capping agent. This combination confines the growth to specific structures. In addition, the segmented flow microfluidic reactor inherently has the ability to operate in a reproducible manner at elevated temperatures and pressures whilst confining potentially toxic reactants, such as CO, in nanoliter slugs. This continuous system successfully synthesised Pd nanorods with an aspect ratio of 6; thin palladium nanosheets with a thickness of 1.5 nm; and Pt nanocubes with a 5.6 nm edge length, all in a synthesis time as low as 150 s.

Understanding the Role of Solvation Forces on the Preferential Attachment of Nanoparticles in Liquid

Optimization of colloidal nanoparticle synthesis techniques requires an understanding of underlying particle growth mechanisms. Nonclassical growth mechanisms are particularly important as they affect nanoparticle size and shape distributions, which in turn influence functional properties. For example, preferential attachment of nanoparticles is known to lead to the formation of mesocrystals, although the formation mechanism is currently not well-understood. Here we employ in situ liquid cell scanning transmission electron microscopy and steered molecular dynamics (SMD) simulations to demonstrate that the experimentally observed preference for end-to-end attachment of silver nanorods is a result of weaker solvation forces occurring at rod ends. SMD reveals that when the side of a nanorod approaches another rod, perturbation in the surface-bound water at the nanorod surface creates significant energy barriers to attachment. Additionally, rod morphology (i.e., facet shape) effects can explain the majority of the side attachment effects that are observed experimentally.

Effects of surface stability on the morphological transformation of metals and metal oxides as investigated by first-principles calculations

Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure-reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and α-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies.

Emergent parabolic scaling of nano-faceting crystal growth

Nano-faceted crystals answer the call for self-assembled, physico-chemically tailored materials, with those arising from a kinetically mediated response to free-energy disequilibria (thermokinetics) holding the greatest promise. The dynamics of slightly undercooled crystal–melt interfaces possessing strongly anisotropic and curvature-dependent surface energy and evolving under attachment–detachment limited kinetics offer a model system for the study ofthermokineticeffects. The fundamental non-equilibrium feature of this dynamics is explicated through our discovery of one-dimensional convex and concave translating fronts (solitons) whose constant asymptotic angles provably deviate from the thermodynamically expectedWulffangles in direct proportion to the degree of undercooling. Thesethermokineticsolitons induce a novel emergent facet dynamics, which is exactly characterized via an original geometric matched-asymptotic analysis. We thereby discover an emergent parabolic symmetry of its coarsening facet ensembles, which naturally implies the universal scaling lawL∼t1/2for the growth in timetof the characteristic lengthL.

Controllable synthesis of In2O3 octodecahedra exposing {110} facets with enhanced gas sensing performance

In₂O₃ octodecahedra enclosed by {110} facets with high concentration of oxygen vacancy have been prepared for enhanced gas sensing performance.

Facet-dependent photocatalytic and antibacterial properties of α-Ag2WO4crystals: combining experimental data and theoretical insights

We have combined experimental results and calculations with new paths to explain the photocatalytic and antibacterial activities of α-Ag₂WO₄crystals.

The spatial diamond-graphite transition in detonation nanodiamond as revealed by small-angle neutron scattering

A spatial transition of the carbon state in detonation nanodiamond (DND) from crystalline diamond inside the particle to a graphite-like state at the DND surface is proposed on the basis of small-angle neutron scattering (SANS) analysis. The SANS contrast variation from concentrated (5 wt%) dispersions of DND in liquids (water, dimethylsulfoxide) reveals a shift in the mean scattering length density of DND as compared to pure diamond, which is related to the presence of a non-diamond component in the DND structure. At the same time, the diffusive character of the particle surface is deduced based on the deviation from the Porod law. The two observations are combined to conclude that the continuous radial density profile over the whole particle volume conforms to a simple power law. The profile naturally suggests that non-diamond states are concentrated mainly close to the particle surface; still there is no sharp boundary between the radial distributions of the two states of carbon in DND.

Direct comparison of kinetic and thermodynamic influences on gold nanomorphology

Under a given set of conditions, nanomaterials can crystallize into structures that are entirely inconsistent with the bulk material and may adopt a range of faceted morphologies that depend on the particle size. A size-dependent phase diagram, a graphical representation of the chemical equilibrium, offers a convenient way to describe this relationship among the size, morphology, and thermodynamic environment. Although creating such a diagram from conventional experiments is extremely challenging (and costly), theory and simulation allow us to use virtual experiments to control the temperature, pressure, size, structure and composition independently. Although the stability and morphology of gold nanoparticles has been add-ressed numerous times in recent years, a critical examination of the literature reveals a number of glaring contradictions. Typically gold nanoparticles present as multiply-twinned structures, such as icosahedra and decahedra, or faceted monocrystalline (fcc) shapes, such as truncated octahedra and cuboctahedra. All of these shapes are dominated by various fractions of {111} and {100} facets, which have different surface atom densities, electronic structure, bonding, chemical reactivities, and thermodynamic properties. Although many of the computational (and theoretical) studies agree on the energetic order of the different motifs and shapes, they do not necessarily agree with experimental observations. When discrepancies arise between experimental observations and thermodynamic modeling, they are often attributed to kinetics. But only recently could researchers analytically compare the kinetics and thermodynamics of faceted nanoparticles. In this Account, we follow a theoretical study of the size, shape, and structure of nanogold. We systematically explore why certain shapes are expected at different sizes and (more importantly) why others are actually observed. Icosahedra are only thermodynamically preferred at small sizes, but we find that they are the most frequently observed structures at larger sizes because they are kinetically stable (and coarsen more rapidly). In contrast, although the phase diagram correctly predicts that other motifs will emerge at larger sizes, it overestimates the frequency of those observations. These results suggest either a competition or collaboration between the kinetic and thermodynamic influences. We can understand this interaction between influences if we consider the change in shape and the change in size over time. We then use the outputs of the kinetic model as inputs for the thermodynamic model to plot the thermodynamic stability as a function of time. This comparison confirms that decahedra emerge through a combination of kinetics and thermodynamics, and that the fcc shapes are repressed due to an energetic penalty associated with the significant departure from the thermodynamically preferred shape. The infrequent observation of the fcc structures is governed by thermodynamics alone.

Organic carbonates as stabilizing solvents for transition-metal nanoparticles

Biodegradable, non-toxic, "green" and inexpensive propylene carbonate (PC) solvent is shown to function as a stabilizing medium for the synthesis of weakly-coordinated transition-metal nanoparticles. Kinetically stable nanoparticles (M-NPs) with a small and uniform particle size (typically <5 ± 1 nm) have been reproducibly obtained by easy, rapid (3 min) and energy-saving 50 W microwave irradiation under an argon atmosphere from their metal-carbonyl precursors in PC. The M-NP/PC dispersions are stable for up to three weeks according to repeated TEM studies over this time period. The rhodium nanoparticle/PC dispersion is a highly active catalyst for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to and 1875 (mol product) (mol Rh)(-1) h(-1) and near quantitative conversion at 4 to 10 bar H(2) and 90 °C. From the PC dispersion the M-NPs can be coated with organic capping ligands such as 3-mercaptopropionic acid or trioctylphosphine oxide for further stabilization.

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.

Manufacturing of inorganic nanomaterials: concepts and perspectives

The present paper aims at extracting key physical and chemical concepts for the development of inorganic nanomaterials with controlled size, shape, and topology. In particular, efforts are made to identify general guiding principles for the rational design of 0D, 1D, 2D and 3D architectures, focusing on selected model systems as representative case studies. To this aim, different strategies and approaches are discussed, in an attempt to unify concepts and ideas common to solid-, liquid- and gas-phase synthetic routes. Furthermore, the importance of tailoring the nanomaterial composition, structure and morphology is also highlighted in relation to their eventual technological applications.

Synthesis of recrystallized anatase TiO2 mesocrystals with Wulff shape assisted by oriented attachment

In this work, we describe a kinetically controlled crystallization process assisted by an oriented attachment (OA) mechanism based on a nonaqueous sol-gel synthetic method (specifically, the reaction of titanium(IV) chloride (TiCl(4)) with n-octanol) to prepare re-crystallized anatase TiO(2) mesocrystals (single crystal). The kinetics study revealed a multi-step and hierarchical process controlled by OA, and a high resolution transmission electron microscopy (HRTEM) analysis clearly shows that the synthesized mesocrystal presents a truncated bipyramidal Wulff shape, indicating that its surface is dominated by {101} facets. This shape is developed during the recrystallization step. The material developed here displayed superior photocatalytic activity under visible light irradiation compared to TiO(2)-P25 as a benchmarking.

Nanosensors: towards morphological control of gas sensing activity. SnO2, In2O3, ZnO and WO3 case studies

Anisotropy is a basic property of single crystals. Dissimilar facets/surfaces have different geometric and electronic structure that results in dissimilar functional properties. Several case studies unambiguously demonstrated that the gas sensing activity of metal oxides is determined by the nature of surfaces exposed to ambient gas. Accordingly, a control over crystal morphology, i.e. over the angular relationships, size and shape of faces in a crystal, is required for the development of better sensors with increased selectivity and sensitivity in the chemical determination of gases. The first step toward this nanomorphological control of the gas sensing properties is the design and synthesis of well-defined nanocrystals which are uniform in size, shape and surface structure. These materials possess the planes of the symmetrical set {hkl} and must therefore behave identically in chemical reactions and adsorption processes. Because of these characteristics, the form-controlled nanocrystals are ideal candidates for fundamental studies of mechanisms of gas sensing which should involve (i) gas sensing measurements on specific surfaces, (ii) their atomistic/quantum chemical modelling and (ii) spectroscopic information obtained on same surfaces under operation conditions of sensors.