A one-pot route to stable Pickering emulsions featuring nanocrystalline Ag and Au.

A simple, one-pot reaction scheme leading to water-in-toluene and toluene-in-water type Pickering emulsions solely stabilized by nanocrystals of Ag and Au is described. Sol properties-ageing and ability to disperse substances are studied. The nature of the solid surfactants and their surface structure is ascertained by transmission electron microscopy and nuclear magnetic resonance spectroscopy.

Deposition of Ag and Ag-Au nanocrystalline films with tunable conductivity at the water-toluene interface.

Thin films of Au, Ag and Ag-Au alloy nanocrystals extending to areas of several square centimetres are obtained by deposition at the interface of water and toluene. Toluene containing chlorotris(triphenylphosphine)silver(i) and/or chlorotriphenylphosphine gold(i) is reacted with aqueous tetrakishydroxymethylphosphonium chloride to obtain nanocrystalline films adhered to the interfacial region. Alloying was induced by varying the composition of the toluene layer. The composition change results in regular and reproducible variation in the transport characteristics of the films, with the initially metallic deposits turning non-metallic with increased Au content. The films at the interface were transferred to different substrates and characterised using atomic force microscopy, UV-visible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy.

Characteristics of nanocrystalline thin films of cadmium sulphide deposited at the water-oil interface.

Thin films of nanocrystalline CdS were obtained at the water-toluene interface by reacting cadmium diethyldithiocarbamate in toluene with aq. Na2S. Three parameters unique to the topical deposition scheme: the effect of column heights, stirring and the action of molecular surfactants are systematically investigated. The obtained nanocrystalline aggregates are characterized by scanning- and transmission electron microscopy, X-ray diffraction and profilometric measurements. Conditions for obtaining smooth device quality thin films have been identified during these experiments.

Assembly of Submicron Sized Ag, Co, and Ni Particles Into Thin Films at Liquid/Liquid Interfaces.

Submicron sized particles of Ag, Co and Ni were synthesised by reducing metal salts in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP). These particles on dispersion in water when held in contact with a toluene layer were found to assemble into dense films extending over large areas at the interface between the two liquids. The effect of reaction conditions (solvent, precursor concentration, temperature) on the synthesis and assembly of the particles was studied. The characteristics of the interfacial deposits and the particulate dispersions were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectroscopy and dynamic light scattering (DLS).

Surface properties of nanocrystalline PbS films deposited at the water-oil interface: a study of atmospheric aging.

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

Thin films of metals, metal chalcogenides and oxides deposited at the water-oil interface using molecular precursors.

The interface between two immiscible liquids is emerging as a powerful medium for the growth and deposition of nanostructured thin films of metals and compound semiconductors. Materials obtained through this exceedingly simple route seem to often adopt fascinating forms and morphologies. In this article, we outline features unique to the interface that shape the striking deposits and highlight the nature and properties of nanostructured films obtained using this technique.

Substituent effects on charge transport in films of Au nanocrystals.

Charge transport (CT) in films of arylthiol-capped Au nanocrystals (NCs) exhibits strong substituent effects, with electron-donating substituents markedly decreasing conductivity. Films suited for measurements were obtained by ligand-exchange reactions on AuNCs grown at the water/toluene interface. Detailed analysis suggests the NCs interact with the ligands by resonance rather than inductive effects. The films were characterized by TEM, SEM, XPS, UV/vis, and AFM. CT characteristics were studied between 15 and 300 K.

Interfacial synthesis of pyramidal lead sulfide crystallites with high energy {331} facets.

Single crystals of PbS in the form pyramids with a high proportion of high energy {331} faces are obtained by the use of a novel reaction technique at the interface of water and toluene. The pyramids spontaneously form a monolayer spread across the entire water-toluene interface. The influence of deposition parameters such as temperature, solution concentration, reaction time on the nature and properties of interfacial films are studied. In addition, the effect of solvo-static parameters such as column height and interfacial area are investigated. The obtained films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM).

Growth of nanocrystals and thin films at the water-oil interface.

The use of the water-oil interface provides significant advantages in the synthesis of inorganic nanostructures. Employing the water-toluene interface, luminescent CdS nanocrystals have been obtained at a relatively modest temperature of 35 degrees C. The diameters of the particulates can be varied between 1.0 and 5.0 nm. In addition, we have devised a new method for transferring thin films at the water-toluene interface onto solid substrates. Using this method, thin films consisting of Au and Ag nanocrystals spread over very large areas (square centimetres) are obtained in a single step. These films are directly usable as ingredients of functional devices. We show this by constructing a working amine sensor based on films of Au nanocrystals. The materials obtained have been characterized by X-ray diffraction, scanning and transmission electron microscopy, absorption and emission spectroscopy and charge transport measurements.

Facile deposition of nanodimensional ceria particles and their assembly into conformal films at liquid-liquid interface with a phase transfer catalyst.

Crystalline nanodimensional particulates of ceria have been obtained in the form of conformal thin films at the interface of two immiscible liquids. For the first time, interfacial deposition has been accomplished by the use of a simple phase transfer process eliminating the need to synthesize metal precursors. The obtained films have been characterized by X-ray diffraction, scanning electron microscopy, as well as low and high resolution transmission electron microscopy. We envisage that the study would simplify the delivery of ions to the interface and pave the way for widespread application of this emerging deposition technique.

Deposition of hierarchical Cd(OH)2 anisotropic nanostructures at the water-toluene interface and their use as sacrificial templates for CdO or CdS nanostructures.

Hierarchical anisotropic structures ranging from triangular 'platelets' to nanofibres of Cd(OH)2 are synthesized at a water-toluene interface; the nanowires are used as sacrificial templates to produce CdO wires and CdS nanostructures, preserving many structural aspects.

Pyramidal lead sulfide crystallites with high energy {113} facets.

A new, generic method to exercise control over the shape of crystallites is reported. Crystals of PbS are grown in the unusual form of pyramids at the water-toluene interface. The pyramids are single crystalline and adopt a unique growth habit (slow growth along [113] direction). The pyramids are exclusive products of the reaction and are obtained in the form of a monolayered film spread across the fluid interface. The origins of the growth habit and assembly lie in phenomena unique to the liquid-liquid interface. The dimensions of the pyramidal base can be controllably varied in the range 575-1260 nm. Crystallites of other forms such as rods and spheres can be obtained by varying the properties of the fluid interface.

Methanolysis of tetraphenylborate (BPh4(-)) as a reaction unit in halotris(2,4-pentadianato) complexes of Zr(IV) and Hf(IV).

The complexes {(C5H8O2)2Zr[(OCH3)2B(C6H5)2]2.1/2C12H10} (M = Zr (1) or Hf (2)) have been obtained from the reaction of [M(acac)3Cl] with sodium tetraphenylborate (BPh4(-)) in methanol. Crystal structures show 8-coordinate complexes with two 2,4-pentanedionato groups and two dimethoxydiphenylborate groups each coordinated through methoxy groups to the metal centre.

The synthesis of inorganic materials at the water-toluene interface.

The synthesis of thin films and nanocrystalline matter at the interface of two liquids is emerging as an important area in synthesis of inorganic materials. The advances in this area are reviewed to highlight potential uses of this method. The current understanding of the deposition process is discussed.

Optical properties of ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions.

ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions were obtained by thermolysis of a family of metal cupferrates. The nanocrystals were characterized by X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscopy, UV-visible, luminescence, and excitation spectroscopy. The band gap of the nanocrystals can be tuned in the range of 2.9-3.8 eV by the use of the dopants. In most cases, the nanocrystals are sufficiently defect-free to exhibit band edge luminescence.

A simple numerical calculation correctly predicts the observed size regime for growth of tetrapodal chalcogenide nanocrystals.

Synthetic schemes yielding monodisperse nanocrystalline particulates in the form of tetrapods, rods, and prisms have been recently discovered. Conditions promoting growth of different shapes of particulates have been defined. However, the rationale for anisotropic growth and the evolution of specific shapes, such as tetrapods, is as yet unknown. On the basis of simple numerical calculations, we suggest that the need to conserve the surface atoms plays a vital role in the growth of nanocrystals in the formation of tetrapods and correctly predicts the size regime in which such structures are obtained.

Nanocrystalline ZnO with ultraviolet luminescence.

Octylamine capped ZnO nanoparticles with mean diameters in the range of 4-14 nm were obtained by thermolysis of single source metal-organic precursors. The nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, and UV-visible, luminescence, and excitation spectroscopies. The nanocrystals are sufficiently defect-free to exhibit band edge luminescence. The size dependence of the changes in the band gap have been calculated by different methods and compared with the Kayanuma model.

Perpendicular magnetization in self-assembled films of citrate-capped gamma-Fe2O3 nanocrystals on Si(100) surfaces.

gamma-Fe2O3 nanocrystals capped with citrate and octylamine have been chemically prepared. The octylamine-capped nanocrystals exhibit a tendency to form ordered lattices. Films of nanocrystals of varying thickness (454, 720, and 1400 microg/cm2 in the case of citrate-capped nanocrystals and 300 microg/cm2 in the case of octylamine-capped nanocrystals) have been prepared on Si(100) substrates by drop casting and have been characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Magnetic measurements have been carried out on the films as well as on nanocrystal powders. The films of citrate-capped gamma-Fe2O3 nanocrystals exhibit enhanced perpendicular magnetization, with the anisotropy depending on the film thickness.

Size-dependent chemistry: properties of nanocrystals.

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.