Birgit Schwenzer

Kinetically controlled vapor-diffusion synthesis of novel nanostructured metal hydroxide and phosphate films using no organic reagents

Nanostructured Co5(OH)8Cl2·3H2O, Co5(OH)8(NO3)2·2H2O, Co5(OH)8SO4·2H2O, Zn5(OH)8(NO3)2·2H2O, Cu2(OH)3(NO3) and Mn3(PO4)2·7H2O thin films have been prepared using a kinetically controlled vapor-diffusion method. Vectorial control by diffusion of ammonia as a base catalyst into an aqueous metal salt solution yields large area (2 cm2) metal hydroxide and metal phosphate films with unique structures. No supporting substrate for growth of the films is necessary in this approach. The films were characterized using X-ray powder diffraction and scanning electron microscopy. The cobalt containing films were studied in more detail using transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption near edge structure and various chemical analysis techniques. For the first time the electronic properties and crystal structure of these materials could be studied in thin films not influenced by the presence of an underlying substrate. For Co5(OH)8(NO3)2·2H2O films, which crystallize in a layered hydrotalcite-like structure that is homogeneous from the nanoscale to the macroscale, unprecedented photoconductivity properties were observed. Resistivity measurements show that this material is a p-type semiconductor with an unusually long minority carrier lifetime and high carrier density.

Preparation of indium nitride micro- and nanostructures by ammonolysis of indium oxide

Indium nitride micro- and nanostructures have been prepared by the reaction of indium oxide and ammonia in the temperature range 600–730 °C. Depending on the reaction temperature, nanoparticles with a diameter of 50–100 nm, nanowires with a diameter of ∼100 nm and of varying length, hollow microtubes or two-dimensional microplates were formed, as observed by transmission electron microscopy and scanning electron microscopy. The microscopy data are consistent with the transformation from one morphology to another by means of a vapor–solid transport mechanism. All crystals possess the wurtzite structure, independent of their morphology.

Kinetic Control of Intralayer Cobalt Coordination in Layered Hydroxides: Co1−0.5xoctCoxtet(OH)2(Cl)x(H2O)n

We report the synthesis and characterization of new structural variants of the isotypic compound with the generic chemical formula, Co(1-0.5x)(oct) Co(x)(tet) (OH)2 (Cl)x (H2O)n, all modifications of an alpha-Co(OH)2 lattice. We show that the occupancy of tetrahedrally coordinated cobalt sites and associated chloride ligands, x, is modulated by the rate of formation of the respective layered hydroxide salts from kinetically controlled aqueous hydrolysis at an air-water interface. This new level of structural control is uniquely enabled by the slow diffusion of a hydrolytic catalyst, a simple technique. Independent structural characterizations of the compounds separately describe various attributes of the materials on different length scales, revealing details hidden by the disordered average structures. The precise control over the population of distinct octahedrally and tetrahedrally coordinated cobalt ions in the lattice provides a gentle, generic method for modulating the coordination geometry of cobalt in the material without disturbing the lattice or using additional reagents. A mechanism is proposed to reconcile the observation of the kinetic control of the structure with competing interactions during the initial stages of hydrolysis and condensation.

Nanostructured ZnS and CdS films synthesized using layered double hydroxide films as precursor and template.

Anion exchange reactions in layered double hydroxide films (M(OH)(2-x)(NO(3))(x).mH(2)O) followed by solid state conversion reactions are shown to yield micrometer-sized unsupported metal sulfide (M = Zn, Cd) films with unique textured morphologies. The characteristic three-dimensional nanostructured film morphology and crystallinity of the initial films are retained in the metal sulfide films although these conversion reactions involve anion exchanges concomitant with significant rearrangements of the crystal structures. Surface areas of 42 m(2)/g for zinc sulfide and 50 m(2)/g for cadmium sulfide thin films are observed. These values correspond to an increase in surface area of 75% for the Zn(5)(OH)(8)(NO(3))(2).2H(2)O to zinc sulfide conversion, while the cadmium sulfide films exhibit more than three times the surface area of their precursor material, Cd(OH)(NO(3)).H(2)O. The three-dimensional morphology of the resulting films is thus observed to combine the physical properties of the bulk materials with the advantages of higher surface areas typically associated with nanostructured or porous materials. The layered double hydroxide materials used in this study to provide both structural and chemical templates were prepared using the mild conditions of a biologically inspired vapor-diffusion catalytic synthesis.

Synthesis of luminescing (In,Ga)N nanoparticles from an inorganic ammonium fluoride precursor

A mixture of cubic and hexagonal indium gallium nitride nanoparticles was prepared by ammonolysis of the precursor ammonium hexafluoroindate-gallate [(NH4)3In1−xGaxF6]. The particles were characterized by transmission electron microscopy. The ratio of the two obtained phases, as well as their composition, was determined using X-ray diffraction, inductively coupled plasma spectroscopy and energy dispersive X-ray spectroscopy. All methods gave consistent results of ∼10% hexagonal material and ∼90% cubic material. The mixture of cubic and hexagonal indium gallium nitride exhibited photoluminescence in the visible region around 735 nm when excited by a 325 nm HeCd laser at room temperature.

Tuning the optical properties of mesoporous TiO2 films by nanoscale engineering.

The optical properties of spin-coated titanium dioxide films have been tuned by introducing mesoscale pores into the inorganic matrix. Differently sized pores were templated using Pluronic triblock copolymers as surfactants in the sol-gel precursor solutions and adjusted by varying the process parameters, such as the polymer concentration, annealing temperature, and time. The change in refractive index observed for different mesoporous anatase films annealed at 350, 400, or 450 °C directly correlates with changes in the pore size. Additionally, the index of refraction is influenced by the film thickness and the density of pores within the films. The band gap of these films is blue-shifted, presumably due to stress the introduction of pores exerts on the inorganic matrix. This study focused on elucidating the effect different templating materials (Pluronic F127 and P123) have on the pore size of the final mesoporous titania film and on understanding the relation of varying the polymer concentration (taking P123 as an example) in the sol-gel solution to the pore density and size in the resultant titania film. Titania thin film samples or corresponding titanium dioxide powders were characterized by X-ray diffraction, cross-section transmission electron microscopy, nitrogen adsorption, ellipsometery, UV/vis spectrometry, and other techniques to understand the interplay between mesoporosity and optical properties.

Correlated Compositions, Structures, and Photoluminescence Properties of Gallium Nitride Nanoparticles

Gallium nitride (GaN, E g = 3.4 eV) is one of the industrially most valuable semiconducting materials. GaN or its alloys are essential components in light emitting diodes (LEDs) and other optoelectronic devices, [ 1 ] and recently the capability of GaN to promote hydrogen production by water-splitting has gained much attention. [ 2 ] But despite the use of GaN in these commercial applications, the structural origin of the light emitting properties of GaN, and in particular the role played by defects, such as nitrogen defi ciencies, is still under theoretical and experimental investigation. [ 3–8 ] NMR analysis of atomiclevel inhomogneities in GaN materials of otherwise seemingly high purity and crystallinity, e.g. when examined by XRD, has sparked debate about the origin of these unexpected features observed in 71 Ga NMR spectra. [ 4–8 ]

Biologically inspired synthesis route to three-dimensionally structured inorganic thin films

Inorganic thin films (hydroxide, oxide, and phosphate materials) that are textured on a submicron scale have been prepared from aqueous metal salt solutions at room temperature using vapor-diffusion catalysis. This generic synthesis approach mimics the essential advantages of the catalytic and structure-directing mechanisms observed for the formation of silica skeletons of marine sponges. Chemical composition, crystallinity, and the three-dimensional morphology of films prepared by this method are extremely sensitive to changes in the synthesis conditions, such as concentrations, reaction times, and the presence and nature of substrate materials. Focusing on different materials systems, the reaction mechanism for the formation of these thin films and the influence of different reaction parameters on the product are explained.

Biologically Inspired Vapor-Diffusion Route to Metal Hydroxide Films at Low Temperatures: Synthesis, Conversion and Applications

Nanostructured thin films have been prepared using a kinetically controlled vapor-diffusion method of catalysis at low temperature. Vectorial control by diffusion of a base catalyst into an aqueous metal salt solution yields metal hydroxide films with unique structures. Post-synthesis treatment of these materials yielded more stable metal oxide films. All films were characterized using X-ray powder diffraction, scanning electron microscopy and a wide rage of chemical and electrochemical methods.

Cd1−xZnxO [0.05 ≤ x ≤ 0.26] synthesized by vapor-diffusion induced hydrolysis and co-nucleation from aqueous metal salt solutions

Nanoparticulate Cd(1-x)Zn(x)O (x = 0, 0.05-0.26, 1) is synthesized in a simple two-step synthesis approach. Vapor-diffusion induced catalytic hydrolysis of two molecular precursors at low temperature induces co-nucleation and polycondensation to produce bimetallic layered hydroxide salts (M = Cd, Zn) as precursor materials which are subsequently converted to Cd(1-x)Zn(x)O at 400 °C. Unlike ternary materials prepared by standard co-precipitation procedures, all products presented here containing < 30 mol% Zn(2+) ions are homogeneous in elemental composition on the micrometre scale. This measured compositional homogeneity within the samples, as determined by energy dispersive spectroscopy and inductively coupled plasma spectroscopy, is a testimony to the kinetic control achieved by employing slow hydrolysis conditions. In agreement with this observation, the optical properties of the materials obey Vegards Law for a homogeneous solid solution of Cd(1-x)Zn(x)O, where x corresponds to the values determined by inductively coupled plasma analysis, even though powder X-ray diffraction shows phase separation into a cubic mixed metal oxide phase and a hexagonal ZnO phase at all doping levels.