Search results for "nanocrystalline"

Probing NiO nanocrystals by EXAFS spectroscopy

2010

Abstract The structure relaxation in nanocrystalline NiO (nano-NiO, 13 nm crystallite size) has been studied by X-ray absorption spectroscopy at the Ni K-edge at 300 K. Conventional single-scattering analysis of the EXAFS signals from the first two coordination shells showed a lattice volume expansion by about 1% and a contraction of the Ni–O bonds by about 0.5% in nano-NiO compared to microcrystalline NiO. A more sophisticated approach, based on a combination of classical molecular dynamics and ab initio multiple-scattering EXAFS theory, allowed us to interpret both static relaxation and lattice dynamics in nano-NiO.

Fabrication and characterization of vacuum deposited fluorescein thin films

2011

Simple vacuum evaporation technique for deposition of dyes on various solid surfaces has been developed. The method is compatible with conventional solvent-free nanofabrication processing enabling fabrication of nanoscale optoelectronic devices. Thin films of fluorescein were deposited on glass, fluorine-tin-oxide (FTO) coated glass with and without atomically layer deposited (ALD) nanocrystalline 20 nm thick anatase TiO2 coating. Surface topology, absorption and emission spectra of the films depends on their thickness and the material of supporting substrate. On a smooth glass surface the dye initially formes islands before merging into a uniform layer after 5 to 10 monolayers. On FTO cove…

Amorphous Silicon Nanotubes via Galvanic Displacement Deposition

2013

Amorphous silicon nanotubes were grown in a single step into a polycarbonate membrane by a galvanic displacement reaction conducted in aqueous solution. In order to optimize the process, a specifically designed galvanic cell was used. SEM images, after polycarbonate dissolution, showed interconnected nanotube bundles with an average length of 18 μm and wall thickness of 38 nm.The deposited silicon was revealed by EDS analysis, whilst X-ray diffraction and Raman spectroscopy showed that nanotubes have an amorphous structure. Silicon nanotubes were also characterized by photo-electrochemical measurements that showed n-type conductivity and optical gap of ~1.6 eV. Keywords: Silicon nanotubes, …

Cooling of Hot Electrons in Amorphous Silicon

1997

ABSTRACTMeasurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data while an energy transfer independent of the temperature difference is not. This contrasts with the situation in crystalline silicon. The measured cooling rates are sufficient to explain the difficulty in observing avalanche effects in amorphous silicon.

Amorphous, nanocrystalline and crystalline calcium carbonates in biological materials

2010

Raman spectroscopy is a powerful tool in identifying different calcium carbonate polymorphs. Here, the method is applied to cultured pearls from freshwater (genus Hyriopsis) and marine bivalve species (Pinctada maxima) as well as to shells of Diplodon chilensis patagonicus bivalves. Raman spectra for vaterite, detected for the first time in an adult shell, and amorphous calcium carbonate (ACC) are discussed. Results for ACC are compared with those of synthetically produced ACC and with the Raman spectroscopic features of stable biogenic ACC from the crustacean Porcellio scaber. Decomposition of the most intense signal of all calcium carbonate polymorphs—the ν1 symmetric stretching mode of t…

Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookite-based) powders prepared by thermohydrolysis of TiCl4 in aqueous chlori…

2008

Abstract Nanocrystalline TiO2 powders were synthesized by thermohydrolysis of TiCl4 in HCl or NaCl aqueous solutions. Rutile, mixtures of brookite and rutile or mixtures of anatase, brookite and rutile were obtained depending on the acidity of the medium. Crystalline phases and composition of the mixtures were identified by using XRD analysis. Pure brookite nanoparticles, separated from the mixtures of brookite and rutile by simple peptization with water, were stable against transformation to rutile up to 750 °C. The prepared TiO2 powders were characterized by thermal analysis, diffuse reflectance spectroscopy and BET surface area determinations. The band gap of bulky brookite was estimated…

2018

We report a novel multi-step method for the preparation of ordered mesoporous titania scaffolds and show an illustrative example of their application to solar cells. The method is based on (monolayer) colloidal nanosphere lithography that makes use of polystyrene nanoparticles organised at a water–air interface and subsequently transferred onto a solid substrate. A titania precursor solution (titanium(IV) isopropoxide in ethanol) is then drop-cast onto the monolayer and left to “incubate” overnight. Surprisingly, instead of the expected inverse monolayer-structure, a subsequent calcination step of the precursor yields an ordered monolayer of hollow titania nanospheres with a wall thickness …

Nanostructured Solids from Freeze-Dried Precursors: Multigram Scale Synthesis of TiO2 -Based Powders

2012

Nanocrystalline TiO2 and Ti1−xVxO2 (x = 0.01) powders have been prepared by thermal decomposition, in air, of amorphous precursors resulting from the freeze-drying of appropriate solutions. In addition, TiO2−xNy (anatase and rutile) and TiOxNy (rock-salt) have been prepared by thermal treatment in ammonia of a crystalline precursor (TiO2 obtained at 673 K). TEM and SEM images, as well as the analysis of the X-ray diffraction (XRD) patterns, show the nanoparticulated character of those solids obtained at low temperatures, with typical particle sizes in the 10–20 nm range when prepared at 673 K. The UV–Vis results indicate both the insertion of V in the anatase lattice and the feasibility of …

Study of phase composition, photocatalytic activity, and photoluminescence of TiO2 with Eu additive produced by the extraction-pyrolytic method

2021

The Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2. The work was also partially supported by the LZP grant 2020/2-0074. R. Burve has been supported by the project “Synthesis of nanostructured materials based on titanium dioxide and tin dioxide and investigation of their physicochemical properties” Nr. MP-2019/7, for strengthening scientific personnel capacity 2019/2020 at the Riga Technical University. Authors are grateful to Dr. K. Šmits for the microscopic measurements and SEM images.

Chemical heterogeneities in nanometric titanomagnetites prepared by soft chemistry and studied ex situ : evidence for fe-segregation and oxidation ki…

2001

Nanocrystalline Fe-based spinels with composition Fe3-xTixO4 are synthesized using soft chemistry. Two steps are involved:  precipitation in an aqueous solution followed by thermal annealing under a reducing mixture of N2/H2/H2O gases. Fe-segregation is found inside stoichiometric particles when the powders are studied ex situ; they exhibit a strong surface iron enrichment. This heterogeneity is related to kinetic effects linked to the difference of mobility between Fe2+ and Ti4+ cations during the partial oxidation of cations occurring ex situ. Stresses in the grains induced by oxidation govern the oxidation kinetics and lead to an abrupt compositional variation inside each particle. These…