H. Van den Rul

Synthesis of ZnO nanopowder via an aqueous acetate–citrate gelation method

The synthesis of nanoparticulate ZnO via an aqueous carboxylate gelation route is presented. Starting from a solution of zinc acetate with citric acid as a complexing agent, a solid glassy gel is obtained after drying that is converted into a fine powder by calcination. It is found that a very homogeneous precursor is indispensable when preparing very fine particles with a narrow size distribution. Cryo-transmission electron microscopy (Cryo-TEM) investigation is used as a feedback tool to prevent early precipitation during gelation. Study of the thermal decomposition of the gel shows that ZnO is formed before the final decomposition step takes place. After removing the organic backbone, very small oxide particles are found. The influence of the thermal treatment parameters on the particle size is investigated and a particle growth process is found. By a proper adjustment of the final calcination temperature in dry air, the mean particle size can be controlled between ∼11 and 175 nm. It was also seen that even in inert atmosphere, ZnO is formed and that particle morphology is greatly influenced by the calcination atmosphere.

Aqueous Chemical Solution Deposition of Ferroelectric Thin Films

Thin films of various ferroelectric multimetal oxides such as (Bi 1 m x La x ) 4 Ti 3 O 12 (BLT), SrBi 2 Ta 2 O 9 (SBT) and PbZr 1 m x Ti x O 3 (PZT) have been prepared by an entirely aqueous chemical solution deposition (CSD) route. Two critical issues related with aqueous CSD have hereby been worked out: in spite of the high degree of hydrolysis of tetra- and pentavalent metal ions (Ti 4+ , Zr 4+ , Ta 5+ , ) we managed to prepare stable aqueous precursor solutions by chemical modification of these individual metals, avoiding phase segregation. Another problem related with aqueous CSD is the wetting of the substrate (both metallic and metal oxide) by the aqueous solution. The hydrophilicity of the substrates is optimized by a chemical treatment of the substrate surface. In this manner, the addition of wetting agents, hence possibly disturbing the gelation reactions, is avoided. In order to study the gelation, decomposition, crystalization and the morphology of the thin films, various characterization techniques ((cryo-)TEM, SEM, EDX, TGA-MS/FTIR, HT-DRIFT, HT-XRD, ) are used.

Aqueous Solution-Gel Synthesis of Strontium Bismuth Niobate (SrBi2Nb2O9)

The synthesis of multimetal oxides containing pentavalent elements like Nb and Ta out of an aqueous solution is very complicated due to the extremely high sensitivity of these metals towards hydrolysis. Moreover the only water-soluble starting compound is the oxalate which is not very suited for gel formation. Nevertheless, from Nb-oxalate it is possible to prepare a water-soluble Nb(V)-precursor by chemical modification. The synthesized precursor remains stable in the pH-range needed for gel formation. This Nb(V)-precursor is used for the synthesis of the ferroelectric material SrBi2Nb2O9 (SBN). An aqueous acetate-citrate solution-gel precursor for SBN was prepared. The chemical homogeneity of this precursor was investigated by means of TEM-experiments. All metal ions were found to be homogeneously dispersed in the precursor gel at least down to 10 nm. The SBN phase formation is followed by means of X-ray Diffraction. It has been shown that the perovskite phase already forms at 550°C.

Chemical Solution Deposition of ZnO Thin Films by an Aqueous Solution Gel Precursor Route

An aqueous chemical solution deposition method was used to prepare thin films of ZnO on SiO2/Si (1 1 1) substrates. Starting from an aqueous solution of Zn acetate, citric acid and ammonia, very thin films could be deposited by spin coating. Heating parameters, necessary for thin film annealing, were determined using FTIR experiments on dried gel precursors, heated up to different temperatures. The morphology and the thickness of the films were investigated by SEM. It is found that homogeneous thin films with grain sizes of about 20 nm are formed. XRD experiments show that there is an indication that the films, crystallized at 500°C, exhibit preferential grain growth along the c-axis.

Synthesis of RuO2 and SrRuO3 powders by means of aqueous solution gel chemistry

Phase pure RUO 2 and SrRuO 3 powders were obtained by a water-based solution-gel route. By reacting ruthenium(III)acetylacetonate with H 2 O 2 in a citric acid solution, a stable aqueous Ru-precursor solution could be prepared. An aqueous Sr-precursor is synthesized by dissolving Sr acetate in a citric acid solution. After mixing the Ru-precursor with the Sr-precursor solution in stoichiometric quantities, a clear aqueous solution for the preparation of SrRuO 3 could be obtained. Gelation resulted in amorphous precursors that were homogeneous down to nanoscale as was confirmed by means of transmission electron microscopy. With the appropriate thermal treatment phase pure RUO 2 and SrRuO 3 powders were synthesised as verified by X-ray diffraction.

Ferroelectric SrBi 2 Nb 2 O 9 Thin Films by Aqueous Chemical Solution Deposition

SrBi 2 Nb 2 O 9 (SBN) thin films on a Pt/Ti/SiO 2 /Si substrate were prepared by aqueous chemical solution deposition. The precursor solution was synthesized by means of an aqueous solution-gel method, starting with stable, inexpensive and easily available inorganic salts which are dissolved in an aqueous solution of chelating or coordinating ligands (acetates and citrates). Afterwards the synthesized precursor was spin-coated. However, problems arose as a consequence of insufficient wetting of the substrate surface by the aqueous solution (poor film-substrate adhesion). Instead of improving surface adhesion by the addition of a surface-wetting reagent, a new strategy was developed: prior to spin-coating the platinum surface characteristics were modified using a UV/ozone technique. In this way the precursor solution was not chemically changed. Wetting/wettability was verified by means of contact angle measurements. A uniform, three-layer thin film with a total thickness of about 200 nm was obtained after thermal treatment, as could be verified using SEM and XRD.

Comparison of two novel solution-based routes for the synthesis of equiaxed ZnO nanoparticles

Due to a dominant one-dimensional growth rate, nanoparticles of zinc oxide often show a rodlike morphology. As a result, the synthesis of small spherical nanoparticles of undoped ZnO remains challenging. This paper presents two procedures that successfully produce a powder consisting of equiaxed zinc oxide nanoparticles: one using a polyethylene glycol- (PEG-) assisted solvothermal method and the other by calcination of zinc oxalate obtained from a microemulsion-mediated method. In the latter, zinc-substituted aerosol OT (AOT) is used as a surfactant. The samples are characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), photon correlation spectroscopy (PCS), and photoluminescence (PL) spectroscopy. Both synthesis techniques produce nanoparticles with similar sizes in the range of 10 to 20 nm. Dense aggregates observed in the calcined powder are infrequent in the case of the solvothermal method.

Synthesis of Tetragonal Zirconia Nanoparticles via an Aqueous Solution-Gel Method

Limburgs Univ Ctr, IMO, Lab Inorgan & Phys Chem, B-3590 Diepenbeek, Belgium. IMOMEC, B-3590 Diepenbeek, Belgium.Geuzens, E, Limburgs Univ Ctr, IMO, Lab Inorgan & Phys Chem, Univ Campus, B-3590 Diepenbeek, Belgium.

Properties of Ultrathin High Permittivity ( Nb1 − x Ta x ) 2O5 Films Prepared by Aqueous Chemical Solution Deposition

Ultrathin Nb1�xTax2O5 films, with thicknesses from 3t o 25 nm, were deposited by chemical solution deposition starting from aqueous precursor solutions. The film’s dielectric properties were characterized by capacitance–voltage and current–voltage measurements. Permittivities ranged from 20 to 31 after annealing at 600°C, with the highest value obtained for pure Nb2O5. With increasing Nb content, increasing leakage currents were observed. The crystallization temperature was determined by in situ X-ray diffraction measurement for films with 15 nm thickness: Nb2O5 was crystalline as deposited 600°C, while the crystallization temperature of solid solutions increased with increasing Ta content, up to 875°C for pure Ta2O5. NbTaO5 showed a marked increase in permittivity from 27 to 38 after crystallization anneal at 600 and 800°C, respectively. For Nb2O5, no significant difference in permittivity was observed between amorphous and crystalline layers.

Phase Formation of Perovskite Lead Magnesium Niobate Prepared by an Aqueous Solution-Gel Method

Limburgs Univ Ctr, IMO, Lab Inorgan & Phys Chem, B-3590 Diepenbeek, Belgium.Storms, A, Limburgs Univ Ctr, IMO, Lab Inorgan & Phys Chem, Univ Campus, B-3590 Diepenbeek, Belgium.