D. Mondelaers

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.

Thermal decomposition of the ammonium zinc acetate citrate precursor for aqueous chemical solution deposition of ZnO

The thermal decomposition of an aqueous chemical solution deposition Zn2+-precursor is studied by HT-DRIFT (high temperature diffuse reflectance infrared Fourier transform spectroscopy), on-line coupled TGA-EGA (thermogravimetric analysis - evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS)), and HT-XRD (high temperature X-ray diffraction). Using these complementary techniques, it is found that the α-hydroxyl group of the citrate ligand plays a significant role in the decomposition pathway of the ammonium zinc acetate citrate precursor. TEM (transmission electron microscopy) shows that crystalline ZnO (zincite) is formed at 390°C, after dehydroxylation of the α-hydroxyl group and subsequent decarboxylation of the Zn2+-precursor complex. Before total calcination, ZnO particles are already formed and a residual organic backbone thereby remains. The results obtained by these complementary techniques clearly indicate the importance of thermal analysis in the preparation of ceramics through chemical solution deposition.

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 SrBi2Ta2O9 (SBT) by means of a soluble Ta(V) precursor

Abstract Aqueous sol-gel processing of pentavalent metals like Ta and Nb is very complicated since the only water soluble compound is the oxalate, which is not very suited for gel formation. Nevertheless, starting from Ta oxalate, it is possible to prepare a water soluble Ta(V) precursor that remains stable in the pH range needed for gel formation. During synthesis the oxalate is oxidized with H 2 O 2 and subsequently complexed with citric acid and as a result the peroxo–citrato tantalum(V) complex is formed. This Ta(V) precursor is afterwards used for the synthesis of the ferroelectric SrBi 2 Ta 2 O 9 (SBT). The formation of the SBT crystalline phases was investigated by means of high temperature X-ray diffraction (HT–XRD).

The Formation of Ferroelectric Bismuth Titanate (Bi4Ti3O12) from an Aqueous Metal-Chelate Gel

Bismuth titanate (Bi4Ti3O12) was synthesized by an aqueous solution-gel process starting from solutions of bismuth acetate and a peroxocitrato-Ti(IV) complex. To gain insight into the thermal decomposition pattern of the gel several thermal analysis techniques were employed: DTA, TGA-EGA (evolved gas analysis by on-line coupling to a FTIR or mass spectrometer) and HT-DRIFT. Transmission electron micrographs showed that the gel is chemically homogeneous down to ca. 5 nm and that this homogeneity is preserved throughout the heat treatment. High-temperature X-ray diffraction measurements were used to make an in situ study of the phase formation. It has been found that single phase Bi4Ti3O12 is formed at 625°C.

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.

Influence of Heat Treatment on Sr0.9Bi2.2Ta2O9 Thin Films Prepared by Aqueous CSD

Sr0.9Bi2.2Ta2O9 thin films were prepared on a Pt-substrate by aqueous chemical solution deposition. The applied heat treatment seems to have a significant influence on the ferroelectric response, crystallinity and surface morphology. For two-layer thin films annealed at 700°C in oxygen, the Pr-value increased from 3.6 until 5.5 μ C/cm2, c-axis preferred orientation was considerably lowered and rod-like instead of spherical grains were grown when the second pyrolysis step at 520°C was omitted.