Zn Acetylacetonates Research Articles

In Situ Observation of ZnO Nanoparticle Formation by a Combination of Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction.

The formation of ZnO nanomaterials from different Zn acetylacetonate precursor solutions was studied in situ by employing simultaneous, time-resolved X-ray diffraction (XRD) and X-ray absorption spectroscopy (EXAFS) at the Zn K-edge. The precursor solutions were heated from room temperature to the desired reaction temperatures in a hermetically sealed cell dedicated to X-ray experiments. In general, the first indications for the formation of hexagonal ZnO were found for elevated temperatures of about 80 °C both by XRD and EXAFS, and the contributions increase with temperature and time. However, no reaction intermediates could be proved in addition to the Zn precursors and the formed hexagonal ZnO materials. Furthermore, the results show that the efficiency of the reaction, i.e., the conversion of the precursor material to the ZnO product, strongly depends on the solvent used and the reaction temperature. ZnO formation is accelerated by an increased temperature of 165 °C and the use of 1-octanol, with a conversion to ZnO of more than 80% after only a ca. 35 min reaction time according to a detailed analysis of the EXAFS data. For comparison, an identical concentration of Zn acetylacetonate in water or dilute alkaline NaOH solutions and a reaction temperature of around 90 °C leads to a smaller conversion of approximately 50% only, even after several hours of reaction. The particle size determined from XRD for different orientations shows a preferred orientation along the c-direction of the hexagonal crystal system, as well in accordance with scanning electron microscopy. The LaMer model explained this highly non-uniform growth of needle-like ZnO crystallites.

The nature of the metal-ligand chemical bond and the electronic structure of Be, Mg, and Zn acetylacetonates and hetero- and β-substituted analogs

The effect of valence molecular orbitals on the nature of the chemical bond of the p-metal ion with chelate ligands in Be(acac)2, Mg(acac)2 and zinc Zn(acac)2 was studied by theoretical methods. The effect of the substitution of atoms and functional groups in the ligands on the chemical bond was estimated. For beryllium, the bond is almost completely covalent with an insignificant ion component: the Coulomb potentials for complexes with beryllium are 1.37 and 1.08 eV for acetylacetonate and hexafluoroacetylacetonate, respectively. For Mg and Zn β-diketonates with a metal-oxygen bond, the calculations showed the essentially ionic nature of binding: about 8 eV for magnesium compounds and 12.3 and 11.3 eV for zinc. However, the O→S substitution significantly reduces the metal-ligand binding iconicity: to 2.8 and 0.9 eV for the thio- and dithio-substitution, respectively. The covalent bond of a metal with ligands is due to the overlapping of only some MOs with s- and p-orbitals of the metal. The covalence of the M–O bond in acetylacetonates decreases in the series Be→Mg→Zn due to an increase in the ionic radius of the metal, leading to two consequences.

Influence of the processing conditions on the structural properties of ZnO layers obtained by PECVD

The plasma enhanced chemical vapor deposition (PECVD) is a powerful and flexible instrument for depositing thin layers, nanocomposites or nanostructures. In this work ZnO layers have been grown by metal-organic PECVD (RF − 13.56 MHz) on Si wafers. Zn acetylacetonate has been used as a precursor and oxygen as oxidant. The influence of the oxygen content in gas mixture, the total pressure, substrate temperature and ZnO seed layer on the structural properties of the layers deposited on Si wafers has been studied. ZnO layer properties were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD data have shown that all layers are crystalline with hexagonal wurtzite structure. The crystallites are preferentially oriented along c-axis direction perpendicular to the substrate surfaces. The results obtained indicate that by controlling the oxygen content in gas mixture, the total pressure and substrate temperature during the film growth one can control the formation of c-axis phase and the crystallite grain size. Nanorods with good alignment, vertically orientated to the substrate surface can be observed in the layers deposited at low content of O2 in plasma at substrate temperature of 400 oC. Due to their structural characteristics these layers are potential materials for preparing chemical- and biosensors where inherently large surface to volume ratio of structured materials are important prerequisite for enhanced sensitivity.

Effect of the substrate surface topology and temperature on the structural properties of ZnO layers obtained by plasma enhanced chemical vapour deposition

In this work thin ZnO layers were grown by metal-organic PECVD (RF 13.56 MHz) on Si wafers. Zn acetylacetonate was used as a precursor and oxygen as oxidant. A system for dosed injection of the precursor and oxidant into the plasma reactor was developed. The influence of the substrate surface topology and temperature on the structural properties of the deposited layers was studied. ZnO and graphite powder dispersions were used to modify the silicon wafers before starting the deposition process of the layers. Some of the ZnO layers were deposited on the back, unpolished, side of Si wafers. Depositions at 400 °C were performed to examine the effect of the substrate temperatures on the layer growth. The film structure was examined by XRD and SEM. The results show that all layers are crystalline with hexagonal wurtzite structure. The crystallites are preferentially oriented along the c-axis direction perpendicular to the substrate surfaces. ZnO layers deposited on thin ZnO seed films and clean Si surface exhibit well-developed grain structures and more c-axis preferred phase with better crystal quality than that of the layers deposited on graphite seed layer or rough, unpolished Si wafer.

Preparation and properties of ZnO nanoparticles in the mesoporous silica matrix

In the present work a novel variant of synthesis of ZnO nanoparticles in the mesoporous silica matrix was suggested. The method is based on the introduction of a hydrophobic Zn compound, Zn acetylacetonate, into the hydrophobic part of silica–surfactant composite, followed by thermal modification. The ZnO content in the as-synthesized composites attains 14.6 wt.%, which exceeds most previously reported values for ZnO/mesoporous silica nanocomposites. The samples were characterized by SAXS, chemical analysis, TEM, XRD, EDX, UV–Vis and photoluminescent (PL) spectroscopy. The UV–Vis spectra of the composites show a significant blue shift of an absorption band demonstrating an increase of ZnO band gap energy due to the quantum confinement effect. The PL spectra under the excitation with 337 nm light show a narrow UV emission band of ZnO at 370 nm, while emission in the visible range of spectra almost disappeared. The intensity ratio IUV/IVis for the as-synthesized nanocomposites is up to 65 (at 77 K).

Synthesis and characterization of Zn/Al and Pt/Zn/Al layered double hydroxides obtained by the sol–gel method

The influence of the nature of the Zn, Al and Pt precursors, and of the temperature of precipitation and aging have been studied in connection with the preparation of Zn/Al and Pt/Zn/Al layered double hydroxides (LDH) by the sol–gel method. Whatever the precursors the XRD analysis shows that LDH is formed at the expense of ZnO when the precipitation and aging temperatures decrease from 353 K to 273 K. Moreover, chemical composition and TG analysis suggest the presence of weak amounts of hydrozincite, hydrozincite-like and Al(OH) 3 phases. When the precursors are Zn acetate-2-hydrate or Al acetylacetonate the amount of LDH reaches a maximum of 50 mol% at 273 K. At variance, about 90 mol% of LDH is obtained when using Zn acetylacetonate and Al isopropoxide as precursors, which are precipitated at 273 K. This proportion is slightly improved for the Pt-containing sample prepared under the same conditions. The specific surface areas of the different samples obtained after calcination at 723 K increase with their LDH content, reaching values of 110–120 m 2 g −1. They make them particularly attractive for catalytic applications.

ZnO thin films on semiconductor substrate for large area photodetector applications

Heterojunctions with transparent and conductive layers of ZnO on Si and InP substrates were prepared and investigated for photodetector applications in order to increase the sensibility of the conventional photodetectors with metallic electrodes, and for large area photodetectors involving lateral photoeffects. The deposition of a ZnO thin film was made on a heated substrate (InP or Si) at 400°C by the thermal decomposition of Zn acetylacetonate. The surface morphology was investigated by AFM and, for the scanning area of 2043.5×2021.5 nm2, the deposited film showed a low roughness (<5 nm). The measured values for the main optoelectrical parameters of 0.52 V for VOC, 120 μA for ISC at EV=1 klx and the external quantum efficiency over 70% demonstrate that the realised ZnO/Si and ZnO/InP heterojunctions can be used for detection applications.

Liquid Phase Methanol Synthesis on Cu–Zn Ultrafine Particles Prepared by Chemical Deposition in Liquid Phase

Abstract Liquid phase methanol synthesis was investigated over ultrafine particle (UFP) catalysts prepared by chemical deposition in liquid phase. The UFP prepared by reduction of Cu and Zn acetylacetonates by LiAlH4 in THF solution showed a very high methanol productivity which is greater than twice that of a Cu–Zn–Al coprecipitated oxide catalyst.

The microdetection of particulates from organometallic compounds: I. Metal acetylacetonate chelates

The new technique of organic particulate analysis (OPA) has been employed to evaluate the thermal decomposition of metal acetylacetonate chelates. Of the 23 compounds thus evaluated, 13 were found to give organoparticulation signals at temperatures <190 °C as indicated by their effect on the output current of an ion chamber detector. In some instances, very strong particulation was observed, particularly with the transition metal acetylacetonates, such as Co(II), Co(III), Fe(III), Cr(III), and Mn(III). In an attempt to characterize the nature of the particulates derived from these compounds, mass-spectral data were obtained on the effluent species arising from the thermal decompositions of the strongest particulate emitting metal acetylacetonates. The results showed that acetylacetone was the major component identified in both particulate and vapor effluents. With the exception of Cr(III) acetylacetonate, no metal was detected in these effluents. The OPA technique enabled the relative thermal stabilities of the metal acetylacetonates to be ascertained. Zn acetylacetonate was found to have the lowest thermal stability and the alkaline earth compounds the highest; the transition metal acetylacetonates exhibit intermediate thermal stabilities. Since a certain critical minimum particulate size (i.e., 25 Å) seems to be necessary to produce a response on the ion chamber detector instrument, vapor-phase association of acetylacetone molecules may be occurring. This association would most probably occur through H-bonded species involving the enol form of the 1,3-diketone.

Study of the degradation of polyurethanes. I. The effect of various metal acetylacetonates on the photodegradation of polyurethanes

AbstractPhotodegradation of polyurethanes based on polyester diol‐diphenylmethane‐p,′p‐diisocyanate in the absence and presence of various metal acetylacetonates (Ti, V, Cr, Mn, Co(II), Co(III), Fe, Ni, Cu, Zn, Mg, Al, and Sn) was carried out; and the effect of these metal compounds on the degradation was examined by investigating the changes in the stress–strain properties and infrared‐ and ultraviolet‐visible spectra of the polymer. The results obtained were as follows: Co(III), Co(II), Cu, Ti, and Sn acetylacetonates substantially enhanced the photo‐oxidation of the polyurethane; and Al, Ni, and Zn acetylacetonates scarcely affected, or slightly retarded, it. The other metal acetylacetonates accelerated more or less the photo‐oxidation of the polymer. Co(III), Co(II), and Cu acetylacetonates extremely degradation. Wavelength of the irradiated light was responsive to photochemical reactions of functional groups in the polymer. CN and CO bonds of urethane groups were very accessible to the photo‐irradiation both without and with a glass filter, while methylene groups were rather stable to the irradiated light longer than 300 nm, through a glass filter. The effect of the metal compounds on the coloration of the polymer was also consistent with that of mechanical strength and infrared spectral changes.