On the experimental and theoretical studies of ZnO precursors: towards Green Chemistry

Abstract

Although an extensive number of investigators, during the arise of this printed materials technology, were devoted to make very interesting discoveries concerning the most used sol-gel precursor to thermally produce ZnO, the motives for which the ZnO material is formed in a certain way from some certain initial conditions is not yet well understood. The first effort carried out was to solve the crystal structure, never reported before, arising from the mixture between the zinc salt (Zinc acetate dihydrate) and the typical stabilizer used (Ethanolamine). This crystal structure was then compared with the molecular models designed and optimized by means of simulations. Simulationes were also useful in the study of the stability of this precursor and explaining the experimental results on its degradation in presence of light and CO2. Regarding the ZnO formation from the precursor’s thermal decomposition, it could be demonstrated the reason why the final ZnO was having traces of nitrogen at high temperatures. In addition, the theory regarding the specimens arising when the precursor is being degraded was also validated. Nevertheless, the presence of nitrogen at high temperatures as well as the toxic species that were evolving from the precursor when decomposing were a serious matter to confront. The aim of the next study was to investigate the substituent effects in this nitrogen-based ZnO precursor.

To do so, up to seven aminoalcohols were used in order to study the dependence of rigidity on the stability and the decomposition process of the precursor. Experiments confirmed that the use of any of the aliphatic aminoalcohols in the precursor produce high stability comparing to those using aromatic aminoalcohols. They bolstered also the conclusion that the aminoalcohol’s rigidity plays and important role. It was additionally confirmed that the use of flexible aromatic stabilizers could be counterproductive, regarding the poor stability and the ZnO purity, while the aliphatic-based aminoalcohols provide better performances. Simulations were useful to comparatively study these different aminoalcohols in terms of stability and molecular dynamics at different temperatures. Dynamic simulations at different temperatures provided conclusive evidences on the nitrogen prevalence in the precursor even at high temperatures, suggesting that the Zn-N bond has effects on the thermal decomposition. For that reason, additives containing nitrogen are not the best solution to stabilize the metal and to produce pure ZnO (or at least with control on the contamination) at the same time.

The next focused was on the desire to increase the number of Zn-O (weaker) interactions at the expense of eliminate the Zn-N bonds (stronger) using the so-popular Green Chemistry. Up to five ZnO precursors were synthesised following these principles, solving their crystal structure and studying their thermal decomposition. In order to increase the stability of the transition metal in the precursor, it was decided to resort to natural occurring carboxylic acids such as salicylic, citric, succinic and lactic acids. Thermal analysis found decomposition processes lasting ca. 150 C less than the required temperature for nitrogen-based precursors. Furthermore, these decomposition processes consisted of cleaner steps and purer ZnO with a lower temperature. Two of the five precursors were chosen, taking into account their delay in synthesizing and their solubility in common solvents (water and ethanol), to compare the behaviour of inks based on these precursors with the ones based on nitrogen compounds from the previous study. Continuous and homogeneous films of highly pure ZnO were obtained at lower temperatures adjusting the dwell time at the maximum

temperature as well as the heating rate. At this point, there was only one proposal to make: the addition of new properties to those already existing in this ZnO. The addition of properties between two metals, well-known in Chemistry, is called molecular hybridization. Thus, the objective of this work was to synthesize an heteropolymetallic precursor that, through thermal decomposition, could generate the Franklinite / ZnO tandem with a variable iron content at will by controlling the atmosphere in which this precursor is decomposing.

La combinación de acetato de zinc y etanolamina conforma el precursor de sol-gel más utilizado para la producción de ZnO con fines de electrónica impresa. Aunque las propiedades de tinta basadas en precursores se han reportado ampliamente, las razones por las que el material de ZnO se forma de cierta manera a partir de condiciones iniciales específicas todavía no se conocen bien. En el presente trabajo, la estructura de la mezcla se caracteriza experimentalmente y se compara con los modelos moleculares optimizados mediante simulaciones. Además, se ha demostrado tanto los procesos que llevan a su degradación a temperatura ambiente como la descripción del proceso de descomposición indica efectos de sustituyentes importantes en este precursor basado en nitrógeno, que se analizan mejor comparando varios amino-alcoholes con diferentes sustituciones de radicales. La falta de control sobre los efectos de estos amino-alcoholes en el material final hace deseable aumentar el número de interacciones Zn-O (más débiles) a expensas de eliminar los enlaces Zn-N (más intensos). En consecuencia, se han diseñado nuevos precursores sobre la base de la Química verde recurriendo para ello a ácidos carboxílicos naturales. Se describen aquí sus estructuras cristalinas y procesos de descomposición térmica, que consisten en pasos más limpios y en la producción de un ZnO más puro a temperaturas más bajas que cuando se usan amino-alcoholes para estabilizar el metal Zn (II). Al agregar solventes comunes (agua y etanol), las tintas obtenidas muestran claras mejoras para las tecnologías de impresión basadas en precursores de ZnO. Como último estudio, se procedió a la incorporación de un segundo metal en este tipo de precursores, el hierro, con el fin de estudiar la conveniencia en la disposición de dos metales que, en función de la atmosfera en la que se descomponga el precursor, generen una proporción controlada de Franklinita (conocida por sus propiedades magnéticas) con respecto a la proporción de ZnO. Un estudio futuro más detallado sobre esta Franklinita podría, incluso, detallar las condiciones en las que encontrar la proporción de Fe(II)/Fe(III), y por tanto su capacidad magnética, deseada para cada fin.