(122d) Time-Resolved Characterization of Customized Aluminum-Doped Zinc Oxide Nanocrystals By Means of Small-Angle X-Ray Scattering

Abstract

The semiconductor aluminum-doped zinc oxide (AZO) has versatile applications due to its special properties, e.g. low electrical resistance, good mechanical stability and high degree of transparency in the visible range. Due to these properties AZO is considered to be a significant cheaper alternative in order to replace existing and finite indium based oxides in the future for diverse scopes e.g. thin-film solar modules, touch-panels, light-emitting diodes or printing electronics. [1–5] Especially the application of AZO in transparency thin-film solar modules has a high potential with regard to the sustainable energy generation from renewable sources. In the future, glass surfaces, e.g. windows of office buildings, could be used as a renewable energy source.

Therefore, the availability of high quality nanocrystals with fitted sizes and shapes of crystals and a defined doping of aluminum to adjust the optical properties (band gap energy) is required. [2, 4] The major challenge is the link between the particular-based properties (size, shape, grade of agglomeration, morphology etc.) and the versatile applications, because the nanoscale area is difficult to access by measurement systems. Furthermore, the measurement methods to evaluate the structure parameters are frequently limited to discrete process steps and states, e.g. the characterization of powdery nanoparticles, or an elaborate post-processing step is required, e.g. in the electron microscopy.

The small-angle X-ray scattering (SAXS) basically enables a non-invasive determination of structural properties and several states of nanoparticles in individual process steps during the entire process chain (synthesis, stabilization, coating etc.) without the requirement of post-processing steps. Only one measurement technique is favorable to combine the measurement results of any process step referring to the evaluation of the performance of AZO-nanocrystals. Additionally, the SAXS-method offers the possibility to determine several characteristic parameters (particle size distribution, particle morphology and the state of aggregation) within only one measurement.

For synthesizing high-crystalline AZO nanostructures the non-aqueous sol-gel-synthesis is used. Here, particle precursors are solved in aromatic solvents, e.g. benzyl alcohol or benzyl amine, and converted to AZO-nanocrystals with defined sizes, narrow particle size distributions and adjustable morphologies at temperatures of 80°C to 200°C. [6–8] The purpose of the solvent is to control the particle growth and the particle morphology. [8, 9] Even the absence of a hydrolysis step, as occurred in the classical sol-gel synthesis, results in moderate reaction rates. This property enables time-resolved studies of the particle formation and growing process during the synthesis by means of SAXS.

The main objective is to adapt the AZO synthesis on our self-developed SAXS instrument. To obtain detailed knowledge about the particle formation and growth mechanism we run time-resolved in situ measurements. Because of measurement limitations of our SAXS instrument during real-time experiments, adjustments of the AZO synthesis are mandatory. On the one hand, an optimal process control is to be adapted to the measurement system. On the other hand, the chemical reaction needs to be modified with regard to reaction kinetics and particle size. Concerning the process control, a specifically developed ideal batch [10] reactor with computer-assisted online pressure- and temperature monitoring is used to ensure exact reaction settings and high reproducibility of particle properties during the synthesis. The ideal batch reactor, with 358 milliliter in capacity, is fitted specifically to the requirements of the non-aqueous sol-gel-synthesis. The heating and the closed vessel are designed for a maximum temperature of 300°C and a maximum pressure of 16 bar. Regarding to the analysis of the reaction solution during the process, a riser with a needle valve is installed to control the flow rate during the sample extraction. The extraction occurs continuously with low-pulsation due to the vessel pressure through a solvent resistant tube into a quartz glass flow capillary, which is located in the X-ray beam throughout the analysis. Hereby, the instrumental set-up provides ideal conditions for an in situ characterization by means of SAXS without interferences of the synthesis.

In this contribution an appropriate adaptation of the synthesis on our SAXS measurement system, special features of the instrumental set-up and time-resolved studies of particle formation and growth mechanisms, dependent on synthesis parameters, will be reported in detail.

References

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[10] Design, construction and manufacturing took place by the Institute for Particle Technology (iPAT) of the Technical University Braunschweig, led by G. Garnweitner.