Evaporation kinetics in highly porous tetrapodal zinc oxide networks studied using in situ SR\xb5CT

Birte Hindenlang,Lena M Saure,Florian Rasch,Martina J Baum,Fabian Schütt,Rainer Adelung,Malte Storm,Berit Zeller-Plumhoff,Sören Kaps,Anna Gapeeva,Julian Moosmann,Jörg Hammel

doi:10.1038/s41598-021-99624-y

Abstract

Tetrapodal zinc oxide (t-ZnO) is used to fabricate polymer composites for many different applications ranging from biomedicine to electronics. In recent times, macroscopic framework structures from t-ZnO have been used as a versatile sacrificial template for the synthesis of multi-scaled foam structures from different nanomaterials such as graphene, hexagonal boron nitride or gallium nitride. Many of these fabrication methods rely on wet-chemical coating processes using nanomaterial dispersions, leading to a strong interest in the actual coating mechanism and factors influencing it. Depending on the type of medium (e.g. solvent) used, different results regarding the homogeneity of the nanomaterial coating can be achieved. In order to understand how a medium influences the coating behavior, the evaporation process of water and ethanol is investigated in this work using in situ synchrotron radiation-based micro computed tomography (SRµCT). By employing propagation-based phase contrast imaging, both the t-ZnO network and the medium can be visualized. Thus, the evaporation process can be monitored non-destructively in three dimensions. This investigation showed that using a polar medium such as water leads to uniform evaporation and, by that, a homogeneous coating of the entire network.

Highlights

ZnO is a direct wide bandgap (~ 3.4 eV) ceramic n-type semiconductor that crystallizes preferentially in the hexagonal wurtzite-type structure and, depending on the synthesis route, exists in multiple nano- and microstructures, e.g., rods, belts, tubes, flowers, and tetrapods[1,2,3]
Due to the hydrophilicity of Tetrapodal zinc oxide (t-ZnO), it is assumed that during the drying process, the water layer becomes thinner while forming menisci between the tetrapod arms and at interconnection points to minimize its surface area, which leads to sail-like formations[7,11]
The investigation using propagation-based phase contrast SRμCT enabled the visualization of the dynamic evaporation process of water from t-ZnO networks for the first time

Summary

Introduction

ZnO is a direct wide bandgap (~ 3.4 eV) ceramic n-type semiconductor that crystallizes preferentially in the hexagonal wurtzite-type structure and, depending on the synthesis route, exists in multiple nano- and microstructures, e.g., rods, belts, tubes, flowers, and tetrapods[1,2,3]. Different strategies have been developed for the synthesis of tetrapodal-shaped particles, including vapor-phase growth, wet chemical and hydrothermal methods, microwave-assisted growth, DC thermal plasma and pulsed laser deposition, as well as flame transport synthesis (FTS)[3] The latter provides a simple solvent-free method for the fabrication of t-ZnO microparticles on a large scale and enables defined control over the aspect ratio (length:diameter) and morphology of the t-ZnO arms[4,5]. We are strengthening the understanding of the evaporation process inside the network by employing in situ synchrotron X-ray radiation-based micro computed tomography (SRμCT) This method enables the dynamic and non-destructive imaging of the whole sample while achieving a high spatial and temporal resolution. The analysis of these 4D datasets enables a better understanding of the evaporation and by that, the coating of the network

Methods

Results

Conclusion