Formation of cerium oxide hollow spheres and investigation of hollowing mechanism

Xudong Wu,Min Wei,Rong Sun,Ching-Ping Wong,Shuhui Yu,Jingyu Huang

doi:10.1007/s42452-019-0178-0

Abstract

Hollow structure materials have found important applications in many fields such as catalysis, energy storage and sensors. A facile and environmentally benign synthesis will favor their applications. Here we prepare a cerium oxide hollow structure with the size of micrometers through a simple one-step hydrothermal method. This process results in the formation of microspheres at 210 °C for only 2 h. Advantages of this method include no template, without surfactant-assistance or subsequent treatment during the synthesizing process. Explanation on the controlled crystal growth and hollowing mechanism via Ostwald Ripening is proposed elaborately based on the detailed experiment and observation. The FESEM images showed that the surface morphology of the sphere shell turned from initially smoothness into roughness with the increase of reaction time, relating to dissolution and re-deposition of nanoparticles onto inner and outside of the shell. The hollowing microstructure evolvement was vividly presented by observing samples taken out at different reaction stages, which makes visualization of Oswald ripening process. The prepared Cerium oxide microspheres were used to modify glassy carbon electrode, which acts as a sensor for sensitive determination of environmental hormone catechol (CC), demonstrating superior performance over the solid oxide. This work helps to deeply understand Ostwald ripening from a new perspective.

Highlights

Micro- and nanoscale hollow materials have attracted growing attention for decades, and various hollow structures [1,2,3,4,5] were prepared in recent years
Hollow materials have shown potential applications such as catalysis [6], energy storage [4, 7],sensor [8], and drug delivery [9],which rely on intrinsic features of hollow structures, such as high surface area, hollow interior space, high stability, low density and good permeability
Template synthesis can be timeconsuming and expensive, and poor crystallinity materials are often produced during the synthesis process [18]

Summary

Introduction

Micro- and nanoscale hollow materials have attracted growing attention for decades, and various hollow structures [1,2,3,4,5] were prepared in recent years. A phenomenon which was first described by Wilhelm Ostwald in 1896, [25] involves the dissolution of small crystals and redeposition of the dissolved species on the surface of large particles, which results in growth of larger domain at the expense of the smaller ones. Unlike previously reported inside-out Ostwald ripening, they proposed a hollowing mechanism termed as “shellinduced Ostwald ripening”, which involves the dissolution and redepositioning of small nanoparticles on the inner surface of the shell to produce cavities. Inspired by the Oswald ripening mechanism, we report a new one-step synthesis of CeO2 hollow microspheres to explore surface morphology evolvement without template, surfactant assistance or subsequent treatment. By focusing on the shell evolvement using FESEM to observe entire particles image, we have concisely depicted a hollowing mechanism that enhances previously reported inside-out Ostwald ripening. The strategy of synthesis and characterization in this study provides general implications for the microscopic morphological evolution of other hollow metal oxide micro- and nanomaterials

Materials

Fabrication of the modified electrode

Results and discussion

XRD and XPS

Electrochemical detection of catechol

Conclusion