Investigation of the morphology of TiO2-CeO2 mixed-oxide nanoparticles for photocatalytic applications

Abstract text (incl. figure legends and references)

Introduction

Titanium(IV)oxide (TiO₂) nanoparticles are of high interest for a broad variety of applications, e.g. (photo)catalysis, self-cleaning and sensing. The photocatalytic performance of TiO₂ can be improved by controlling its crystal structure [1] as well as by the addition of noble metals [2] or oxides [3]. Combination of TiO₂ and other oxide especially with CeO₂ is one of the promising ways for effective electron-hole separation due to easy fluctuation of oxidation states of Ce [3].

Objectives

The presented research aims to find connections between the real structure of TiO₂-CeO₂ particles (i.e. crystal structure, morphology and composition) and their photocatalytic properties. Understanding these connections, allows to purposefully adapt the particles to the respective application and optimize their performance.

Materials & Methods

A custom-made gas aggregation cluster source (GAS) was used to synthesize mixed-oxide nanoparticles. Ti and Ce were sputtered from a custom build Ti-Ce target, agglomerated under admixture of O₂ as reactive gas and were deposited on Si wafer pieces. After deposition, the particles were heat treated at 700 °C under ambient conditions.

For TEM preparation the particles were scratched off the substrate and transferred to a Cu lacey TEM grid (Plano GmbH).

The particles were analyzed in a Jeol NeoArm JEM-ARM200F STEM equipped with two windowless EDX detectors. STEM-HAADF imaging in combination with EDX spectroscopy provides information on the particle morphology and the distribution of Ti and Ce. HRTEM as well as HRSTEM-HAADF images can reveal the nature of the interfaces between TiO₂ and CeO₂ crystals.

Results

First measurements on TiO₂-CeO₂ nanoparticles with a high concentration of Ce have shown demixing of a Ti-rich and a Ce-rich phases. Separate Ti-rich and Ce-rich particles have been observed as well as a core-shell type particle with a Ti-rich core covered by a Ce-rich shell. Ce-rich as well as Ti-rich phases where found to be crystalline. The crystal phase of CeO₂ was determined to be Fm-3m by electron diffraction. Due to the low Ti concentration, the crystal phase of TiO₂ was not determined by electron diffraction. Instead, the phase of TiO₂ was observed to be brookite phase in FFTs of HRTEM images. The investigation of further samples with different Ti/Ce-ratios synthesized at different process parameters are expected to show a broader variation in particle morphology and give an insight into the dependence of the particle morphology on Ti/Ce-ratio as well as GAS process parameters.

Conclusion

At high Ce concentration, demixing into different morphologies with a Ce-rich and a Ti-rich phase can be observed in the mixed-oxide nanoparticles. Both phases are crystalline. To achieve anatase phase, which is desirable for high photocatalytic activity, the GAS process parameters need to be adjusted. Earlier studies with pure TiO₂ particles have shown that the crystal phase can be varied by changing the magnetron voltage.

For further investigations, the magnetron voltage window for the synthesis of anatase phase TiO₂ in the mixed-oxide system will be investigated. In addition, the Ti/Ce ratio will be varied to study the influence of the particle composition on the morphology.

References

[1] A. Sclafani, J. M. Herrmann, J. Phys. Chem., 1996, 100, 13655–13661

[2] S. Veziroglu et al., ACS Appl. Mater. Interfaces, 2020, 12, 14983–14992

[3] S. Veziroglu et al., Nanoscale, 2019, 11, 9840-9844