Decoupling the Mechanistic Effects of CeO2-x-Based Catalytic Heterojunctions: Chemisorbed MoO3 vs Physisorbed RuO2

Access & Terms of Use
embargoed access
Embargoed until 2023-06-17
Copyright: Zheng, Xiaoran
CeO2-x and CeO2-x-based catalysts are emerging as important functional materials in many energy- and environment-related applications. However, there remain uncertainties and misconceptions in the interpretation of the fundamental function of defects in determining the characteristics of materials. The present work explores this relationship in detail by considering the critical role of defect equilibria in terms of the effects of solid solubility and charge compensation mechanisms on the resultant physicochemical properties and catalytic performance of bulk CeO2-x as well as MoO3-CeO2-x and RuO2-CeO2-x heterojunctions. Electrodeposition was used to synthesise holey nanosheets and heterojunctions were created using wet chemistry. Analyses consisted of XRD, Raman, SEM, HRTEM, EDS, SAED, AFM, XPS, EPR, PL, KPFM, and UV-Vis. DFT was used to calculate the optical indirect band gap (Eg) values for the different solubility mechanisms for the dopant valences. The catalytic performance was assessed by HER and ozonation testing. The combination of XPS data, their detailed and extensive analyses, and consideration of all possible defect equilibria represents a powerful tool to interpret the physicochemical properties and catalytic performance of bulk materials and heterojunction nanostructures based on them. With this information, it is possible to decouple multifarious data for disparate materials such as bulk materials, chemisorbed heterojunction nanostructures, and physisorbed heterojunction nanostructures. A key outcome of the present work is that the primary factor in both the properties and performance unambiguously is Ce3+ ions, not oxygen vacancies. This is manifested through the solubility mechanisms of the dopants, which are interstitial, and the charge compensation mechanisms, which are ionic for Mo doping and ionic + redox for Ru doping. The latter mechanisms may be altered by three F centres (viz., colour centres), which derive from oxygen vacancies, and intervalence charge transfer (IVCT) in the case of Mo doping. The F centres and metal interstitials also are key factors in raising the Fermi level (Ef) of the doped materials, effectively reducing the Eg, particularly for Mo doping. The hydrogen evolution reaction (HER) performance was dominated by the heterojunctions, where the strong bonding from chemisorption, IVCT, and homogeneous and high distribution density of small heterojunction particles with Mo doping resulted in enhancement such that this performance is the best yet reported for CeO2-x-based materials. In contrast, the HER with Ru doping was relatively poor owing to the weak bonding from the inhomogeneous and low distribution density of large physisorbed heterojunction particles. The ozonation performance was outstanding but adversely affected by cerium vacancies. While this performance for Mo doping was improved by reduction owing to IVCT, that for Ru was uniformly poor owing to the high cerium vacancy concentration. The performance for bulk CeO2-x was poor owing to structural destabilisation during reduction, thus suggesting stabilising effects from the heterojunction particles.
Persistent link to this record
Link to Publisher Version
Link to Open Access Version
Additional Link
Zheng, Xiaoran
Koshy, Pramod
Sorrell, Charles
Seifi Mofarah, Sajjad
Conference Proceedings Editor(s)
Other Contributor(s)
Corporate/Industry Contributor(s)
Publication Year
Resource Type
Degree Type
Masters Thesis
UNSW Faculty