Catalytic Materials: X-ray Absorption Fine Structure (XAFS) for the Characterisation and Optimisation of Semiconductor Photocatalyst Performance
Importance
The efficiency of semiconductor heterocatalysts is determined fundamentally by surface chemical reactions that define both their activity and stability. Through understanding these processes systems can be developed and optimised for particular applications. Probing and describing the chemistry and crystallography of surfaces is technically challenging – more so, if in-situ time-resolved studies are conducted. This project, conducted in collaboration with the Singapore Synchrotron Light Source (SSLS) focuses on the development and application of X-ray Absorption Fine Structure (XAFS) as a tool for enhancing the synthesis and performance of titania based photocatalysts to be applied to the destruction of organic pollutants and the production of hydrogen by water splitting.
While a vast photocatalyst literature has arisen (see the recent compilation of Blake, 2001) significant questions remain concerning those critical features of the physical, crystallographic and electronic structures that are rate limiting in terms of performance. Indeed, such questions cannot be satisfactorily addressed by a single group of scientists, but requires the combined efforts of researchers from several disciplines. The purpose of this project is to build on the island-wide credibility and expertise of CARE researchers to lead a team of collaborators from SSLS, IMRE, NUS and Stanford to develop the first XAFS synchrotron beamline in Singapore and apply it, along with the complimentary techniques of high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD), to gain new and fundamental insights into the operation of semiconductor photocatalysts. This will ultimately lead to the design, synthesis and fabrication of novel materials with enhanced performance.
The project will build on the substantial expertise of CARE researchers to fabricate in a controlled fashion titania photocatalysts with predetermined size, phase chemistry and surface structure (Li et al, 2003a, b). Using HRTEM these studies will allow the direct observation of catalyst surfaces (Fig. 12) including the elucidation of surface roughness, surface termination states (anion or cation), and crystal perfection. Significantly however, the chemical state cannot be verified. This project will exploit the intense X-rays from the SSLS to study the characteristic X-ray lines of titanium (4.51 keV) by XAFS to collect information concerning the speciation and bonding of near-surface atoms. Data acquisition will be across the XANES (X-ray absorption near-edge structure) or EXAFS (extended X-ray absorption fine structure) regions of the absorption spectra to reveal information concerning metal valence states, bond density out to second or third nearest neighbours, and physical state (i.e. adsorption versus chemical bonding).
