Miao, Tina Jingyan; (2021) Time resolved absorption and vis-NIR spectroscopic investigation of heterogeneous photocatalysts. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

A transient absorption (TA) spectrometer with 1 ns temporal resolution and a probe wavelength range of 490-1000 nm has been constructed. Accompanying software for controlling data acquisition as well as for specifying electronic delays and probe wavelengths has been developed in LabVIEW, which allows real-time data acquisition. Furthermore, a script was developed in Python to process the as-acquired data, which reduces noise by about an order of magnitude and eliminates oscillatory interference signals. The performance of the presently developed instrument and accompanying software was tested using anatase TiO2 as reference material. The TA data is qualitatively comparable to literature reports and in agreement with existing knowledge on typical TA characteristics reported for anatase TiO2. However, discrepancies exist between different literatures, and between literature and TA data obtained in the present work, which are attributable to variable measurement conditions and sample characteristics. The TA instrument was applied to understand the difference between two carbon nitride samples, one of which performs as a photoanode and the other as a photocathode. The photoanode material exhibits positive TA signals attributed to absorption of deep trapped electrons with little reducing power, whilst the photocathode exhibits negative TA signals attributed to electrons in shallow emissive states with reduction potentials close to that of the conduction band. Based upon long-lived charge carriers observed in the photoanode material, some limitations of the current TA instrument were attempted to be addressed through further software developments. Subsequently, a benchtop spectrometer was demonstrated to have extended capability of measuring absorption characteristics of photoexcited charge carriers in powder photocatalysts. Using this spectrometer, it is demonstrated that 1) methane is directly oxidised by photoexcited holes in TiO2, 2) O2 is more easily activated than methane, and 3) most of the charge carriers scavenged by O2 are free conduction band electrons.

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