Author: Abdulrahman Alotabi
Alotabi, Abdulrahman, 2023 Investigating the physical, chemical and electronic properties of chromium oxide layers and gold clusters on surfaces: Implications for photocatalysis, Flinders University, College of Science and Engineering
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The scientific community is researching alternative renewable and clean energy sources to shift from fossil fuels. The conversion of solar energy into useful chemical energy such as hydrogen production via photocatalysis is a promising process to contribute to these alternatives, with much effort being focused on the development of photocatalysts for photocatalysis. The deposition of noble metal nanocluster co-catalysts can modify the properties and improve the efficiency of photocatalysts. The use of noble metal nanoclusters consisting of a few atoms as co-catalysts has attracted considerable attention due to their unique electronic and catalytic properties. However, it is challenging to maintain the cluster co-catalysts’ size due to the clusters’ tendency to agglomerate to form larger particles and lose their cluster-like properties. Another challenge that co-catalysts face is the back reactions of water splitting such as water formation from hydrogen and oxygen on the co-catalysts.
An overlayer of thin metal oxide on a co-catalyst-modified photocatalyst plays a critical role in photocatalysis reactions by stabilising the co-catalysts and suppressing the back reactions. This thesis makes an original contribution to our knowledge on the chromium oxide layer formed onto photocatalysts and gold cluster-modified photocatalysts by performing physical, chemical, electronic and catalytic studies on this layer.
The stability of the CrOx layer photodeposited onto different crystal phases of TiO2 films and P25, BaLa4Ti4O15 and Al:SrTiO3 particles with subsequent annealing at temperatures up to 600 ⁰C was investigated (Please note that the layer is labelled “CrOx” until the nature of the Cr component is confirmed (vide infra)). After annealing, for TiO2 films, CrOx layer diffuses into the amorphous and anatase phases of TiO2 film but remains stable at the surface of the rutile phase. Moreover, for particles, CrOx mostly diffuses into P25, while for BaLa4Ti4O15, the degree of CrOx diffusion is less than that into P25. Interestingly, the CrOx layer is very stable on the surface of Al:SrTiO3 particles. This diffusion is attributed to the strong metal–support interaction effect between CrOx and different photocatalysts. Moreover, some of the CrOx was reduced to metallic Cr after annealing but there was no observation of the high oxidation state of Cr. The particles’ bulk and surface band gap structures were also investigated, along with the photocatalytic water splitting activity.
For the first time, chemically synthesised phosphine-protected gold clusters, Au9(PPh3)8(NO3)3, deposited onto RF-sputter deposited TiO2 film are prevented from agglomeration after the removal of ligands. This effect is achieved by photodepositing the CrOx layer on the top of the clusters as a protective layer. Further, the influence of heat treatment on the surface roughness of two different thicknesses of RF-sputter deposited TiO2 films and the effect this has on size-specific Au9 clusters deposited on the surface was also investigated. It is found that the high mobility of the thick TiO2 film after heating leads to a significant agglomeration of the Au9 clusters, even when protected by the CrOx layer. However, the thin TiO2 film has much lower mobility when heated, resulting in non-agglomerated clusters with CrOx coverage.
In the last part of this thesis, the stability of Au9 clusters beneath a Cr(OH)3 layer onto SrTiO3 particles under conditions of photocatalytic water splitting (i.e., UV irradiation) was investigated. After a photocatalytic water splitting reaction for seven hours, Au9 clusters without the CrOx layer were found to agglomerate into large particles, while the application of the CrOx layer resulted in the inhibition of the agglomeration of Au9 clusters.
Keywords: Overlayer, Chromium Oxide Layers, Al:SrTiO3, Cr2O3 layer, P25, BaLa4Ti4O15, Al-doped SrTiO3, Photodeposition, Photocatalytic water-splitting, Back reaction, water splitting
Subject: Chemistry thesis
Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Professor Gunther Andersson