Exploring the Properties and Stabilisation of Nanoscale Metal Cluster/Overlayer Architectures

Abstract

The shift from fossil fuels to renewable energy sources is a major focus in the global effort to reduce CO2 emissions, with photocatalytic hydrogen production being a promising approach for harvesting energy from sustainable energy sources. Photocatalysts absorb energy from sunlight to drive the water splitting reaction, producing H2. The deposition of a co-catalyst, such as noble metal clusters, can modify and improve the efficiency of the photocatalyst. Metal clusters, consisting of only a few atoms, have gained attention as co-catalysts due to their unique electronic and catalytic properties. However, maintaining their size and stability is challenging, as they tend to agglomerate into larger particles, losing their unique properties. Another challenge is the occurrence of the back reaction during photocatalysis, when H2 and O2 react to form water on the co-catalyst surface. The back reaction reduces the efficiency of photocatalytic water splitting. Atomic Layer Deposition (ALD) of thin metal oxide overlayers on cluster-modified photocatalysts offers a strategy to stabilise the clusters and suppress the back reaction. The self-limiting nature of ALD allows for deposition of an ultrathin overlayer with a controllable thickness. This thesis investigates how ALD-AlOx overlayers can be used to preserve the integrity of noble metal clusters, particularly Au clusters. It is examined how an ALD-AlOx overlayer grows on the surface of a photocatalyst formed by depositing Au clusters on TiO2 surfaces, as well as the distribution and stability of the clusters on the TiO2 surface before and after the ALD overlayer. The growth of ALD-AlOx overlayers on Au101/TiO2 was investigated as a model photocatalyst system to understand how the overlayer grows on the Au clusters and the TiO2 substrate. The investigation determines the overlayer thickness after applying several ALD cycles on a planar TiO2 substrate. The study demonstrated that the ALD-AlOx resulted in evenly deposited overlayers for the system of Au101/TiO2 with a slight tendency to be thicker on the Au cluster than on the TiO2. The layer thicknesses were found to be 2.0 Å, 3.5 Å, and 5.5 Å for 1, 5, and 10 ALD cycles, respectively. A comprehensive study of the stability of Au9(PPh3)8(NO3)3 deposited onto TiO2 by depositing an ultrathin overlayer of ALD-AlOx at various deposition temperatures, 25 °C, 100 °C, 150 °C, and 200 °C was conducted. It was found that ALD-AlOx stabilised Au9 clusters on the TiO2 surface across various temperatures. Notably, the phosphine ligands desorb during the ALD overcoating process at elevated temperatures, while the Au9 cores remained protected beneath the AlOx overlayer. The ALD-AlOx overlayer on Au metal clusters on TiO2 was studied by a combination of microscopic and spectroscopic techniques. It was revealed that the Au101 clusters were distributed randomly across the entire TiO2 surface. The roughness of the Au101/TiO2 system increases as the Au concentration increases, while ALD overcoating smooths the clusters, as the roughness was found to decrease, indicating a uniform coating on clusters by forming thicker overlayers on interstitial regions between clusters. This work provides an understanding of the role of ALD-AlOx overlayer on the stabilisation of Au metal clusters on TiO2 as a photocatalyst model system, with direct relevance of designing and improving photocatalytic water splitting for green hydrogen production.