Spectroscopic studies of size-selected Ru and Pt clusters on Titania

Author: Liam Howard-Fabretto

Howard-Fabretto, Liam, 2021 Spectroscopic studies of size-selected Ru and Pt clusters on Titania, Flinders University, College of Science and Engineering

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Metal clusters are small groups of bound metal atoms which can have distinctly different electronic and catalytic properties, unique from those of a bulk sample of the same metal. The main motivation for research on clusters is their potential for use in catalysis and photocatalysis. The physical and chemical properties of metal clusters are dependent on size, as well as the cluster-substrate interaction. It is difficult to predict the properties of a cluster/substrate pair if studies have not been performed on that specific combination. Fundamental measurements into cluster properties have previously been performed on a range of cluster/substrate combinations, where Au clusters and TiO2(110) surfaces have received a large focus. However, there are many important cluster and substrate materials which have not had their surface properties measured, which is vital for the informed design of efficient catalytic materials.

The original contribution to knowledge in this field is through the novel experimental measurements and analysis of the surface properties and electronic properties of 3-atom Ru3 clusters supported on radio frequency (RF)-sputter deposited titania substrates (RF-TiO2). Ru3 was chosen due to its potential catalytic and photocatalytic applications. RF-TiO2 is a cost-effective alternative to the commonly used TiO2(110) substrate and was chosen due to its ease of production and applicability to industrial applications. The aims were to determine details about the cluster-surface interaction and temperature stability of the clusters, in addition to measuring their electronic density of states (DOS). Additional cluster and substrate systems were analysed for comparison to Ru3/RF-TiO2. This study provides a strong basis for future work on the catalysis of RF-TiO2-supported Ru3.

Ru3 clusters were deposited onto RF-TiO2 and analysed using temperature programmed desorption (TPD) to probe Ru-CO binding sites, and X-ray photoelectron spectroscopy (XPS) was used to provide chemical information. Differences in cluster-support interactions were compared for bare Ru3 deposited using an ultra-high vacuum (UHV) cluster source (CS), and the chemical vapor deposition (CVD) of ligated Ru3(CO)12 clusters. For CS-deposited Ru3, heating to 800 K caused partial oxidation of clusters on both non-sputtered and sputter-treated RF-TiO2. Furthermore, on sputter-treated RF-TiO2 all Ru-CO binding sites on the clusters were blocked immediately after deposition for CS-deposited Ru3, but for CVD-deposited Ru3(CO)12 the clusters were protected by their ligands and the Ru-CO binding sites were only blocked once the sample had been heated to 723 K. The mechanism for complete blocking of Ru-CO binding sites on sputtered RF-TiO2 could not be directly determined from this first study.

To further probe the Ru3/RF-TiO2 system, additional temperature-dependent (TD) measurements were performed to analyse the effects of heat treatment. Samples were analysed with TD-XPS, angle-resolved (AR) XPS, and low energy ion scattering spectroscopy (LEIS). Ru3 was deposited onto RF-TiO2 using 3 methods: solution submersion of Ru3(CO)12, CVD-deposition of Ru3(CO)12, and CS-deposition of bare Ru3. TD-XPS showed that after heat treatment the Ru 3d BE was very similar between the methods suggesting the cluster oxidation state was the same. TD-LEIS showed the encapsulation of CVD Ru3(CO)12 on sputter-treated RF-TiO2 by a titania overlayer after heat treatment. The average overlayer thickness was calculated to be 0.35 nm ± 0.08 nm, which is thin enough that there is the potential for catalytic or photocatalytic reactions to occur. The encapsulation of the clusters help explain the mechanism behind the complete blocking of Ru-CO sites, suggesting CO is sterically hindered from accessing Ru by the overlayer.

The electronic DOS for Ru3 on sputter-treated RF-TiO2 was measured using ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). Bare Pt3 clusters were CS-deposited for the purpose of comparison to a different cluster of the same size. Measurements both Ru3 and Pt3 were after sample heat treatment. UPS measures the top several layers while MIES has perfect surface sensitivity and measures only the top atomic layer. The electronic DOS of Ru3 was very similar for Ru3 deposited using solution submersion and CVD of Ru3(CO)12. UPS suggests that RF-TiO2-supported Ru3 clusters have metallic characteristics and Pt3 clusters have non-metallic characteristics. MIES suggests that the encapsulating layer above Ru3 clusters is composed of reduced titania with a bonding structure such as Ru-Ti-O, while Pt3 was not encapsulated. The mechanism for the encapsulation of Ru3, but not Pt3, is possibly an energetic benefit for Ru3 encapsulation in terms of the minimisation of surface energy.

As an extension of previous studies, the effects of changing the supported Run cluster size by a single atom was studied by depositing size-selected Ru4 clusters onto sputter-treated RF-TiO2 and comparing results to previous experiments. Clusters were CVD-deposited with H4Ru4(CO)12 and analysed using TD-XPS, TPD, and UPS. Based on TD-XPS, H4Ru4(CO)12 begins to lose ligands at a slightly lower temperature than Ru3(CO)12, most likely related to the loss of H bridging ligands at low temperatures. Other results were very similar between the clusters. CO-TPD showed that after heat-treatment all Ru-CO binding sites were blocked, caused by encapsulation of the clusters. UPS provided evidence that the encapsulated Ru4 clusters have metallic properties, and the valence DOS was similar for both Ru3 and Ru4. This suggests that the resultant properties of CVD-deposited Ru4 after heat treatment are very similar to Ru3, and that both clusters have the same potential catalytic and photocatalytic benefits when supported on RF-TiO2.

Keywords: catalysis, nanotechnology, physics, chemistry, clusters, metal clusters, nanocluster, ruthenium, platinum, spectroscopy, temperature programmed desorption, ion scattering

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Gunther Andersson