Binary clusters produced with cluster beam deposition for electrochemistry and heterogeneous catalysis

Niu, Yubiao (2019). Binary clusters produced with cluster beam deposition for electrochemistry and heterogeneous catalysis. University of Birmingham. Ph.D.

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Abstract

This thesis describes the production, characterisation and catalytic performance of nanoclusters fabricated by cluster beam deposition using the magnetron sputtering, gas condensation technique. $MoS_{2}$ -based clusters and Au-based clusters are demonstrated in electrochemistry (HER) and gas phase heterogeneous catalysis (CO oxidation), respectively. The atomic structure analysis of the clusters was performed with aberration-corrected scanning transmission electron microscope with high angle annular dark field (HAADF-STEM). Size-controlled $(MoS_{2}$ $)_{300}$ clusters deposited on amorphous carbon present an incomplete multi-layer structure with the absence of extended crystalline order. Such a layered structure was also found in $Ni-MoS_{2}$ hybrid clusters [with a mass corresponding to $(MoS_{2}$ $)_{1000}$ ] produced by dual target magnetron sputtering. Compared with $MoS_{2}$ clusters, a significant enhancement in HER activity by $Ni-MoS_{2}$ hybrid clusters was found. However, both $MoS_{2}$ clusters (Mo:S = 1:0.9) and $Ni-MoS_{2}$ clusters (Mo:S = 1:1.8) present a sulphur-deficient composition. In order to overcome the sulphur deficiency of the $MoS_{2}$ clusters, a sulphur-enrichment technique based on a combination of sulphur addition (by sublimation) and annealing inside the cluster beam vacuum chamber was performed on size-selected $(MoS_{2}$ $)_{1000}$ clusters. This process led to a notable increase in extended crystallinity and a moderate increase in size (from 5.5 nm to 6.0 nm in diameter). Compared with $Ni-MoS_{2}$ clusters, the sulphur-enriched $MoS_{2}$ clusters show even more enhancement on the HER activities with more than 30-fold increases in exchange current densities.
We have demonstrated a method of inhibiting the sintering of Au clusters in Au-based catalysis by exploring the stabilisation of supported Au clusters against sintering by alloying with Ti. Size-selected $Au_{2057}$ (405, 229 amu) clusters and similar mass Au/Ti nanoalloy clusters (400, 000 amu) were produced by cluster beam deposition onto thin silica films. A strong anchoring effect was found in the case of Au/Ti clusters by HAADF-STEM experiments, consistent with DFT calculations by collaborators. Different sintering mechanisms were revealed between Au cluster dimers and Au/Ti cluster dimers. Preliminary CO oxidation measurements on Au and Au/Ti clusters indicates that Au/Ti clusters are promising as catalysts. Au/Ti clusters show catalytic activity on CO oxidation while Au clusters are non-active due to the serious sintering and the support effect.