Investigation of nanoscale properties of graphene-based interfaces for possible application in catalysis

The focus of my research activity during three years of PhD has been on the nanoscale properties of graphene-based interfaces to grow metal or metal oxide clusters for possible application in catalysis. Graphene has emerged as most promising material since its first isolation in 2004, by A. Geim and K. Novoselov and triggered an exponentially rising interest in this topic which grew over the last decade due to the uniqueness of its properties. The most striking feature of graphene stems from its electronic band structure giving rise the high mobility of electrons plus other excellent properties like thermal and mechanical stability thus making it an attractive material for its application in catalysis. The first part of this thesis work is devoted to investigate the growth processes of graphene on transition metal surfaces and study the interaction mechanism between graphene and the metal substrates. The primary objective of this research activity is to find the suitable metal substrate for graphene growth giving us almost free-standing graphene to study properties of metal clusters. In recent years, great progress have been achieved in the field of supported transition metal oxide nanoclusters due to its promising application in the field clean and renewable energy sources. In this respect, the second part of this thesis work focuses on the employment of graphene/Ir(111) interface, where the moiré patterns will act as template for self-assembly of metal oxide clusters. We grew high-quality, thermally stable graphene-supported Co oxide clusters on graphene/Ir(111) interface. We studied the chemical state of the Co clusters prepared on graphene /Ir(111) using spectroscopic techniques (HR-XPS) and secondly, to understand the growth mechanism and cluster size and distribution using Low Energy Electron Microscopy. Graphene’s unmatched electron mobility was further exploited with TiO2 for inhibition of electron-hole recombination. We studied the reaction of titanium surface with oxygen above and at the interface of graphene by following in-situ the evolution of the surface atoms using core-level photoemission spectroscopy with synchrotron radiation and demonstrate the co-existence of sub-oxide moieties along with titania. We successfully tested one of the interfaces for photocatalytic hydrogen production and found that titania-graphene is 20 times more active than titania without graphene. Beside this, I was also involved in the investigation about the use of coronene as precursor, not only for epitaxial Gr growth, but also for the production of carbon nanoflakes which could have potential applications, especially in photocatalysis, energy conversion, and sensing. This part of the thesis is devoted to a detailed understanding of the dissociation mechanism of large molecule like coronene (C24H12) adsorbed on Ir(111) using a variety of experimental techniques, (HR-XPS, NEXAFS and ARPES). The last part of my thesis work is dedicated to CO adsorption on Rh-nanoclusters grown on the Gr/Ir(111) interface. The natural corrugation of Gr grown on Ir(111) enables the deposited clusters to arrange themselves into extended, periodic superstructures by adsorbing at the minimum–energy sites. By means of core level photoemission and DFT calculations (carried out by theoretician at UCL), we found that the CO molecules tend to absorb at the basal cluster edges where the Rh atoms have low coordination number. We also found that at low coverages CO molecules adsorb in on top sites while at higher coverages the bridge-edges of Rh nanoclusters are the places where the adsorption of CO is preferred. This kind of studies can be exploited to the design new graphene-based materials with improve fundamental properties at nanoscale for their application in field of catalysis.