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Accessing the mechanisms for plasmonic photosensitization of ceria

Giovedì 6 dicembre 2018, ore 15.00, aula Seminari, terzo piano, edificio Fisica, Dipartimento FIM, Modena

Relatore: Jacopo Stefano Pelli Cresi

Abstract: The possibility to sensitize wide band gap oxides to visible light has stimulated the research community in view of efficiently converting solar to chemical energy, with the aim of obtaining optimized materials for photocatalysis and sensor applications. Nowadays, the use of plasmonic NPs represents one of the most promising way to induce an efficient electron-hole separation in semiconductors.
As an alternative to titanium oxide, cerium oxide has been investigated due to the possibility for Ce ions to easily and reversibly switch between the 4+ and the 3+ oxidation states depending on the ambient conditions. In cerium oxide, the presence of localized Ce 4f states between the filled O 2p valence band and the empty Ce 5d conduction band can make the material a very sensitive probe to identify possible charge transfer to/from neighboring metal atoms. The occupation of the 4f levels is in turn expected to modify the material properties, decreasing the oxygen vacancy formation energy and modifying its optical response.
Pure CeO2 absorbs light in the ultraviolet range (band gap 3.2 eV). To achieve a high efficiency for solar to chemical energy conversion, the material can be coupled with silver nanoparticles (NPs). Irradiation with photon energies which excite the localized surface plasmon resonance (LSPR) can induce energy and/or charge transfer from the metal to the oxide, through a rich variety of mutual interactions, still largely unexplored in terms of the atomic scale description of the processes involved, though undoubtedly responsible for the enhancement of the activity of the material [1,2]. A part of the literature ascribes this enhancement to hot-carriers injection (indirect mechanism) in the semiconductor conduction band, however other mechanisms (as direct electron injection mediated by plasmon decay) must be considered.
Cluster assembly in an inert gas condensation chamber combined with mass filtering allowed us to obtain silver NPs with different LSPR excitation energies. These nanostructures were fully embedded in the CeO2 matrix in order to maximize the interface area between the oxide and the metal and to prevent the oxidation of the NPs.
Transient absorption spectroscopy (TAS) was used to explore mechanisms of energy/charge transfer at femtosecond/picosecond time scale in the Ag/CeO2 system. These studies reported an unexpectedly high efficiency of electron injection, which could not be explained only in terms of hot-electron injection.

[1] B. Li, et al. ACS NANO 8, 8152 (2014).
[2] S. M. Kim et al. JPCC 119, 16020 (2015).

Host: Massimo Rontani massimo.rontani@nano.cnr.it

[Ultimo aggiornamento: 27/11/2018 15:30:45]