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Jacopo Stefano Pelli Cresi1 2 Maria Chiara Spadaro1 Sergio D'Addato1 2 Sergio Valeri1 2 Daniele Catone3 Patrick O'Keeffe3 Lorenzo Di Mario3 Paola Luches2

1, Università di Modena and Reggio Emilia, Modena, Emilia Romagna, Italy
2, CNR-NANO, Centro di Ricerca S3, Modena, , Italy
3, Istituto Struttura della Materia, CNR, Roma, , Italy

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 photo-catalysis and sensor applications. Cerium oxide has been started to use considering the presence of localized Ce 4f states between the filled O 2p valence band and the empty Ce 5d conduction band that can make the material a very sensitive probe to identify possible charge transferred 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. The material can be coupled with plasmonic nanoparticles (NPs). Irradiation with photon energies which excite the localized surface plasmon resonance (LSPR) and hot carries generation can induce energy and/or charge transfer from the metal to the oxide, though undoubtedly responsible for the enhancement of the activity of the material [1]-[3].
Cluster assembling in inert gas condensation chamber and mass selection filter allows to obtain silver NPs with different size down to 10 nm. NPs were characterize using TEM in order to control the morphology and the crystallinity. These nanostructures were fully embedded in CeO2 matrix grown on MgO and Quartz substrates (material with higher band gap) using molecular beam epitaxy in order to maximize the interfaces between the oxide and the metal and to prevent the oxidation of NPs. More layer of CeO2- Ag NPs- CeO2 with different thickness of ceria were realized to have also information on possible collective excitations. The oxidation state of the cerium were obtained using XPS in situ experiment during the growth.
Transient absorption spectroscopy was used to explore mechanisms of energy/charge transfer at femtosecond/picosecond timescale. Towards observing charge/energy transfer, we performed these pump&probe experiments pumping first at 410 nm to excite the LSPR and then at 275 nm in order to excite cerium oxide intraband; for all the excitation were acted using different fluencies of the pump in order to excite more hot electron and so to observe higher signal of transfer.

[1] B. Li, et al. ACS NANO 8, 8152 (2014).
[2] S. M. Kim et al. JPCC 119, 16020 (2015).
[3] D. C. Ratchford et al. Nano Lett. 17, 6047-6055(2017)

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