New materials for photocatalytic application. Cerium-doped Zirconium dioxide

Chiara Gionco^a, Maria Cristina Paganini^a, Elio Giamello^a

^aUniversità degli Studi di Torino, Via P. Giuria 7, Torino, 10125, Italy

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Elio Giamello, presentation 209
Publication date: 5th February 2015

Among the semiconductors employed in photocatalytic reactions, transition metal oxides play a paramount role due to their qualities in term of stability in various media often accompanied by low or reasonable cost. The search for innovative materials in these field is oriented to select systems having a suitable electronic structure capable of harvesting solar light (which means essentially visible light) and excellent potential to perform the desired redox process. This is practically impossible to be found in a unique system. Solid materials with large band gap values correspond, in principle, to good reduction and oxidative potentials but inevitably, to perform the charge separation, need high energy photons (UV light, scarcely present in solar irradiation at the earth surface). On the other hand semiconductors with smaller band gap value, compatible with visible light absorption, may have unsatisfactory potentials for both reduction and oxidation. A possibility to overcome this drawback consists in modifying a semiconductor with relatively large band gap in order to make possible the absorption of visible light. This has been done intensely in the case of titanium dioxide, following more than an approach including doping with non metal atoms [1]. In the present contribution we intend to analyze the modification of the light absorption mechanism induced by engineering the band gap of ZrO₂ and show how using low energy photons it is somehow possible to convey electrons in the conduction band generating holes in the valence band in spite of the relatively large band gap value of the oxide (5eV). In our study we synthesized ZrO₂-CeO₂ mixed oxides with different CeO₂ loading (0.5-1-5 wt%) [2] and with different processes. Encouraging results have been obtained especially with the sample having lower ceria loading (0.5%). Both reduction and oxidation photoactivity of the sample have been verified through paramagnetic resonance (EPR). The photoformation of both electrons (reacting with oxygen and producing superoxide anions) and holes necessary to entail photocatalytic processes has been demonstrated irradiating with visible light (l>420nm). This phenomenon, which is not observed for bare ZrO₂, is due to the presence of empty Ce⁴⁺ states in the zirconia band gap.

[1] S. Livraghi, M.C. Paganini, E. Giamello, A. Selloni, C. Di Valentin, G. Pacchioni, Journal of the American Chemical Society. 128 (2006) 15666-15671. [2] Gionco C.; Paganini M.C.; Giamello E.; Burgess R.; Di Valentin C.; Pacchioni G.; J Phys.Chem.Lett., 2014, 5, 447

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