Institut des
NanoSciences de Paris
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Soutenance de thèse de Miao Zhang - Lundi 16 octobre 2017 à 14 h

Miao Zhang, doctorante dans l’équipe Oxydes de basses dimensions, soutient sa thèse le lundi 16 octobre 2017 à 14 h.

UPMC - 4 place Jussieu - 75005 Paris - Amphithéâtre Charpak (R-d-C, barre 22-23)

« Defects in ZnO nanoparticles obtained by gas-phase syntheses »

Abstract

By far, the assignment of defects-related spectroscopic features of zinc oxide is still a matter of great controversy. This is probably due to the variety of possible defects in ZnO as well as to their still uncertain formation energies and positions within the band gap. Uncontrolled measurement conditions and impurities related to some synthesis methods can additionally mislead interpretations. The aim of this work is to identify the intrinsic native defects in pure ZnO or formed upon different kind of post-synthesis treatments. To fulfill this goal our strategy was to : i) prepare model zinc oxide nanoparticles using two different vapor-phase synthesis techniques (Combustion and CVS) ii) identify, assign and discuss the occurrence of the defects in line with the synthesis and post treatments conditions by combining in situ PL and EPR measurements together with other complementary spectroscopies (Raman, UV-vis, FTIR) and iii) reveal the surface reactivity of defective ZnO samples by studying the interaction with water or 2-methyl-3-butyn-2-ol (MBOH). We observed that in all as-synthesized ZnO samples VO2+ and Zni+ are the predominant native defects with relative amounts depending on the partial pressure of oxygen used during the synthesis. Neutral oxygen vacancies (VO0) are additionally detected in samples prepared in conditions particularly rich in zinc. The formation of Vo+ is demonstrated in ZnO smoke upon post treatment (annealing in high vacuum or zinc vapor) while the associated electron release is shown to participate to the reduction of Zni+ into Zn0. On the contrary, annealing in oxygen leads to reverse processes while if used in an excess, to creation of Oi-related defects. Dissociation of water on vacuum annealed surface leads to the filling up of VO+, and reduction of Zn2+ into Zn+. Such redox processes controlled by the pretreatment conditions affect the surface reactivity through the change of the acid base balance, as revealed by MBOH conversion catalytic tests.

Jury

  • Myrtil KAHN, Directrice de Recherche (Université de Toulouse) Rapporteur
  • Lubomir SPANHEL, Professeur (Université de Rennes 1) Rapporteur
  • Maria Cristina PAGANINI, Associate professor (Università di Torino) Examinateur
  • Hélène PERNOT, Professeure (UPMC) Examinateur
  • Frédéric AVERSENG, Maître de conférences (UPMC) Co-encadrant (invité)
  • Slavica STANKIC, Chargée de Recherche (UPMC) Co-encadrante
  • Guylène COSTENTIN, Directrice de Recherche (UPMC) Directrice de thèse