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Unravelling the complex formation mechanism of HfO2 nanocrystals using in situ pair distribution function analysis

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Hafnia, HfO2, which is a wide band gap semiconducting oxide, is much less studied than the chemically similar zirconia (ZrO2). Here, we study the formation of hafnia nanocrystals from hafnium tetrachloride in methanol under solvothermal conditions (248 bar, 225-450 °C) using complementary in situ powder X-ray diffraction (PXRD) and Pair Distribution Function (PDF) analysis. The main structural motif of the precursor solution (HfCl4 dissolved in methanol) is a Hf oxide trimer with very similar local structure to that of m-HfO2. Different measurements on precursor solutions show large intensity variation for the Hf-Cl correlations signifying different extents of HCl elimation. A few seconds of heating lead to a correlation appearing at 3.9 Å corresponding to corner-sharing Hf-polyhedra in a disordered solid matrix. During the next minutes (depending on temperature) the disordered structure rearranges and the nearest neighbour Hf-Hf distance contracts while the Hf-O coordination number increases. After approximately 90 seconds (at T = 250 °C) the structural rearrangement terminates and 1-2 nm nanocrystals of m-HfO2 nucleate. Initially the m-HfO2 nanocrystals have significant disorder as reflected in large Hf atomic displacement parameter (ADP) values, but as the nanocrystals grow to 5-6 nm in size during extended heating, the Hf ADPs decrease toward the values obtained for ordered bulk structures. The nanocrystal growth is not well modelled by the Johnson-Mehl-Avrami expression reflecting that multiple complex chemical processes occur during this highly nonclassical nanocrystal formation under solvothermal conditions.

Sider (fra-til)12711-12719
Antal sider9
StatusUdgivet - 7 aug. 2021

Bibliografisk note

Funding Information:
Funding from the Villum Foundation and the Danish Agency for Science, Technology, and Innovation (Danscatt) is gratefully acknowledged. We also thank the Deutsche Electronen-Synchrotron (DESY), a member of the Helmholtz-Gemeinschaft (HGF), for the beamtime allocated at Beamline P21.1, PETRA III, and thank the support from its staff, Martin von Zimmermann and Oleh Ivashko for support and assistance during beamtimes. We are also grateful to Ida Gjerlevsen Nielsen, Nils Lau Nyborg Broge, Frederik Søndergaard-Pedersen, Niels Juhl and Andy Sode Anker for assistance during beamtime. Affiliation with the Aarhus University Center for Integrated Materials Research (iMAT) is gratefully acknowledged.

Publisher Copyright:
© 2021 The Royal Society of Chemistry.

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