BPI Seminar by Dr. Thorsten M. Gesing

From Crystal Structure to Photocatalytic Application

Location: CHBE #202, 2360 East Mall Vancouver BC V6T 1Z4

Agenda

• 10:00 AM Intro by Prof. Johan Foster
• 10:05 AM Presentation by Prof. Thorsten M. Gesing, Heisenberg Professor & Chair of Solid-State Chemistry, University of Bremen 
• 10:50 AM Q&A

*Light refreshments will be provided.

Abstract

The thermal decomposition of lead dioxide results in the formation of six different ambient condition polymorphs, known for a century. Among these, the two black minium phases, Pb₁₂O₁₉ and Pb₁₂O₁₇, remain subjects of structural discussion due to the stereochemical activity of the 6s² lone electron pairs (LEP) of lead. The first phase is now described using machine-learning, density functional theory, and pair distribution function (ML-DFT-PDF) refinements. The also observed red minium, Pb₃O₄, where one lead atom can be substituted by tin to form a full solid solution of thermochromic compounds, belongs to the mullite-type family of phases, providing interesting physical properties. The band-gap type and energy of these semiconductors can be analyzed using the reflection-absorption-Tauc-DASF (RATD) method. The LEP-containing mullite-type phases exhibit partially unusual thermal expansion behavior, such as axial negative thermal expansion with a turnover to positive, which is described using the Debye-Einstein-Anharmonicity (DEA) approach. However, the thermal expansion up to a certain phase transition could not be described this way. Therefore, in the last decade, the model was extended by a gliding component (DEA+G), enabling the calculation of the additional energy necessary for a second-order phase transition, either of nuclear or magnetic type. Additionally, most of the mullite-type compounds are colored phases with a band-gap transition energy in the range of visible light, making them suitable as sustainable photocatalysts. Tuning the band-gap energy within different solid solutions is a key factor for this application. Another important factor is the precise synthesis, tuning, and analysis of the average crystallite size compared to the average particle size to understand the catalytic performance. This can be investigated by combining TEM investigation with the Envelope average crystallite size (EnvACS) analysis, which provides precise information not only on the average crystallite size of single-phase catalytic active phases but also on heterogeneous catalysis, such as quantum-crystalline platinum on a SiO₂ support, enabling oxidation reactions on a chip. For the latter, the quantum-crystalline structure was determined using delta-PDF (ΔxPDF or ΔgPDF) in-house Mo-radiation data, which can determine not only short-range order but also medium- and long-range order of nano- or micro-crystalline phases, like in defect-rich olivines as model phases to describe part of Martian regolith.

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