Mesoporous single crystals have been a matter of intense discussion in the last years in the context of solar energy harvesting, since it is expected that this material class can contribute significantly to the improved design of highly efficient solar energy conversion devices . Especially for photocatalytic or photoelectrochemical hydrogen generation high surface area and good charge-transport properties are key features to enhanced device performance [2, 3]. Good conductivity is usually obtained in large defect free structures such as single crystalline materials, where consequently the surface area is small. High surface area, however, is obtained best by porous agglomerates of nanoparticles, where the conductivity is low because of multiple grain boundaries. The possibility to achieve performance improvement by combining both concepts has been demonstrated via the fabrication of large single crystals on the micrometer scale with a mesoporous structure using a template based approach . In comparison to nanocrystalline materials, the improved charge carrier conductivity has been shown in this material class as well as its competitive surface area. Moreover, other template-free synthesis routes are known, where porous structures in solids are formed spontaneously. One example is the solid-gas phase reaction carried out for the synthesis of oxynitrides, i.e. thermal ammonolysis . However, the control of pore size and density requires a detailed understanding of the reaction mechanisms during synthesis.
Some of these oxynitride materials i.e. LaTiO2N (LTON) or LaTaON2 are photocatalytically active [6-8]. The main characterization techniques used to evaluate the pore quantity and quality have been powder techniques (for example physisorption), giving information about the open porosity, and qualitative scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which suggested that open and closed pores are formed [6-8]. However, little is known about the size and shape distribution especially of the closed porosity or about the pore formation process.
In this contribution we will focus on microscopic pore characterization of LTON as a function of the synthesis method by combining several TEM techniques (Figure 1, 2) [9,10]. The pores themselves were explored mainly by electron tomography and by scanning TEM (STEM) with an high angle annular dark field (HAADF) detector, while the crystallinity was investigated using a combination of high resolution transmission electron microscopy (HREM), selected area diffraction (SAD) and nanobeam diffraction. The local chemical composition was studied by electron energy loss spectroscopy. With the improved understanding of the pore formation mechanism in LTON we enabled porosity tuning in large oxynitride single crystals leading to enhanced performance in photocatalytic and photoelectrochemical water-splitting.
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Acknowledgements: The authors thank the SNSF for the PrecoR grant 20PC21_155667.
To cite this abstract:Simone Pokrant, Stefan Dilger, Steve Landsmann; Design and characterization of mesopores in photocatalytically active oxynitride single crystals using structural and chemical TEM analysis. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/design-and-characterization-of-mesopores-in-photocatalytically-active-oxynitride-single-crystals-using-structural-and-chemical-tem-analysis/. Accessed: December 6, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/design-and-characterization-of-mesopores-in-photocatalytically-active-oxynitride-single-crystals-using-structural-and-chemical-tem-analysis/