Type of presentation: Oral

MS-10-O-1558 Mesoporosity in photocatalytically active oxynitride single crystals characterized by electron tomography, HREM, nanobeam diffraction and EELS

Pokrant S.¹, Cheynet M.², Irsen S.³, Maegli A.¹, Erni R.⁴

¹Laboratory of Solid State Chemistry and Catalysis, Empa, Dübendorf, Switzerland, ²SIMAP, PHELMA-Campus, Grenoble, France, ³Electron Microscopy and Analysis, caesar, Bonn, Germany, ⁴Electron Microscopy Center, Empa, Dübendorf, Switzerland

simone.pokrant@empa.ch

Mesoporosity in single crystals has lately received highest attention [1], since it is expected that this type of material can contribute significantly to the improved design of highly efficient solar cell devices. Especially for photocatalytic or photoelectrochemical applications high surface area and good charge-transport properties are key features to enhanced device performance [2,3]. Good conductivity is usually obtained in defect free structures such as single crystals, where the surface area is small. High surface area, however, is obtained best by porous agglomerations of nanoparticles, where the conductivity is low because of multiple grain boundaries. One possibility to achieve performance improvement consists in the fabrication of large single crystals (up to 1 µm) with a mesoporous structure using a template based approach [4]. In comparison to nanocrystalline materials, their improved electron conductivity has been demonstrated as well as their large surface area. Other methods to fabricate mesoporous structures in a single crystals without using a template are available like solid-gas reactions, as carried out for the synthesis of oxynitrides, i.e. thermal ammonolysis [5].
Some of these materials i.e. LaTiO₂N (LTON) or LaTaON₂ are photocatalytically active [6-8]. Up to now the main characterization techniques used to evaluate the pore quantity and quality have been bulk methods (BET, BJH) giving information about the open porosity and qualitative SEM and TEM imaging, 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 by a combination of several transmission electron microscopy techniques. The pores themselves were explored mainly by electron tomography, while the crystallinity was investigated using a combination of HREM and nanobeam diffraction. The local chemical composition was studied by EELS. With the improved understanding of the pores in LTON we would like to open up a door to further tune the porosity in large oxynitride single crystals for enhanced performance in photocatalysis.

[1] C. Ducati, Nature 495 (2013) 180
[2] T. Hisatomi, J. Kubota and K. Domen, Chem Soc Rev, (2014) in press
[3] A. Kay, I. Cesar and M. Grätzel, JACS, 128 (2006) 15714
[4] E.J.W. Crossland et al. Nature, 495 (2013) 215
[5] S.G. Ebbinghaus et al., Prog. Solid State Chem., 37 (2009) 173
[6] A.E. Maegli et al., J. Phys. Chem. C, (2013) in press
[7] F. Zhang et al., JACS, 134 (2012) 8348
[8] N.-Y. Park and Y.-I. Kim, J. Mater Sci., 47 (2012) 5333

The authors would like to thank Prof. A. Weidenkaff for drawing our attention to the oxynitrides.

Fig. 1: 3D isosurface plot of an LTON particle with open and closed pores projected in the xy plane. The particle dimensions are about 400 nm in x and 200 nm in y direction.

Fig. 2: HREM image of an LTON particle with pores. The lines are guides to the eye to show the pore shape and their orientation in the LTON crystal.