The morphology of semiconductor-electrodes has been shown to significantly affect the performance of photoelectrochemical (PEC) water splitting devices. A favorable morphology can lead to increased surface area, enhanced solar absorption, decreased charge recombination, and reduced mass transport limits [1]. However, for complex 3D geometrical morphologies, it is challenging to quantify the performance enhancement attributed to the semiconductors’ geometrical features and to identify morphological limitations to the performance.  

We used a coupled experimental-numerical approach for the characterization of morphologically-complex photoelectrodes consisting of nano- to micrometer thick semiconductor-films. A 3D-microscopy method, focused ion beam nanotomography (FIB-nt) [2], was applied with a high resolution of 4x4x4 nm3 voxel sizes. By post-processing of the data obtained by the secondary electron (SE) and the energy selective backscattered (EsB) detectors in the FIB-nt, the exact 3D geometries of the semiconductor-films were digitally reconstructed and performance-related morphological parameters were analyzed and quantified.  

The method was applied to two photoelectrodes, different in structure, composition, and scale: i) a particle-based lanthanum titanium oxynitride electrode with a film thickness of a few micrometers [3], and ii) a ‘cauliflower-like’ structured hematite electrode with a film thickness of a few hundred nanometers [4]. The digitalized morphology of the two films was used to quantify morphological parameters such as material loading and the specific surface, as well as the density distribution normal to the electrode plane. A representative elementary volume was defined to assess the homogeneity of the films. The geometrical features of each film, particles or ‘cauliflower-like’ structures, were characterized by the distributions of the feature sizes and orientations. For the particle-based electrode, the contact points of the particles, as well as the pores within the particles, were quantitatively analyzed.  

The FIB-nt, with its high resolution in the nanometer range, reveals precise structural information of semiconductor-films at the submicrometer scale. The methodology proofs to be applicable to photoelectrodes and provides a unique insight into their morphologies. The analysis of the 3D-data obtained allows for the qualitative and quantitative assessment of performance-related morphological parameters and will subsequently be used to characterize relevant transport characteristics.

[1] P. Zhang, L. Gao, X. Song, J. Sun, Adv. Mater. 2014, 27, 1521.

[2] M. Cantoni, L. Holzer, MRS Bull. 2014, 39, 354.

[3] A. E. Maegli, S. Pokrant, T. Hisatomi, M. Trottmann, K. Domen, A. Weidenkaff, J. Phys. Chem. C 2013, 118, 16344.

[4] S. D. Tilley, M. Cornuz, K. Sivula, M. Grätzel, Angew. Chemie 2010, 122, 6549.