Probing Interfacial Electronic Structure Evolution in Fe₂TiO₅/ZnO Core–Shell Nanodendrites by Soft X-ray Ptychography and X-ray Absorption Spectroscopy

Kuan-Hung Chen^1*, Sambhu Charan Das¹, Hsiao-Tsu Wang¹, Wei-Xuan Lin¹, Shu-Ang Teng¹, Chung-Li Dong¹, Chih-Wen Pao², Shih-Chang Weng², Jian Wang³, Jih-Jen Wu⁴, Wing-Kwan Ho⁴, Sekhar Chandra Ray⁵, Jau-Wen Chiou⁶, Way-Faung Pong¹

¹Department of Physics, Tamkang University, New Taipei City, Taiwan
²National Synchrotron Radiation Research Center, Hsinchu, Taiwan
³Canadian Light Source Inc., University of Saskatchewan, Saskatoon, SK S7N 5A2, Canada
⁴Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
⁵Department of Physics, CSET, University of South Africa, Florida Park 1710 Johannesburg, South Africa
⁶Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan

* Presenter:Kuan-Hung Chen, email:a22551778@gmail.com

The spatially-resolved interfacial electronic/atomic structure of epitaxially grown core-shell Fe₂TiO₅/ZnO/SnO₂ [shell-Fe₂TiO₅ (FTO), core-ZnO and substrate-SnO₂ heterojunction nanodendrite (i.e FTO/ZnO/SnO₂) has been systematically investigated using synchrotron-radiation-based X-ray absorption spectroscopy (XAS), scanning transmission X-ray microscopy (STXM) combined with Ptychography, and complementary structural characterization techniques to elucidate the origin of its enhanced photoelectrochemical (PEC) performance. Detailed structural and electronic analysis reveals that lattice mismatch between FTO (020) and ZnO (002) planes primarily induces interfacial tensile strain, while the work function difference generates a built-in electric field (BIEF) via type-II band alignment. XAS analysis reveals elongation of nearest-neighbor (NN) Fe-O and Ti-O bonds, and contraction of next-nearest-neighbor (NNN) Fe-Fe, Fe-Ti and Ti-Ti bonds at the interface, indicative of interfacial tensile strain and defects in the FTO/ZnO/SnO₂. These local distortions at Fe/Ti sites along with oxygen defects, particularly at Ti sites, contribute to efficient charge separation. The Fe/Ti L₃-edge XANES STXM-Ptychography, together with O K-edge spectral modifications, disclose charge redistribution in Fe/Ti 3d (t_2g and e_g) states via super-exchange, which promotes spin-polarized charge transport and enhances interfacial reaction kinetics. The observed oxygen defects and orbital anisotropy in O K-edge Ptychography-XANES spectra further support efficient charge separation and facilitate directional charge transfer. This work highlights the critical roles of interfacial tensile strain, defects (oxygen vacancies/dangling bonds) and spin-charge interactions in optimizing PEC performance, offering fundamental insights into the design of advanced nanostructured heterojunctions for efficient solar water-splitting catalysts.

Keywords: Nanostructures, Heterojunction, Water Splitting, X-ray Absorption Spectroscopy (XAS), Scanning Transmission X-ray Microscopy (STXM)