Disclosing the nanostructure of hematite nanoplatelet-films for active PEC water splitting

Hematite (?-Fe2O3) composite thin films are among the most promising materials for the preparation of photoanodes for photoelectrochemical (PEC) water splitting [1-3] allowing one to obtain good theoretical solar-to-fuel conversion (14-17% and photocurrent of 11-14 mA cm-2) and a favorable band gap (2 eV) for solar harvesting. Moreover, the bare material is not classified among Critical Raw Materials since it's abundant, inexpensive and chemically stable. The PEC activity is anyway strongly limited by low conductivity, low light penetration depth and small hole diffusion length (few nanometers). In order to enhance the practical performance of hematite films, a convenient approach is the structural optimization by nanostructuring: in this way the electron-hole recombination is minimized and the photocurrent increased [4]. In this work we present the characterization of a hierarchical nano-engineered hematite film, synthesized by PE-CVD (plasma enhanced chemical vapor deposition) [5]. Initial TEM and STEM studies showed an organized, highly crystalline, and oriented complex structure where single small nanosheets (the building blocks) were assembled together in nanolayers which in turn are organized in bigger nanoplatelets. The fine structure was disclosed by the analysis of a FIB (focus ion beam) lamellae of a vertical section of the film, preserving the original spatial orientation with respect to the substrate. Thus, the 3D hierarchical platelet features were studied by a 3D STEM tomography. A detailed analysis of the structure is essential to understand the electron dynamics and path responsible to the photo-efficiency and PEC activity of this film (J=1.20 mA cm-2 @1.55V (RHE)) which make our system a promising platform for customized multilayer architecture.