Rolling dopant and strain in Y-doped BiFeO epitaxial thin films for photoelectrochemical water splitting.

We report significant photoelectrochemical activity of Y-doped BiFeO (Y-BFO) epitaxial thin films deposited on Nb:SrTiO substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2θ-ω scans of the symmetric (00 l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L) is preserved on larger nanoregions/nanodomains. For higher thickness values L is decreasing while domains tilt angles (α) is increasing. The photocurrent value obtained for the thinnest sample was as high as J = 0.72 mA/cm, at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film's thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.