Exploring Burstein-Moss type effects in nickel doped hematite dendrite nanostructures for enhanced photo-electrochemical water splitting.

The Burstein-Moss (B-M) effect, which suggests that the optical band gap of degenerately doped semiconductors increases when all states close to the conduction band get populated due to shifting of an absorption edge to higher energy, is important, as it gives a chance to obtain different optical properties for the same material. Here, we report our observations of the similar shift in the optical band gap in NixFe2-xO3 nanocomposites as a function of composition with the help of cyclic voltammetry (CV) and XPS valence band (VB) position measurements. The conduction band edge (CBE) position of the NixFe2-xO3 nanocomposites as determined using CV was noted to move towards more negative potential with increasing Ni-concentration. A similar shift is also noted in the CBE estimated using XPS measurements (by subtracting the VB position from the optical band gap values). The observed shift in the optical band gap along with the CBE position gives the corresponding shift in the Fermi level, which is found to move closer to the CBE position, suggesting the observation of an effect similar to the B-M shift. Also, the extent of band bending estimated from the deviation of the CBE from the flat band potential (measured through Mott-Schottky plots) is found to increase with increasing Ni-concentration. Moreover, the Ni-composition has been observed to enhance the current density as well as to facilitate water splitting at a much lower onset potential compared to pure hematite. The NixFe2-xO3 nanocomposite with an 11 mol% Ni-composition shows the highest photo-electrochemical response with an almost ten times enhancement in the current density at 1.9 V vs. RHE in alkaline medium, as compared to the dark current. This enhanced performance is attributed to the improved charge separation and higher charge carrier density as a result of the higher extent of band bending in the NixFe2-xO3 nanocomposites.