Hematite and its hybrid nanostructures for photoelectrochemical water splitting: how do properties affect functionality?

The concept of water splitting in photo-electrochemical cells (PEC) dates back to 1972 when Fujishima and Honda developed the titania electrode which splits water into H2 and O2 under the presence of light and bias. The water splitting reaction is well established in nature. For instance, green plants and cyanobacteria used this water splitting machinery to run the very important photosynthetic process. Hematite films are suitable for PEC because of their stability, suitable band gap, and energy band positions.
I have dedicated my PhD thesis to this field and will present here my results. The final PEC properties of the films have been studied by running current vs. potential measurement and it is found that one dip coated layer gives rise to around 250 μA /cm2. Optimization of their photocurrent densities has been achieved either through doping or by morphological modification. Doping with silicon resulted in the formation of dendritic nanostructures and a significant increase in the incident photon to- current conversion efficiency of 42% at 320 nm wavelength. Following this, Si doping have been performed and it is found that 0.5% silicon doped hematite film showing maximum photocurrent density of around 2.4 mA/cm2. After this surface modification strategy has been employed in order to get hybrid nanostructure. In this regard, a simple one pot hydrothermal method is described for converting a dip coated hematite nanoparticulate film into an array of nanorods with superimposed flower like structures suitable for the water splitting in photoelectrochemical cells. The hydrothermal treatment of the dip-coated hematite film with FeCl3•6H2O and L – Arginine enhances the photocurrent by a factor of two. The photocurrent density of the pristine film reached 218 μA /cm2 after 48 hours of hydrothermal treatment and this increase was attributed to the higher specific surface area of the modified film and changes in the optical properties pristine film after hydrothermal treatment. Another way of increasing the photocurrent of hematite by covalent cross-coupling with phycocyanin has been developed. For this, a hematite – phycocyanin integrated system is assembled by consecutive adsorption and cross coupling of protein molecules separated by an agarose layer and a linker molecule on the top of a mesoporous hematite film. The hematite - phycocyanin assembly shows a two-fold increased photocurrent in comparison with the pristine hematite film. The increase in the photocurrent is attributed to the enhanced light absorbing of the hematite film after integration with protein, as is evident from the UV-Vis spectra and from the photocurrent action spectrum. The assembly shows long term stability and thus constitutes a promising hybrid photoanode for photoelectrochemical applications. Finally, a multiple band gap photoelectrode was fabricated by employing the hydrothermal deposition of NiO nanostructure on the top of hematite electrode. This system showed good gas evolution and a current density of around 16 mA /cm2 and systems works by the redox reaction of Ni and Fe.