Investigating the charge separation and transport in iron based photoanodes for solar-driven water splitting
Bassi, Prince Saurabh
Date of Issue2016-04-25
School of Materials Science and Engineering
Solar fuel production using Photoelectrochemical Cell has developed after Fujishima and Honda demonstrated in 1972, for the first time, solar water splitting using Titanium Dioxide (TiO2) as photoanode. After that, efforts have been made to employ cheap, abundant and non-toxic semiconducting materials as photoanodes and to improve overall efficiency and performance of PEC devices. In the last decade, hematite (α-Fe2O3) based PEC devices have attracted many scientists in exploiting its advantages like good visible light absorption and stability but the efficiency achieved (only 1-2%) is still far from the theoretical limit of Solar-to-Hydrogen (STH) efficiency (15%). This is due to the limitations of hematite like small minority carrier diffusion length, poor charge transport properties, slow water oxidation kinetics etc. To counter this, hematite nanostructures with elemental doping are used in PEC devices and are thermally activated through sintering at high temperature to achieve significant photocurrent. But high temperature annealing also deforms the substrate morphology which in turn degrades the conductivity and hence trade-offs with overall performance of PEC device. Here, we have demonstrated annealing profile which proves optimal in tackling bulk recombination and simultaneously keeping intact the crystallinity of Fe2O3 nanorods and conductivity of FTO substrates. Through measurements performed under frontside and backside illumination, we have shown electron transport drives the photocurrent under backside illumination for the optimized annealing profile. To inhibit the surface recombination probability, we employed surface passivation technique by coating hematite nanorods with a very thin Atomic Layer Deposition (ALD) grown TiOx overlayer which resulted in manifold enhancement in its performance. Owing to inability of Fe2O3 to reduce water due to its lower conduction band level than the water reduction potential, iron based photoanodes based on Fe-Ti-O systems were explored. Crystalline nanoporous single phase Fe2TiO5 (without secondary phases) thin films are obtained and characterized for the first time. This material is stable in water and has a bandgap suitable for visible light absorption (2.1eV). Using XPS, the work function was calculated to lay around 4.77 eV with respect to vacuum level and the conduction band is estimated to lie or higher than water reduction level based on XPS data. Hall measurement showed a mobility of around 6 cm-2V-1s-1 which demonstrate good charge transport properties of Fe2TiO5 thin films. To enhance charge separation and transport in pristine Fe2O3 nanorods, it was coupled with Fe2TiO5 thin films to form a Fe2O3/Fe2TiO5 heterojunction, which according to their band level positions share a type II band alignment. Synthesized using low-cost hydrothermal technique, these heterojunction films show high photocurrent, reaching 1.4 mA/cm2, as compared to pristine Fe2O3 and Fe2TiO5 films. Characterization techniques like impedance spectroscopy and PEC characterization with hole scavenger revealed the charge dynamics of the system. It is proposed that surface mediated water oxidation was enhanced for the heterojunction films which allowed firstly the trapping of holes in the surface states and secondly facile transfer of these holes into the electrolyte for water oxidation. In sum, the favorable band alignment and the improvement in catalytic activity due to integration of Fe2TiO5 film with Fe2O3 nanorods was beneficial in providing low onset voltages and higher photocurrent density for heterojunction based PEC devices.