Hematite (α-Fe2O3) is a promising material for photoanode. However, it has a few drawbacks as low concentration of charge carriers and low OER kinetic. These disadvantages fix by morphology control and doping.
The study is dedicated to the photoelectrochemical behavior of films based on α-Fe2O3 hollow submicron spheres. Spheres with an average size of 970 nm and wall thickness of about 300 nm were received by the spray-pyrolysis method. The structure of the synthesized particles implies the presence of pores inside the particles. Well-known that the concentration of vacancies near the pore is more than equilibrium. Also, oxygen vacancies in α-Fe2O3 are donor dopants. Therefore, the bottom of the conduction band near the interface wall/pore is lower than at the interface wall/electrolyte. This structure provides a higher charge separation.
Films were produced by Dr. Blade method. FTO-glass was used as a substrate, a mixture of 20 % (wt.) aqueous solution of Fe(NO3)3 and hollow submicron sphere α-Fe2O3 in ratio 2 : 3 was used as slurry. Samples were annealed at 400 ? for 1 hour with heating rate 2 ?/min at air. Morphology and phase composition of samples were obtained by scanning electron microscopy (SEM) (Vega 3 Tescan, Czech Republic) and X-ray diffraction (XRD) (Difrey-401, Russia). At the cross-section SEM-image was seen two-level structure consisting of nanoparticles layer and submicron particles layer with film thickness 5 μm. At the surface was found “necks” between submicron particles. Nanoparticles layer and “necks” were formed as result Fe(NO3)3 decomposition. XRD analysis was shown that only hematite phase in the film.
Photoelectrochemical behavior of the film was studied in 0.1M KOH aqueous solution using hand-made three electrode photoelectrochemical cell (SCE electrode was used as reference, graphite electrode was auxiliary, sample was work electrode), potentiostat P-45X (Electrochemical Instruments, Russia) and Xenon solar light source (Newport, USA) (solar illumination 100 mW/cm2). Contact area between the sample and electrolyte and illuminated area was 1 cm2. Photovoltage was determined as difference of open circuit potential in dark and under illumination (VOC). Current-voltage (J-V) in the dark, stationary current-time curves (J-t) with chopped illumination were recorded. VOC of the sample was 150 mV. J-V plots in the dark shown that onset potential is about 0.87 V vs SCE (1, 13 V vs RHE). J-t plots recorded at 0 – 0.8 V vs SCE with chopped illumination shown that photocurrent density was stable and equal 1 – 4 μA/cm2 respectively. Long-term tests at 1 V vs SCE showed that when the sample is illuminated for 4 hours, the current increases according to the equation J = a*exp(-t/b) + c, the value of the photocurrent was about 1 mA/cm2. After this time, a change in current density was observed at a stationary potential also for 4 hours, the value of the current in this time range decreased according to the same equation. The b parameter was interpreted as reaction rate constant.
Based on the studies performed, it is assumed that the increase in current density during long-term tests at a potential of 1 V is associated with the formation of Fe(IV) on the surface of the photoanode. The formation of Fe(IV) is explained by the high concentration of holes at the photoanode/electrolyte interface due to the distribution of vacancies in the wall of the α-Fe2O3 hollow sphere.
- The study is devoted to one of the most promising materials for photoanodes.
- The study talks about obtaining film with two-level structure.
- The study describes the photoelectrochemical behavior of films based on hollow microspheres and presents the relationship between the structure and properties of the material.