Mesoscopic thin films of hematite as photoanode for water photoelectrolysis

Due to the limiting amount of fossil fuel available and to the continuous growth of the world energy consumption, it becomes important to find alternative energy sources. Hydrogen produced by the photoelectrolysis of water is a perfect candidate as a clean energy source since it can be produced and used efficiently without generating any pollution. The water oxidation to oxygen is a slow reaction that requires an important overpotential in order to reach a significant current. This problem is crucial in water photoelectrolysis: a high overpotential means more solar cells in series in order to obtain high enough conversion efficiency. In order to lower the number of solar cells in series, a photoactive anode can be used. One potentially good material for a photoanode is hematite: it is cheap, stable and it has a bandgap small enough to absorb a part of the visible light. The objective of the present work is to develop and characterize highly photoactive semi transparent thin films of hematite that could be used efficiently to photo oxidize water. Three different deposition methods have been tested: Spray Pyrolysis (SP), Ultrasonic Spray Pyrolysis (USP) and Electrostatic Spray Deposition (ESD). The films obtained were characterized using spectroscopic (UV-Vis absorption, SEM and Raman spectroscopy) and electrochemical techniques (current potential curves in dark and under light, IPCE spectra, photopotential measurements and electrochemical impedance spectroscopy). The films made by ESD were not photoactive. The films made by SP were weakly photoactive and the films made by USP gave the highest photocurrent. After an optimization step, films with a photocurrent of 0.5 and 1.07mA/cm-2 at 1200mV vs RHE and under 1.5AM were produced by SP and USP respectively. Ti[IV] doping improved the photoresponse of the films deposited by SP. An inverse effect was observed for the films deposited by USP: the introduction of a dopant lowered the photocurrent and increased the dark current. The properties of layers made by USP were compared to those deposited by SP. Although both types of films show similar XRD, UV-visible and Raman spectra they differ greatly in their morphology and in their photoactivity. The mesoscopic α-Fe2O3 layers produced by USP consist mainly of 100 nm- sized platelets with a thickness of 5-10 nanometers. These nanosheets are oriented perpendicularly to the FTO support, their flat surface exposing (001) facets. The films made by SP consist of spherical hematite particles of 50-100nm in diameter densely packed. Open circuit photovoltage measurements indicated a lower surface state density for the films made by USP as compared to the films made by SP. This factor explained the much higher photoactivity of the USP compared to the SP deposited α-Fe2O3 films. Addition of hydrogen peroxide to the alkaline electrolyte further improves the photocurrent-voltage characteristics of films generated by USP and SP indicating hole transfer from the valence band of the semiconductor oxide to the adsorbed water to be the rate limiting kinetic step in the oxygen generation reaction. Electrochemical impedance spectroscopy (EIS) experiments have been performed on the most photoactive USP films. The construction of a frequency independent Mott-Schottky plot showed that the hematite flat band potential lies at 0.26V vs RHE in 1M NaOH and that the donor concentration is 5.6 1017cm-3. EIS allowed the identification of a deep donor level 0.3V above the flat band potential. This level explains the difference between the onset potential and the flat band potential. The electrochemical and photoelectrochemical behaviors of the most photoactive hematite thin films deposited by USP were found to be really close to those of single crystal hematite indicating that the influence on the photoactivity of the grain boundaries can be neglected.