Photoelectrochemical Responses Of Hematite Films Deposited By Ultrasonic Spray Pyrolysis

A photoelectrochemical (PEC) cell has attracted continuous interest as a potential route to produce hydrogen from water under sunlight irradiation, since Fujishima and Honda used TiO2photoanode to split water in1972. It seems to be a broad agreement that a suitable semiconductor photoanode for a photoelectrochemical cell should have a band gap close to2.0eV. Hematite is a promising material for the storage of solar energy by the photoelectrochemical water splitting into hydrogen and oxygen because of its photochemical stability, abundance, non-toxicity, and capability to absorb photons in the visible spectral range (band gap, Eg=2.0cV). However, the overall hematite photocurrents produced by solar light have been severely limited by several disadvantages, such as the poor conductivity, the high surface recombination and the short hole diffusion length. In this paper, we proposed some methods to increase the hematite bulk conductivity and reduce the surface recombination.Pure hematite is Mott insulator. In most cases, doping is done intentionally for improving the hematite conductivity and photoresponses. Usually, the dopant, such as Si4+or Ti4+, is assumed to act as an electron donor due to the substitution of Fe3+in the hematite lattice and thus improves its donor concentration and electrical conductivity. To further increase the donor concentration, we proposed a Si4+and Ti4+codoping idea. Our research results show that codoping is superior to Si-doping or Ti-doping, and enhances the hematite photoresponses significantly. IPCE of the codoped film is34%at365nm and0.6V vs. Ag/AgCl, much higher than10%,20%and22%for the undoped, Si-doped and Ti-doped film, respectively. The mechanism of codoping bringing the donor concentration to a new level may be the ions radius of Si4+<Fe3+<Ti4+.Besides tetravalent cations doping, bivalent cations doping, like Mg2+, has also been investigated. In the case of hematite doped with Mg2+, which was intended to get a p-type hematite, anodic and cathodic photocurrents were observed simultaneously when sweeping the voltage from positive to negative. The photocurrent direction switching performance is ascribed to be an intrinsic feature related with competition between the electron concentration and the hole concentration of hematite. It reveals the inadequacy prevailing in the literatures nowadays that judging the semiconductor is p-type based on the cathodic photocurrents.Another factor that deteriorates the hematite photoresponses is the slow OER (oxygen evolution reaction) kinetics. Normally, surface loaded some co-catalysts, such as CO3O4, RuO2, IrO2,"Co-Pi", etc. have been induced to increase the OER kinetics. There is another limiting factor other than slow OER kinetics in regard to the surface. Surface traps have been suggested as an additional loss mechanism in hematite photoelectrodes prepared using various methods by several groups. Here, we reported a novel and simple strategy to passivate trapping states by scanning photocurrent-time (i-t) curves of hematite photoanode for-10h in saturated NaCl aqueous electrolyte in three-electrode cell under intense light illumination. Electrochemical impedance and photoluminescence spectroscopy reveals a significant change in the surface capacitance and radiative recombination, respectively, which confirms the passivation of surface states. This method was discovered in hematite phototstability test in saturated NaCl aqueous electrolyte. Unlike other methods, it is facile to manipulate without inducing any co-catalyst or passivation layer.For PEC cells, the semiconductor liquid junction (SCLJ) is often treated as a simple Schottky barrier, and the photoresponses are usually described by the Schottky barrier model. However, this model would be invalid when the depletion layer width is very small, such as for the doped hematite, whose donor concentration could reach above1020cm-3and depletion layer width is ca.5nm. Moreover, the Schottky barrier model cannot reflect the influence of the semiconductor electrode film thickness and conductivity on the photoresponses. Therefore, we gave a modified model to predict the hematite photoresponses based on the transport equation for the minority carriers (holes) under illumination in an n-type semiconductor. The simulation results by this modified model matched well with the experimental results of the hematite photoresponses.