Controllable Preparation And Photoelectrochemical Water Splitting Of α-Fe₂O₃ Nano-films

Hematite（α-Fe₂O₃） has been extensively investigated as one of the best candidates for photoanode semiconductor due to its excellent chemical stability in aqueous solution, abundance in the earth, nontoxicity and low cost. The theoretical total photocurrent as high as 12.6 m A/cm² of water splitting has been predicted under AM 1.5 G solar irradiation with a band energy gap of 2.1 e V and visible light absorption up to 590 nm. However, several factors have greatly limited the practical performance of hematite for solar water splitting, such as poor conductivity(0.2 cm²· V^-1· s^-1), short hole diffusion length（<4 nm） and poor oxygen evolution reaction kinetics. Recently, doping elements, such as Mg²⁺, Ti ⁴⁺, and Sn⁴⁺, are being reported to improve the electric properties of hematite. Some strategies of nanostructured architectures including nanorods, nanowires, and nanotubes are developed for shortening the pathway of photoexcited electrons/holes to surface. Co-catalysts such as IrO₂ and Co-Pi were coated on hematite films to reduce the overpotential and enhance photocurrent dramatically. In this thesis, hematite films were synthesized through hydrolysis method. The photocurrent was improved via nanostructured architectures and surface modification. The effects of nanostructure, morphology, and surface modification on the photoelectrochemical properties of the hematite thin films were investigated. To design and synthesize efficient photoanode, a comprehensive understanding on the structure-property relationship of α-Fe₂O₃ film and electrode is necessary.The α-Fe₂O₃/TiO₂ films on conductive glass FTO substrate were synthesized through a separated two-phase hydrolysis-solvothermal reaction. After optimization of conditions, 1.0 mg/mL TiO₂ in ethanol, 140 ℃ hydrolysis temperature, 500 ℃ calcinations temperature in furnace and four cycles deposition were determined. A photocurrent of 0.683 m A/cm² was obtained at +1.5 VRHE when the hematite film was examinated under visible light（100 m W/cm²） illumination. Moreover, the α-Fe₂O₃/TiO₂ films were very stable since the photocurrent of the hematite remained almost the same value without degradation. Through characteristic of XRD and TEM, TiO₂ act as an intermediary to increase α-Fe₂O₃ crystallinity and as a support of large surface areas to coat greater amounts of α-Fe₂O₃. In order to absorb the light passing over the thin film, four combined film photoanodes were used in tandem and a total photocurrent of up to 1.53 mA/cm² was obtained at +1.5 VRHE under visible light（100 mW/cm²） illumination. The result demonstrates that thin films combined in tandem enhance light absorbance, which provides a possible process of full use of sunlight for ultrathin film.The optimised photoanode based on 7 deposition cycles with a 175-nm-thick α-Fe₂O₃ film afforded a comparable photocurrent of α-Fe₂O₃/TiO₂ film. The effect of deading layer can be ignored when hematite film was enough thick. The photocurrent can be improved by reducing graphene oxide（RGO） coated on the hematite film through a simple spin coating process. RGO was synthesized from a solution of 7 mg/mL GO in water, then sintered at 400℃ for 30 min. The effect of RGO on photoelectrochemical properties of the hematite thin films were investigated in details. The performance enhancement are attributed to the promoting effect of RGO which acted as hole collector and transporter in the photoanodes. This work shows that graphene as one of carbon nanomaterials can be used as an inexpensive co-catalyst with environmental friendliness to enhance the photoelectrochemical water splitting activity.Due to phosphate groups adsorbed on certain planes of the initial FeOOH nuclei, hydrolysis of FeCl₃ solution results in the formation of nanocube structure of α-Fe₂O₃. The effect of phosphate ions on the morphology was studied. The hydrolysis reaction conditions were optimized and the ratio of Na₂HPO₄:FeCl₃ was determinated to be 1:100. A photocurrent of 0.788 m A/cm² was obtained at +1.5 VRHE for phosphate modified hematite film. The effect of morphology, types of phosphate ions existed in the hematite films and surface characteristics on the photoelectrochemical properties of the photoanode were further investigated. Because of nanocube structured α-Fe₂O₃ ultrathin film（<50 nm）, the distance of photogenerated electrons and holes reaching the interface is very short, which is beneficial to facilitating charge transfer and minimizing bulk charge recombination. A negative electrostatic field formed by phosphate ions also promotes holes transfer resulting in enhancement in charge injection yield and increasing photocurrent.