Effects Of Anneal Conditioning And Mg/Ti Co-doping On Water Oxidation Performance Of Hematite Photoanode

Energy issues are still a hot topic widely discussed worldwide.Eco-friendly and renewable new energies to replace traditional energy like coal and petroleum for alleviating the process of environmental degradation and filling the energy shortage are urgently demanding.Solar power is regarded as an ideal alternative energy source because of its wide distribution,huge reserves and environmental friendliness.However,the energy density of solar energy is too low,and its distribution are variable with times and locations,and thus it is not a stable and continuous energy source.A suitable method to effectively store solar energy is of importance.Among many solar energy storage solutions,photoelectrochemical?PEC?water splitting to produce hydrogen is a very attractive and challenging way to capture and store solar energy.Hydrogen is very clean energy since it has zero-pollution during uses.Therefore,the PEC water splitting has attracted more and more researchers'attention.Photoanodes,a place for solar energy collection,charge separation and water oxidation reactions,are the key component of PEC water splitting cell.It is critical to have an ideal photoanode to offer a high performance PEC cell.The available materials for ideal photoanodes is often semiconductor materials with suitable band gap like metal oxides,metal sulfides,metal selenides,metal nitrides,metal phosphides and so on.However,the decomposition potential of most of materials is above the water oxidation potential,which is not suitable as photoanodes.Among those alternative semiconductor materials with suitable band gap,only metal oxides are stable during PEC water splitting,of which hematite??-Fe₂O₃?is regarded as a promising material possesses a suitable band gap,stable chemical properties,abundant crustal reserves and eco-friendly nature.On the other hand,?-Fe₂O₃ itself also has some unfavorable characteristics as photoanodes,such as poor conductivity,full of surface state which stop hole transfer to electrolyte and poor water oxidation catalytic performance,etc.,Therefore,the photoelectric conversion efficiency of the untreated?-Fe₂O₃ photoanode is extremely low,which is far from the actual requirements as a photoanode in PEC cell.In the preparation methods of various?-Fe₂O₃ photoanodes,?-FeOOH nanorod films fabricated on the FTO glass by chemical bath deposition,and then annealed to obtain?-Fe₂O₃ photoanode,is a convenient and large-scale preparation method.Therefore,this thesis work carried out investigations to obtain?-Fe₂O₃ photoanode with high water oxidation performance:1.Effect of annealing temperatures preparing?-Fe2O3 photoanode on the water oxidation performanceThe crystal phase structure of the sample prepared by chemical bath deposition converts?-FeOOH to?-Fe₂O₃ after annealing at a phase transition temperature?550°C?.In order to improve the water oxidation performance of?-Fe₂O₃ photoanode,a rapid annealing process was applied to“activate”the?-Fe₂O₃ prepared in the previous step at a higher annealing temperature over 700°C and 800°C.The as prepared products were characterized for their physical properties and water oxidation performance.The water oxidation performance of the two prepared photoanodes were compared and discussed,and reasonable explanation were proposed.Moreover,the merits and demerits of the two annealing temperatures for the prepared?-Fe₂O₃photoanodes were analyzed.2.Effect of annealing atmosphere of preparation of?-Fe2O3 photoanode on the water oxidation performanceThe annealing process is important in preparation of?-Fe₂O₃ photoanodes from?-FeOOH nanorod film,since it could greatly affect the water oxidation performance.Apart from the annealing temperature,the annealing atmosphere is also a critical factor.Therefore,the prepared Ti:?-FeOOH films under air and argon atmosphere were annealed to obtain Ti:Fe₂O₃-Air and Ti:Fe₂O₃-Ar photoanode,respectively.Photoelectrochemical tests demonstrate that the latter photoanode has a better water oxidation performance.The physical and electrochemical characterizations were used to analyze the performance difference between two photoanodes and a mechanism is proposed to explain and explore the scientific insights behind the results.3.Effect of Mg/Ti co-doping on water oxidation performance of?-Fe₂O₃photoanodeIn addition to the excellent conductivity,surface state distribution on electrode surface is also an important factor responsible for the water oxidation performance of photoanode.Based on above discussion,in addition to improving the conductivity of the?-Fe₂O₃ photoanode by Ti doping,the Mg element is also incorporated into the lattice of the Ti:Fe₂O₃ photoanode by ex-situ doping method for?Mg,Ti?:Fe₂O₃photoanode.After a series of photo-electrochemical tests,it is discovered that the conductivity of?-Fe₂O₃ photoanode is greatly improved after Mg/Ti co-doping,and its surface state distribution is also more favorable for water oxidation.With synergistic effect from Mg and Ti,the?Mg,Ti?:Fe₂O₃ photoanode exhibits excellent water oxidation performance compared to the?-Fe₂O₃ photoanode without Mg/Ti co-doping.