A Study On Surface Passivation Of Hematite Thinfilms Photoanode For Solar Water Splitting

With the continuous increase of the population,the use of energy has become more and more intensive,and the consumption of petroleum,coal,and various ores has increased dramatically.Solving the energy problem has become a priority task in the world today.A series of new energy such as solar energy,wind energy,and hydropower have gradually entered our lives.Solar energy comes from the solar radiation,which can be used through three approaches:photo-electro,photo-thermal and photo-chemical transformation.Among them,solar hydrogen production has been developed in the last 30 to 40 years.So far,the main research technologies of solar hydrogen production are divided into:photoelectrochemical decomposition of water to produce hydrogen,thermochemical hydrogen production,and photocatalytic hydrogen production.Photoelectrochemical(PEC)water splitting produces hydrogen by photo assisted electrolyzing water.The PEC system consists of photoanode,photocathode and electrolyte.The main source of photoanode material is semiconductor material.Electron-hole pairs are generated after being excited by light.A closed loop is formed by transferring electrons from the anode to the cathode by applying a voltage in the electrolyte solution.Photoanode is a key factor affecting the efficiency of hydrogen production,and the research on anode materials has drawn great attention.Among them,?-Fe₂O₃ has become one of the most promising photoanode materials due to its small bandgap(2.2 e V),abundance,good stability.In this thesis,surface modifications of hematite were conducted to address the slow water oxidation kinetics and fast photo-electro and hole recombination issues.In this paper,we use two precursors to modify the surface of?-Fe₂O₃ thin films.The precursor(dopamine hydrochloride(abbreviated as DA)or Na H₂PO₂)was placed in the upstream of a tube furnace for thermal decomposition,and then blown onto the surface of?-Fe2O3 in the downstream under a nitrogen environment.The experimental section mainly contains two contents.The first part is the study on the DA treated hematite.By weighing DA in different grams and placing them in a tube furnace for decomposition,the carbon element in the products will be deposited on the surface of?-Fe₂O₃.It was found that the photocurrents were increased after DA treatment.To elucidate the role of carbon overlayers,Scanning electron microscopy(SEM),Transmission electron microcopy(TEM),X-ray photoelectron spectroscopy(XPS),electrochemical impedance spectroscopy(EIS)and Mott-Schottky(M-S)were conducted.It turned out that,the carbon overlayers has two main positive effects.One the one hand,it enhanced the carrier density of the bulk of hematite,increasing the electrical conductivity;on the other hand,it effectively passivated the surface states,decreasing the surface photo electon-hole reconbination.of the material,thereby improving the photoelectric conversion efficiency.It is found that increasing the amount of DA will greatly change the morphology of?-Fe₂O₃ thin films.Higher amount of DA results in bigger figure size of the nanotube.It is expected,higher amount led to formation of thicker carbon overlayers as observed by HRTEM.Furthermore,the optimum thickness of the overlayers is about 3-5 nm and the photocurrent density reached 1.64m A/cm3@1.23 V_RHE.The second part is the study on the effect of Na H₂PO₂ treatment for hematite.Similar procedure is used and the DA is replaced by Na H₂PO₂ By heating at 300°C for 1 hour in a tube furnace,the PH₃ gas in the decomposition product of Na H₂PO₂will react with?-Fe₂O₃.The reaction produces a substance called Fe-Pi,which passivated the surface state of hematite effectively.To date,most surface passivation layers have been metal oxides.However,oxygen vacancies exist in most metal oxides.In this study,the surface capacitance and resistances were greatly reduced and the surface state is effectively suppressed by the Fe-Pi films.The starting potential of the corresponding photoanode was negatively shifted by 0.15V,thereby increasing the photocurrent density to 1.997 mA/cm3@1.23 RHE.