Study On The Synthesis Of Doped BiFeO₃ Thin Films And Its Photoelectrocatalytic Properties

Energy crisis and environmental pollution are the two key scientific and technological problems that restrict people's sustainable development.The development of new energy and new technology for clean energy is imminent. Nowadays, semiconductor materials as photocatalysts for the development and utilization of solar energy which is green pollution-free resources are considered to be one of the effective means to solve the problem of energy crisis.Among them, the photocatalysts under the sun light for water splitting into hydrogen and oxygen as energy are the focus of the academic research. Therefore, developing new, highly efficient photocatalyst materials has become the main way to solve the energy crisis and environmental pollution.As a noticeable photocatalyst, perovskite BiFeO₃ has a narrow band gap?about 2.2e V?, so that it has a wider absorption range for sun light. It can effectively use the solar energy for water splittinginto oxygen. BiFeO₃ as photocatalyst also has some advantages, for example, low-cost, stability, non-toxicity. However, due to the low efficiency of the separation of photogenerated electron-hole pairs, the efficiency of BiFeO₃ photocatalyst using solar energy for water splitting is still low, therefore, it is very important great significance to further study the photocatalytic properties of BiFeO₃ to improve the efficiency. Here, in this paper, we synthesized and studied the photoelectrocatalytic properties of BiFeO₃, Ti-BiFeO₃ samples from the morphology, crystal structure, band structure and photogenerated carriers concentration. And the possible reasons for the improved photoelectrocatalytic performance after Ti⁴⁺ doping are analysed.The main research contents of this thesis are as follows:1 The BiFeO₃ photocatalyst was prepared by a simple, cost-effective drop-casting method, using ferric nitrate and bismuth nitrate as iron source and bismuth source, respectively. The results of XRD, EDS, XPS and the element mapping show thatpure BFO films without impurities can be prepared by drop-casting method. The SEM results of the obtained BiFeO₃ show that it has a dense thin film structure formed by the nanoparticles closely. The results of BiFeO₃ about optical performance test show that compared with the traditional, Ti O2 semiconductor, BiFeO₃ has better absorption of sun light. The value of photocurrent is about 0.014 m A/cm2 in the 1M Na OH solution and the set voltage is 0.4V?Vs.Ag/Ag Cl?. It was found that the BiFeO₃ catalyst with dense thin film structure has good stability. And the photocurrent is about 80% after 3600 s under illuminate.2 In order to further improve the photocatalytic performance of BiFeO₃ semiconductor, in this paper, we have aresearch about using the metal ion doping to modifyit. BiFeO₃ photocatalyst doped with metal ion was prepared by the drop-casting method. The experiment results show that BiFeO₃ doped with Ti⁴⁺ ions can obviously improve the photoelectrocatalytic properties. XRD results showed that BiFeO₃ doped with Ti⁴⁺ ion induced distortion of crystal structure, but there are not impurities. SEM results showed that the morphology did not change significantly when BiFeO₃ doped with Ti⁴⁺ ion, and both show tense films structures. Compared with pure BiFeO₃ photocatalyst, the photocurrent of 5% Ti⁴⁺ doped BiFeO₃ tested in 1M Na OH solution, the set voltage is 0.4V, increased by 85%.3 In this work, we analysed the reasons and mechanism of improved BiFeO₃ photoelectrocatalytic property after Ti⁴⁺ doping form the chemical composition, crystal structure, morphology, optical properties and physical properties. The results of UV-Vis absorption spectroscopy show that Ti⁴⁺ doping increases the absorption range of BiFeO₃ thin film samples of sun light. The band gap of BiFeO₃ and 5% Ti⁴⁺ doped BiFeO₃ samples were calculated for 2.13 e V and 1.97 e V respectively. The narrower band gap after BiFeO₃ doped with Ti⁴⁺ increased the absorption about sun light. The PL test results show that the Ti⁴⁺ doping effectively prevents the recombination of photogenerated electron-hole pairs. According to the results of the tested Mott-Schottky curves, we can get the value of valence and conduction band of BiFeO₃ respectively. The value of valence has a negative potential which is beneficial towater oxidation. The calculated photogenic charge carrier concentrations of BFO, Ti?0.05?-BFO and Ti?0.1?-BFO were 5.07×1019cm-3, 6.32×1019cm-3and 5.59×1019cm-3, respectively. The reduced band gap and increased photogenic charge carrier concentration are major factors contribute to the enhancement of photoelectrocatalytic performances of Ti-BFO films.