Preparation,microstructure And Photoelectric Properties Of Bismuth Based Oxides

The ferroelectric oxide has been widely concerned due to its lead-free composition, good chemical stability and high Curie temperature, making it a good candidate in the applications of memory storage capacitors and electro-optic devices. Recently, photovoltaic effect of ferroelectric oxide has been widely concerned due to its ferroelectric polarization adjustable and its higher open circuit voltage larger than the optical band gap. In this thesis, bismuth based oxides, such as BiFeO3(BFO), Bi4Ti3O12(BTO), Bi4Ti3O12/BiFeO3(BTO/BFO) and BaBiO3(BBO) were selected to be investigated.Chapter 1 reviews the fundamental properties and the research progress of bismuth based oxide, which includes crystal structures, the development history of ferroelectric materials and preparing techniques. Finally, advance research about structure and optoelectronic properties of BFO, BTO and BBO were concentrated.Chapter 2 introduces that BFO ceramics were successfully sintered at about 850 oC in for 15 hours by a conventional solid state reaction method and then the BFO film was deposited on FTO glass substrate by PLD technique. XRD measurement shows the film has pure phase perovskite structure. The film has a flat and dense surface, with a uniform grain size. The film with a thickness of around 550 nm has an obvious hysteresis loop. Its band gap(Eg) is fitted to be about 2.76 eV using Tauc equation. From the J-V curves, photoconductivity of the BFO film was observed. Optical injection resulted in increasing conductivity. It is proposed that the contribution of polarization to the photoconductivity is negligible.In Chapter 3, BFO precursor was analyzed by using DSC-TGA method. Therefore, baking at 150 oC, pretreating at 300 oC and crystallizing at 650 oC were adopted to obtain pure phase BFO film. BFO thin film was successfully grown on the FTO glass substrate by a sol-gel method. The film has good surface quality and its thickness is around 484 nm. Surface morphology and local ferroelectric property of the film were mesured by AFM method combined with PFM technique. Its RMS is about 22.5 nm in an area of 10 ×10μm2. Macroscopic ferroelectric characteristic of the film was also studied. It was observed that the remnant polarization and coercive field of the films are gradually increased with the increase of the applied field. Its Eg is fitted to be about 2.76 eV using Tauc equation. From the J-V curves under the light illumination, it is found that the open-circuit voltage and short-circuit current are approximately 0.25 V and-36.1 μAcm-2, respectively. The calculated maximum power efficiency is about 0.00223%. It can be observed that repeatable and stable instantaneous responses of photocurrent to the light illumination by ON-OFF. In order to understand photovoltaic(PV) effect, possible schematic energy band diagrams are constructed. We give the description of the PV effect mechanism of BFO thin films. Self-polarization and Schottky barrier at electrode and flim interface co-determine the PV effect.In Chapter 4, BTO precursor was analyzed by using DSC-TGA method, Therefore, baking at 150 oC, pretreating at 400 oC and crystallizing at 650 oC were adopted to obtain the best preparation technique. Pure BTO and BTO/BFO films were successfully prepared on the FTO glass by a sol-gel method. Both BTO and BTO/BFO films have relatively good surface quality and BFO layer has no obvious influence on BTO layer. Local ferroelectric characteristics of the films were tested by PFM technique. RMS of BTO(BTO/BFO) film is about 11.5(5.36) nm in an area of 10×10(5 ×5) μm2. The macroscopic ferroelectric property of the film was also studied. The optical transmittance spectrum reveal direct band gap(Eg) of the bilayer film of 2.72 and 3.41 eV, which are almost equal with Eg of single BFO and BTO, respectively. Thus, it is reasonably inferred that the UV-vis light can pass BTO through BFO, which may favor the enhancement of PV effect. Under the light illumination, compared to Pt/BTO/FTO structure, whose open voltage(Voc) and short circuit current density(Jsc) are about 0.278 V,-20.44 μAcm-2, respectively, Pt/BTO/BFO/FTO structure shows enhanced Voc and Jsc, being about 0.38 V,-56.24 μAcm-2, respectively. It is found that the Power conversion efficiency of the Pt/BTO/BFO/FTO structure has an improvement of 3.83 fold(0.00548%) in comparison with that of the Pt/BTO/FTO structure(0.00143%). In order to deeply understand PV effect, possible schematic energy band diagrams are constructed. Based on the PFM experiment result, we give the description of the PV effect mechanism of BTO/BFO thin film. Self-polarization and the electric field in p-n junction formation together determine the PV effect. In the visible region, the transmittance of the ITO layer is about 85%. In order to examine the transparent conductive oxide thin film electrode effect on the PV effect, ITO/BTO/FTO sandwich structure was prepared in the same BTO film. Compared to Pt/BTO/FTO structure, the magnitude of PV output of the structure with the ITO top electrode is about 7.25 times larger than that with the Pt electrode. Pt/BTO/BFO/FTO structure show repeatable and stable instantaneous responses of photocurrent and photocurrent to the light illumination with light ON-OFF(J-t) and(V-t) curves, respectively.Chapter 5 reports that BBO powders were successfully sintered at different temperatures in 36 hours by a conventional solid state reaction method. XRD characterization confirms the pure phase of the BBO powder. BBO powder shows an agglomeration behavior, with a diameter in the range of 1-2 μm. The absorption spectrum shows the absorption edge of 620 nm. Its Eg is fitted to be about 1.8eV using Tauc equation. Without electric field, the BBO powders have two surface photovoltage response peak, located at 400 and 470 nm, respectively. The surface photovoltage decreases with the decrease of sintering temperature of the BBO powders. The photoluminescence properties of the BBO powders were investigated upon excitation at 350 nm. The emission intensity decreases with the increase of sintering temperature of the BBO powders. BBO powder showed photocatalytic activity after the photocatalytic performance test.