Study On The Synthesis,Microstructure And Visible-light Photocatalytic Activity Of Single-crystal TiO₂/PbTiO₃,Heterostructure

Titania has been under extensive research and wide application as a semiconductor photocatalyst with excellent photocatalytic performance in the ultraviolet light region. However, the utilization efficiency of solar-energy during photocatalytic activity is still under-cultivated for TiO2. Thus, methods including doping, hydrogenation, dye-sensitization and the formation of junctions with other semiconductors have been employed to expand applications of titania to the visible light range and improve the light harvesting efficiency. Perovskite oxides with a wide variety have shown great chemical stability, and many of them have ferroelectricity. The combination of TiO2 with perovskite oxides can extend applications of TiO2 to the visible light region and decrease the recombination of the photogenerated electron-hole pairs. Furthermore, a sharp and tightly combined heterostructured interfaces between the two phases of the composites should be significant in facilitating the separation of photogenerated charge carries and their transfer, thus improving the final photocatalytic performance. In this paper, single-crystal TiO2/PbTiO3 nanofiber composites and nanoplate composites with high-quality interfaces were prepared by a facile secondary hydrothermal synthetic strategy. Furthermore, the microstructure of the heterostructured interfaces and the photocatalytic degradation performance of the samples under visible light irradiation were systematically investigated. Some novel results have been achieved and listed as follows:(1) Single-crystal TiO2/PbTiO3 heterostructured nanofiber composites with excellent visible-light photocatalytic performance have been successfully synthesized with ammonia as mineralizer by using secondary hydrothermal reaction and subsequent heat-treatment process. The size of TiO2 nanocrystals in the nanofiber composites was about 30 nm in diameter and 50-80 nm in length. In the nanofiber composites TiO2/PbTiO3 TBOT:0.4 ml, anatase TiO2 grew on the surface of tetragonal perovskite PbTiO3 nanofiber and formed sharp TiO2/PbTiO3 interfaces. The interfaces were not epitaxially grown, and there was a disordered region of the width of 1～2 nm within the heterostructured interface. Its photocatalytic degradation rate was 1.4352 h-1. Such photocatalytic activity of the heterostructured nanofiber composites was, supposedly, attributed to the large-scale formation of the sharp interfaces and monocrystalline component phases.(2) Single-crystal TiO2/PbTiO3 heterostructured nanoplate composites with excellent visible-light photocatalytic performance have been successfully obtained with single-crystal and single-domain perovskite PTO nanoplates as substrates and ammonia as mineralizer. In the nanoplate composites TiO2/PbTiO3 TBOT:0.4 ml, the anatase TiO2 with (100) facets exposed have been grown epitaxially on the (100) facets of perovskite PTO nanoplates with the formation of a heterostructured interfaces of high-quality. With the increase of the amount of TBOT from 0.4ml to 1.0ml, TiO2 nanocrystals on the nanoplate composites changed from single face growing to double-faced growing. Correspondingly, the photocatalytic efficiency parameter KMB decreased from 1.4096 h-1 to 1.3690 h-1. The epitaxially grown interfaces of high quality, and the synergistic effect of ferroelectric polarization and the p-n junction which can greatly promote the separation of electron-hole pairs at the heterostructured interfaces, resulted in excellent visible-light catalytic activity of the nanoplate composites.(3) Other single-crystal TiO2/PbTiO3 heterostructured nanoplate composites have been achieved by using KOH instead of ammonia as mineralizer to adjust the pH values of the hydrothermal system. TiO2 nanocrystals in the nanoplate composites changed from double-faced growth to single face growth with the increase of the pH value from 7 to 10. Correspondingly, the photocatalytic degradation efficiency parameter KMB decreased from 3.4200 h-1 to 3.1320 h-1. When the pH value was further increased to 12, the single-faced growth phenomenon of TiO2 nanocrystals on PTO nanoplates remained while the morphology of the TiO2 has developed into nanofibers with a diameter of 15 nm and length of 100～200 nm. The corresponding visible-light catalytic activity has been determined to be 3.1224 h-1. The major cause of the greatly enhanced photocatalytic performance of the nanoplate composites after mineralizer alteration can be attributed to the significant decrease in transition length of the carriers when the crystal length of TiO2 was reduced to 20 nm with an increased total surface area.