The Synthesis Of Polar Structure TiO₂ And Its Enhanced Physical Mechanism Of Photocatalysis, Gas Sensitivity, Photovoltaic And Piezoelectric Properties

The TiO2 nanosheet based flowerlike micro-nanostructures with a large surface area,SnxTi1-xO2-TixSn1-xO2 core-shell structure by composite of Sn4+ and Ti4+ metal ions and nanotubes/nanowires assembled from TiO2 nanoflakes exposed with {111} facets were prepared through a solvothermal and solution fast combustion method.They were annealed in H2 atmosphere to obtain hydrogenated TiO2 flowerlike micro-nanostructures,hydrogenated SnxTi1-xO2-TixSn1-xO2 core-shell structure and hydrogenated TiO2 nanotubes/nanowires.[001]orientated Ti02 nanorod array were grown by two-step method.The morphology,surface structures,crystal structure,pore structure and specific surface area analyses of the as-fabricated TiO2 products were performed on field scanning electron microscope(FESEM),X-ray photoelectron spectra(XPS),X-ray diffraction(XRD),field transmission electron microscope(TEM),atomic force microscope(AFM),infrared spectroscopy(IR)and absorption-desorption porosimetry.The properties of gas sensing and photocatalytic reduction CO2 by hydrogenated TiO2 samples.After hydrogenation treatment,it was found that all exhibited enhanced redox reaction of gas sensing and catalysis.The nanotubes/nanowires assembled from TiO2 nanoflakes exposed with {111} facets has achieved the higher yield about synthesizing CH4(1708.1 μmol g-1 h-1)in the photocatalytic reduction CO2.Meanwhile,the effect of[001]orientated TiO2 nanorod array on piezoelectric and photovoltaic properties was studied.The impact of the surface structure on the sensing,photocatalytic,piezoelectric and photovoltaic effect of TiO2 micro-nanostructures was discussed,and a new mechanism was proposed.These results lay a foundation for the preparation of nanomaterials,optimizing of their chemical and physical properties.And their applications are in opto-electronics,lithium-ion batteries,gas sensing,solar cells,photocatalysis and other fields.(1)Heating Ti(C4H9O)4-C2H6O-C3H8O3 aqueous mixtures at 110 ℃ for 48 h through a solvothermal method,TiO2 nanosheet based flowerlike architectures with a surface area as high as 171 m2.g-1 was prepared.And the thickness of TiO2 nanosheet is about 10.5 nm.Then the Ti02 nanosheet based flowerlike architectures were hydrogenated at different times for 600℃ to obtain hydrogenated Ti02 flowerlike architecture.It was found that the hydrogenated TiO2 flowerlike architectures shown the excellent gas sensing properties than the samples without hydrogenation,and the response further increases with the extension of hydrogenation time.Therefore,We proposed a concept of the unsaturated Ti atom on the Ti-TiO2 surface serving as the sensing reaction active sites,and described the sensing mechanism at the atomic and molecular level in detail.(2)The nanotubes/nanowires assembled from TiO2 nanoflakes exposed with {111}facets were prepared via heating a mixture of TiOSO4·xH2SO4-CH3COOH-H2O.The TiO2 nanotubes/nanowires were hydrogenated at different times for 650℃ to obtain hydrogenated TiO2 nanotubes/nanowires.It was concluded that photocatalytic CO2 reduction activity and gas sensing properties of the as-prepared TiO2 nanoflake based nanotubes/nanowires was remarkably increased through hydrogenation.The CH4 evolution rate of the hydrogenated TiO2 nanotubes/nanowires is as high as 1708.1 μmol g-1 h-1,which is the highest of all Ti02 photocatalysts and noticeably higher than the majority of TiO2 based photocatalysts,and the sample hydrogenated at 650℃ for 12 h has the best gas sensing performance.Atomic structure of TiO2 {111} high active facets were calculated by using periodic density functional theory(DFT).On the basis of polar structure of the exposed TiO2 {111} crystal facets,a charge separation model between TiO2 polar {111} facets and spontaneous electric field were proposed.They can supply a clear insight to understanding of photocatalysis of the high performance of photocatalytic C02 reduction activity,and proposed a concept of the unsaturated Ti3c,Ti4c,Tisc atoms on the Ti-TiO2(111)surface serving as the sensing reaction active sites.(3)The SnxTi1-xO2-TixSn1-xO2 core-shell structure were successfully synthesized by a solvothermal method heating Ti(C4H9O)4-CH3CH2OH-SnCl4·5H2O aqueous mixtures.The influence of different Sn/Ti ratios on the gas sensing performances of SnxTi1-xO2-TixSn1-xO2 core-shell structures were studied,and it could be seen that the Sn/Ti ratios with 1.8/3.0 show higher sensing performance than others.For the SnxTi1-xO2-TixSn1-xO2 core-shell structures of Sn/Ti ratios with 1.8/3.0 annealed in H2 atmosphere to obtain hydrogenated TiO2 core-shell structures,It was seen that the hydrogenated TiO2 core-shell structures shown the further increased sensing performance than the samples without hydrogenation.The response further increases with the increase of hydrogenation temperature.The relationship between the surface atomic structure changes of SnxTi1-xO2-TixSn1-xO2 core-shell sample and the gas sensing properties were studied.We propose that the unsaturated Ti atoms and Sn atoms collectively act as the sensing reaction active sites.(4)A layer of TiO2 seeds was grown on FTO,then different[001]orientations of TiO2 rod arrays were obtained by changing the temperature of hydrothermal reaction by two-step method.The influence of different[001]orientated TiO2 rod arrays on photovoltaic and piezoelectric effect was researched.It was conclued that with the increase of the TC(001)of TiO2 rod arrays,its photovoltaic,piezoelectric and rectifying properties are gradually enhanced.The relationship between the Ti-TiO2(001)surface atomic structure and piezoelectric,photovoltaic effect was researched.We propose that the concept of spontaneous electric field existing between TiO2 {001}polar planes and a physical mechanism of photon-to-electron conversion.The hydrogenated TiO2 flowers structure,nanosheet assembled wire/tubular structure and SnxT1-xO2-TixSn1-xO2 core-shell structure show the increased sensing performance.Therefore,it is proposed that the unsaturated metal atom with a dangling bond on the surface actes as the gas sensing reactive site,which better explains the gas sensing mechanism at the molecular atomic level.At the same time,spontaneous polarization electric field theory is proposed for the enhanced the CO2 reduction performance about the line/tubular structure assembled by the TiO2 nanosheets exposed by {111} crystal planes and the enhanced photovoltaic,piezoelectric and rectifying characteristics of different[001]oriented TiO2 rod arrays.