| Since the photocatalytic effect of TiO2semiconductor materials was discovered in1972, the technologies of photocatalysis have been sufficiently developed. The photocatalyst materials are applied in pollution treatment, solar energy conversion, self-cleaning coating and other fields. It may help mankind to solve two major problems:environmental pollution and energy crisis. Among the photocatalyst materials, TiO2was widely investigated due to its high stability, low cost and relatively high reactivity. However, its intrinsic high photoinduced recombination rate and large bandgap are limiting the efficiency of pristine TiO2materials. Therefore, TiO2should be modified to increase its photocatalytic efficiency and enhance the utilization of solar energy. The modification strategies have become one of the hotest topics in the field of materials, physics, chemistry and environmental science.In the dissertation, the TiO2photocatalysts and the modification technologies were briefly introduced. The mechanism and principles of TiO2-based photocatalysis were reviewed. The existing modification technologies were summarized.And the modification by designing TiO2based heterostructure composites was highlighted.Then, to inhibit the recombination of photoinduced charger carriers and enhance the photo-response, TiO2photocatalysts were modified by the heterostructuring technologies. By biphases or triphases heterostructuring, four types of TiO2-based heterogeneous composite photocatalysts were designed and prepared via hydrothermal or sol-gel methods. The as-prepared heterostructure composites possess high UV, visible and near-infrared light response, respectively. Hence, the light absorption was greatly enhanced and the utilization efficiency toward solar spectrum was increased. By the design of heterostructure, the bandgap alignment was achieved, the p-n internal electric field was formed, the transport of charger carriers was enhanced, the adsorption toward organic degradation objectives was improved and consequently the photocatalytic activity of the heterostructure photocatalysts were enhanced. The details are listed below:1. A novel TiO2isomer heterostructure was designed. A controllable shell composed of anatase nanocrystals were grown on the surface of TiO2(B) nanobelts, to achieve this TiO2(B)@TiO2(anatase) heterogeneous photocatalyst. Compared to the methods reported elsewhere, the synthetic process is facile and controllable. The shell thickness was tunable in the range from0to32nm, meanwhile the one-dimensional skeletal structure could be remained. Then the heterostructure interface was observed and the bandgap alignment was estimated. The results indicated that the bandgap of TiO2(B) is located between the conduction and valence band of anatase, which benefits the transfer and separation of photoinduced electron-hole pairs. The lattice mismatch of designated crystal facets of TiO2(B) and anatase is small enough so that the continuous interface could be formed. The photocatalytic experiment results indicated that, the heterostructure sample showed better activity than each of its components. The photocatalytic degradation rate constant of the heterostructure composite toward5mg/L methyl orange solution was4.9times and3.2times higher than that of TiO2(B) and anatase, respectively. The result of the photoluminescence measurement showed that the heterostructure interface greatly inhibited the recombination of the photoinduced electron-hole pairs, which indicated the high activity of the heterostructure samples. Besides, a novel TiO2(B)@TiO2(anatase)/Au triphase heterostructure was built by depositing gold nanoparticles on the surface of TiO2(B)@TiO2(anatase) composite. By this modification, the photocatalytic degradation efficiency was further increased by26%.2. A TiO2/YFeO3p-n type heterostructure was designed. First, YFeO3narrow band-gap photocatalysts were prepared by sol-gel method. The crystal phases, morphologies and visible-light photocatalytic activities of a series of these YFeO3samples were tuned. Compared to the orthorhombic samples calcined at high temperature, the hexagonal YFeO3, prepared at low temperature, possessed narrow bandgap, high surface area, small grain size and unique porous structure, which would result in its high photocatalytic activity. Then, the active p-type hexagonal YFeO3, as skeletal substrate, was heterogeneously combined with n-type TiO2semiconductor to obtain this novel TiO2/YFeO3p-n type heterostructure. A series of heterostructure composites were prepared by tuning the reactant ratio. By adding more YFeO3, the initial rutile TiO2was gradually transformed to anatase phase, which resulted in potential higher activity and the average grain size was meanwhile decreased. Rutile, anatase and hexagonal YFeO3phases were all observed in the composite with certain ratios. The photocatalytic activity of a series of the heterostructure composites was measured and evaluated. The results showed that YT-5sample (rutile/anatase/YFeO3=85:10:5) has highest activity. The photocatalytic degradation efficiency toward10mg/L methyl orange was18times as that of pure rutile sample and1.8times as that of physical mixture of TiO2and YFeO3in the same ratio.3. A novel TiO2@C/Graphene heterostructure was designed. Amorphous carbon layer was applied as the interface between anatase TiO2nanocrystals and two-dimensional graphene planar sheet. The contact of TiO2and graphene became compacter and the photocatalytic activity of the heterostructure was enhanced. TiO2nanocrystals were prepared by hydrothermal method. The thickness and disorder arrangement of the amorphous carbon on the nanocrystals surface could be tuned by heat-treatment. Then the as-prepared TiO2@C and graphene were combined to achieve the TiO2@C/Graphene heterostructure. The results showed that, the carbon layer with designed thickness and disorder arrangement could be used as excellent heterogeneous interface and cause the separation of photoinduced electron-hole pairs. The heterostructure sample, which had been heated at360℃, was the optimal one, because of its relatively thin and ordered carbon layer interface. After4h visible-light irradiation, the photocatalytic degradation efficiency of this sample was5.2times as that of the worst one (heated at450℃) and1.9times as that of P25/graphene composite. A charger carrier transfer model for this optimal heterostructure was demonstrated. By the carbon layer connection, TiO2nanocrystals and graphene sheets could be sufficiently combined. Consequently, the dispersion of nanocrystals on graphene sheets was enhanced and efficiency of electron transfer through the interface was increased. More photoinduced electrons on TiO2lattice could be transferred to graphene plane and recombination was inhibited.4. NaYF4:Yb,Tm@TiO2core/shell heterostructure near-infrared photocatalysts were designed. First, the lanthanides-doped NaYF4nanomaterials were synthesized by hydrothermal method. The influence of the crystal phases, morphologies and doping ratio to their upconversion photoluminescence properties were investigated. The doping ratio and synthetic parameters for high UV emission efficiency were optimized. In the next step, NaYF4@Ti02core-shell heterostructure composites with highly crystalline and tunable anatase TiO2shells were prepared under mild conditions. The heterostructure possessed highly compact interface between the fluorides and the anatase shells. A series of heterostructure composites with different shell thickness were prepared by tuning the concentration of Ti precursor solution. The results of upconversion photoluminescence measurements showed that, the heterogeneous shells greatly absorbed UV emission of the fluorides cores. The energy transfer mode on the interface was determined as "radiation-reabsorption" by photoluminescence time-decay measurements. The generation of free radicals was proved by the detection of hydroxyl radical in the solution. The as-prepared core/shell heterostructure composite showed high photocatalytic activity under near-infrared light irradiation. After12h irradiation,90%methylene blue was degraded in the presence of the composite. The comparison experiments proved the synergistic enhancement caused by both upconversion effect in core and photocatalysis effect in shell. Moreover, the influence of shell thickness toward photocatalytic activity was investigated. A moderate shell may benefit the dual transfer of light energy. Consequently, the catalytic efficiency of the composite with moderate shell (prepared with0.04mol/L TiF4) was1.8times of the one of composite with relative thicker shell (prepared with0.08mol/L TiF4). |
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