Synthesis And Photocatalytic Activity Of TiO₂Hybridized With Conjugated Molecules

Semiconductor photocatalysis is one of the most effective technologies fordealing with global environmental pollution and the energy crisis. As a typicalsemiconductor photocatalyst, anatase TiO₂has been the most promising candidatesbecause of its strong oxidizing powers, nontoxicities, and low cost. However, theband gap energy of TiO₂is about3.2eV, leading to a threshold wavelength of only387nm and a poor utilization for solar energy （3%-5%）. On the other hand, theserious recombination of photogenerated electron-hole pairs always occurs whenTiO₂is used in photocatalysis. Therefore, the practical applications of this excitingphotocatalyst are greatly restricted due to its inherent defects. To overcome thesetwo defects, herein, we synthesized a series of TiO₂hybridized with differentconjugated materials surrounding the heterostructuring of TiO₂. The photocatalyticactivity and other physicochemical properties of obtained samples werecharacterized in detail. More importantly, in-depth study of the mechanisms duringphotocatalytic process was carried out under UV and visible light irradiation,respectively. The main contents of this thesis are as follows:Nano-sized anatase TiO₂with reactive {001} facets was prepared using lamellarprotonated titanate （LPT） as a new precursor through a facile solvothermal method.XRD and TEM were carried out to study the crystal structure and micrograph ofTiO₂samples obtained from different precursors. On the other hand, the effect of HFon the crystal growth of TiO₂was also studied. It was found that, TiO₂nanoparticlessynthesized with LPT as precursor was smaller than that obtained with conventionalprecursor of tetrabutyl titanate （TBT）, and exhibited markedly superiorphotoactivity under UV light illumination. Moreover, the dosage of HF had asignificant impact on the size, surface area and the proportion of exposed {001}facets of the TiO₂sample.A novel incorporating style of polyaniline/TiO₂（PANI/TiO₂） composites （Ns）was developed via a designed two-step route. The discrepancies of crystal structureand micrograph of novel and conventional PANI/TiO₂composites were studiedthrough XRD and TEM. The light-absorption property of Ns and the interactionbetween PANI and TiO₂were also analyzed by UV-vis DRS, IR and XPS. At last,the photostability and circle photodegradation of Ns were tested. After theintroduction of one-dimensional PANI nanofibers in the step of TiO₂crystal growth,PANI could act as an effective template to stabilize TiO₂particles at nanoscalethrough interfacial chemical bonds. In this way, the transition of some newly formed anatase crystals to rutile phase was partly suppressed. Also, the as-preparedcomposites showed obvious light response in both UV and visible light range due tothe sensitizing effect of PANI. Compared to TiO₂and conventional PANI/TiO₂composite, Ns exhibited improved visible photocatalytic property with scarcedecrease of UV photoactivity over TiO₂.Anatase TiO₂nanosheets supported on reduced graphene oxide （RGO） weresynthesized through a one-step, solvothermal method. The crystal structure andmicrograph of RGO/TiO₂composite were studied through XRD, TEM and AFM.The interaction between RGO and TiO₂was studied by XPS, Raman and IR. Finally,the photocatalytic mechanism under UV light was also deeply studied by PL,trapping test and EPR. The results showed that, during the solvothermal step,graphene oxide （GO） was reduced to RGO with the in situ growth of anatase TiO₂with exposed {001} facets on the surface of the RGO nanosheets. Compared withpure TiO₂, RGO/TiO₂hybrids displayed improved photoactivity as a result ofeffective photoinduced electron transfer from TiO₂to RGO through interfacialinteractions. The photogenerated holes were confirmed as the main oxidizingspecies in the RGO/TiO₂photocatalytic system.Graphitic carbon nitride （g-C₃N₄） and anatase TiO₂nanosheets with exposed {001}facets were used as starting materials for the fabrication of g-C₃N₄/TiO₂hybridsthrough solvent evaporation method. Crystal structure and micrograph of the hybridwere analyzed through XRD, SEM and TEM. The light-absorption property ofhybrid and interaction between g-C₃N₄and TiO₂were studied by UV-vis DRS, IRand PL. Finally, the photocatalytic mechanisms under UV and visible light werestudied by trapping test and EPR measurements. The composition of g-C₃N₄did notimpact the crystal structure of TiO₂, and the coating g-C₃N₄layers interacted withthe TiO₂nanosheets. It was proposed that an effective charge separation existedbetween g-C₃N₄and TiO₂in the photocatalytic process, i.e. the transferring ofphotogenerated holes from valence band （VB） of TiO₂to the highest occupiedmolecular orbital （HOMO） of g-C₃N₄under UV light, and the injecting of electronsfrom the lowest unoccupied molecular orbital （LUMO） of g-C₃N₄to conductionband （CB） of TiO₂under visible light. Due to this synergistic effect, theenhancement of photoactivity over the hybrid was achieved. Furthermore, it wasrevealed that holes were the main reason for the improved photoactivity ofg-C₃N₄/TiO₂under UV light while the OH radicals gained the predominance fordegradating organic dyes under visible light.