| Titanium dioxide (TiO2), as a wide bandgap semiconductor, has been widely used in thefields of clean energy and environmental protection. Because of its highly photoactivity,chemical stability, non-toxicity, low cost, etc., it is considered as the most importantphotocatalyst. However, the wide bandgap (3.2eV) of TiO2leads to its lower utilization ofsolar spectrum, which greatly limits the photocatalytic application of TiO2. Thephotocatalytic efficiency of catalyst has been affected by many factors, such as the opticalabsorption, energy levels, and the generation, combination, separation, migration and thecapture of photo-generated carriers. For this reason, researchers have conducted a lot ofworks to confirm that many modifications could effectively improve the solar-drivenphotocatalytic activity of TiO2, such as metal-doping, nonmetal-doping, binary or multiplecombined semiconductor, surface sensitization, etc. In this paper, the sol-gel technique hasbeen employed to prepare both TiO2nanopowders and TiO2nano-films. We have applied avariety of modifications to focus on how to improve the photocatalytic activity of TiO2nanocrystals, including transition metal doped biphasic TiO2; mono transition metal dopedTiO2; binary core-shell structure and the internal disorder-engineered TiO2; metal andnon-metal codoped TiO2. Meanwhile, we have systematically researched these modifiedTiO2nanocrystals on their structures, morphologies, chemical compositions, optical proper-ties, and surface wettabilities. The main studies of this paper are as follows:(1) We have prepared the biphasic TiO2powders with different proportions(Rutile/Anatase, R/A) by controlling the sintering temperature. The proportion of rutilephase increased with the sintering temperature increasing. The biphasic TiO2with28.76wt%rutile showed the highest efficiency of photo-degradation among the biphasic TiO2samples, by the comparison of photo-degrading20mg/L methyl orange. In this case, the28.76wt%rutile content was taken as the reference proportion of Fe-doped biphasic TiO2in the following sythesization. The biphasic TiO2powders with different Fe-dopingconcentrations were synthesized by adjusting the heat-treatment temperature, to consider thedecrease of phase transition temperature caused by Fe-doping. From the experimentalcharacteractions, doping of Fe in biphasic TiO2powders induced a significant red-shift ofoptical absorption band-edge, which could extend the optical response to visible region. Andthe photocatalytic activities of Fe-doped biphasic TiO2powders were improved by furtherprolonging the lifetime of photo-generated carriers, under the synergy of the biphasicstructure and Fe-doping.(2) Mo-doped TiO2nanoparticles were prepared and studied on their correspondingproperties. To combine with first-principle calculations, we analyzed the modified reasonson both optical property and photocatalytic activity of Mo-doped TiO2. From theexperimental and calculated results, Mo atom has been doped into anatase TiO2lattice as asubstitution to influence the binding states of its round atoms. The Mo-doping did not induceany changes of the phase structure but induced a decrease in the grain size, and caused thechanges of both lattice parameter and lattice strain with a certain regularity. Meanwhile, Modoping in TiO2caused on a shift of optical absorption threshold to the longer wavelength.Thus the optical bandgap energies were decreased with the increase of Mo-dopingconcentration. Because of the4d orbits induced by Mo-doping close to the CBM of TiO2,they caused little perturbation at the CBM and affected little on the CBM level splitting,which leaded to a small narrowing of optical bandgap. The results were corresponding to thechanging trend of bandgap energies calculated by first-principle. And the first-principlecalculations also illustrated that the Mo-doping induced the Fermi level to move upward toincrease the concentration, and the distribution of the symmetry of HOMO and LUMOcontributed to the seperation of photo-generated electron/hole pair. These helped to improvethe photocatalytic activity of Mo-doped TiO2.(3) In the present work, we proposed a simple and green synthetic pathway,abandoning additional carbon sources but utilizing the organic group in the Ti precursor, toprepare nanostructured TiO2-carbon (C/TiO2) core-shell nanoparticles. Furthermore, we decarburized C/TiO2by annealing it in air to obtain defect-engineered TiO2-x(reduced TiO2).These two modified TiO2samples contained a large number of oxygen vacancies, whichleaded to structural disorders in them. These structural disorders induced the valence bandtails which made the band edge extending within the bandgap, therefore, the bandgap wasnarrowed and the optical responce was increased. Furthermore, the band edges were tailoredto match with the hydrogen and oxygen production energy levels in the water-splitting. As aresult, the two structure-disordered C/TiO2and reduced TiO2nanocrystals showed excellentsolar-driven photocatalytic activities: the C/TiO2performed the best on the photodegradationof water pollutants attributed to the rapid charge transfer of the amorphous carbon shell;while the reduced TiO2displayed an excellence in the hydrogen generation rate byphoto-splitting water, because of its proper band edge positions. The findings of our studymay provide a facile way to develop a nanostructurally disordered TiO2photocatalyst toenhance photocatalytic activity, to be applied both in water pollution treatment and in thegeneration of a clean energy source.(4) To prepare Cu/N-codoped TiO2nanoparticles, we employed the sol-gel techniqueby two rounds of sintering. We characterized the as-prepared samples on their structure,morphology and surface chemical composition to prove that Cu atoms and N atomssuccessfully replaced the sites of Ti atoms and O atoms in the TiO2lattice, respectively.With the increase of codoping content, the crystal growth of anatase TiO2was inhibited,causing the decrease of grain size and the increase of lattice distortion. However, thedeviation of periodic lattice arrangement in the structure of TiO2by a certain Cu-dopinghelped N atoms to dope into TiO2lattice, that is, Cu-doping in an adequate amount promotedN atoms occupying O atoms as the substitutions, while the excessive Cu-doping played asinhibitors. The Cu/N-codoping reduced the energy of photon transition and narrowed theoptical bandgap of TiO2. The optical absorption results revealed that the Cu/N codopingcould greatly enhance the absorption in whole visible region and disarrange the lattice cyclepotential field, thereby to greatly increase the optical absorption from the impurities andlattice defects. The PL spectra of Cu/N-codoped TiO2samples mainly concentrated in the blue-green region. In addition, the emission region showed a certain red-shift, which wasattributed to the effect of surface states and impurity levels. The PL intensity ofCu/N-codoped TiO2was significantly reduced, meanwhile, the photocatalytic activities ofsome typical samples had negative correlation with their PL intensities. In this case, wecould make a preliminary rapid assessment of the photocatalytic activities based on PLspectra in our experiments. The Cu/N-codoping significantly enhanced the solar-drivenphotocatalytic activity of TiO2on degrading methylene blue solution, with an enhancingdegree much higher than that of mono-element doping.(5) Nanosized Cu-doped TiO2film with good surface wettability, was prepared by solspin coating technique. As a result, the Cu-doped TiO2thin film possessed asuperhydrophilic surface with a water contact angle (WCA) only5.7°, comparing with41.1°of the undoped TiO2film. In addition, the Cu-doped TiO2thin film exhibited excellentanti-fogging behavior. The Cu-doping caused the transmittance of TiO2to decrease to60%,considering90%of undoped TiO2. Meanwhile, a red-shift existed in the transmittancespectrum of Cu-doped TiO2compared to that of undoped TiO2, thus leaded to a decrease intransmittance region of Cu-doped TiO2film. The Cu-doped TiO2thin film also exhibited amuch better photocatalytic activity than undoped TiO2thin film, as evaluated by thedegradation of10mg/L methylene blue (MB) solution under the simulated solar-drivenirradiation. |
PDF Full Text Request