| This article studies the fabrication and photoelectrochemical property of TiO2 thin films doped by Mo and Mo-C, respectively. The result indicates:1. C-TiO2 thin films were fabricated by two different approaches, named cosputtering and implantation. Targets applied in cosputtering are TiC and TiO2 ceramic plate. As to the second approach, a various content of carbon ions were implanted into TiO2 films deposited in advance by sputtering. XRD patterns indicate all films obtained demonstrate anatase phase only, where the crystallite diameter decreases gradually as dopant content grows. A red-shifted absorption edge was observed in UV-vis spectroscopy after carbon doping, yet doping realized by implantation gives rise to a more notable red shift. The subsequent XPS analysis reveals different roles played by carbon ions in TiO2 lattice. In specific, carbon doped by cosputtering would substitute both Ti and O, and then a considerable amount of Ti-C and Ti-O-C are formed simultaneously. On the other hand, implanted carbon ions prefer to substitute Ti and form Ti-O-C bond, while oxygen substitution is disfavored. As to both cases, a large number of interstitial carbon atoms were confirmed, probably due to the small size of carbon. Carbon-doped TiO2 films demonstrate significantly enhanced current density and catalytic activity under visible light. Compared to TiO2 films as deposited, cosputtering-driven carbon doping yielded a maximum current density that is 2.5 times higher, while implantation-driven carbon doping raised the current density by 2.6 times.2. Mo-TiO2 films and Mo-C codoped samples were deposited by cosputtering of a Mo-embedded TiO2 target, a TiC target and a TiO2 ceramic target, respectively. Only anatase phase was observed by XRD, while a remarkable red-shift of absorption edge was confirmed in UV-vis. spectroscopy. XPS spectra demonstrate that parts of Mo atoms substituting Ti atoms in the TiO2 lattic of Mo-Ti02 thin film. For such a codoping case, some Ti atoms were replaced by Mo, while lattice oxygen was mainly substituted by carbon. For Mo-TiO2 thin films, the best doping content is around 0.95 at% Mo, and for (Mo,C)-TiO2 thin films, the best doping content is around 0.32 at% Mo and 9.78 at% C, respectively. Under this concentration, photocurrent increased to its maximum value which 3 and 8 times those before doping, respectively.3. Two three-layer structures M1 and M2 were prepared by mutlitargets magnetron sputtering. M1 includes an undoped TiO2 surface layer, a middle layer slightly doped by Mo, and an ultrathin heavy doped bottom; while in M2 the middle layer was replaced by a Mo-C codoped one. The results of XRD demonstrate that only anatase phase was observed and the grain size in every layer decreased with the doping contents decreased. The photocurrent tests under visible light indicate that photocurrent change effect of M1 enhanced obviously and its photocurrent value is 5 times than those before doping. But the photocurrent of M2 is not as good as M1. Although the middle layer (Mo-TiO2 thin film) in M1 was replaced by Mo-C codoped TiO2 thin film in M2 could increase the generation ratio, but the decrease of Fermi level drops at interfaces has weaken internal electric fields and slow down carrier separation. Our observation confirms carrier density in a multilayer structure is dominated by interfacial depletion layer, rather than the carrier concentration of a certain sublayer.4. A Mo-doped base deposited by cosputtering was etched into a trench array by reactive ions etching (RIE). The double layers structure with channel was formed with the TiO2 thin film on the top by magnetron sputtering method. This array has a distance of 50 μm between two adjacent trenches. By doing this, planar depletion layers in section Three are curved into the new base here to increase carrier collection along the parallel direction. Although width of depletion layer, which is dominated by Fermi level drops, didn’t change much for the new structure, carrier collection probability is remarkably enhanced, which we believe is due to a smaller effective mass as well as a much larger diffusivity along the parallel direction. Based on the creative application of anisotropic carrier transport properties resulted from quantum confinement, this novel structure is able to put minor carriers accumulated at film bottom into the undoped surface layer, and transport them to film surface at a relatively smaller recombination ratio. Specifically, when a large etching depth like 660 nm is given, current density increased by 19 times under visible light, which is 56 times higher than TiO2 films as deposited. Good photocatalytic activity of samples with a trench array was observed in the process of degradation of methylene under visible light. The reaction rate constant is 3.8 times undoped TiO2 film. |
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