| Titania(TiO2)photocatalysis is one of the potential alternatives to tackle with the environmental and energy problems encountered nowadays.The low quantum efficiency is a main reason for its limited practical applications.It is well known that by constructing branched one-dimensional nanostructures(nanotrees)and by applying a bias voltage,the fast separations of photogenerated carriers and hence the high photocatalytic activity can be achieved for TiO2.In this thesis,we constructed series of TiO2 nanostrees,and studied in detail their photocatalytic(PC)and photoelectrocatalytic(PEC)performances in relating with the various factors of the morphology and length of the trunk,and the branch length.The effects of crystalline phase on the PC and PEC performances were also investigated by changing the phase of trunk and branch in TiO2 nanotrees.Follows are the main results obtained:1.The effects of nanostructures on the PC and PEC performances of TiO2 nano trees:Alkali-thermal treatments were applied on metallic Ti substrates to achieve anatase TiO2 nanobelts and nanowires,respectively,using the various NaOH concentrations of 5 M and 1.25 M.Anatase TiO2 nanowires with various lengths of 1,3 and 8 ?m were also synthesized using various alkali-thermal durations.A solution combustion technology in combination with chemical precipitation was applied to prepare the precursor solutions for the precipitations of the anatase branches.The nanobelts possess a thin dimension which favors the rapid migration of photogenerated charges to the very surface to be involved in the photocatalytic reaction;therefore,they exhibited a PC activity superior to that of nanowires.On the contrary,the nanowires trunk showed better PEC performances towards degradations of phenol in water because of the merits of one-dimension nanostructures,which enhance the light harvesting and provide a direct path for carriers transfer,and also a thinner middle layer between TiO2 and the Ti substrate that reduces the resistance.TiO2 nanotrees constructed with the 3?m long anatase nanowires exhibited the best PEC activity,which can be attributed to its one dimensional structure with suitable length,the thinner middle layer and the larger specific surface area,etc.The branch length affected differently the performances of TiO2 nanotrees with different trunks.It is found that an appropriate branch length contributes positively to the PC and PEC performances.2.The effects of crystalline phase on the PC and PEC performances of TiO2 nanotrees:Precipitations of rutile TiO2 nanorods on Ti foils were achieved by a hydrothermal route based on the HCl solution.A subsequent calcination improved the crystallinity and the adhesion of the nanorods with the Ti substrates,which contributed to a largely improved PEC efficiency that is 269 times that of the PC efficiency,inducing degradations of phenol in water within 2 h.Anatase branches were further precipitated on the rutile nanorod arrays to construct a TiO2 nanotree with rutile/anatase phase junctions.The PC activity of the TiO2 nanotree increased a littile,thanking to the increased light harvesting and the phase junction that favors the charge separations.However,the PEC efficiency declined upon the introduction of the branch,which might be ascribed to the increased deficiency resulted from the chemical procedure to precipitate the branch.Rutile TiO2 branches were precipitated on the 3 ?m long anatase nanowires derived from the alkali-treatment to construct a TiO2 nanotree with anatase/rutile phase junctions.It is also found that the phase junctions contribute to the charge separation and hence the PC activity.However,the PEC efficiency towards degradations of phenol in water decreased,because the charge separation direction in the phase junction is on opposite to that induced by the applied bias potential. |
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