Modification and Photoelectochemical Water Splitting Properties of Nanostructured Titanium Oxide Photoanodes

Author:Liu Qiang

Supervisor:ding dong yan


Degree Year:2016





Titanium dioxide(TiO2)is an important semiconductor metal oxide,which has wide applications in solar cells,photoelectrochemical water splitting(PEC),gas sensors and disinfection.Due to its relatively large band gap width,TiO2 has low utilization of sunlight when it was used as the catalyst in hydrogen production from water splitting.In addition,TiO2 has low light quantum efficiency because the electrons and holes were easy to recombine,which will weaken the PEC performance.To improve the water splitting performance of TiO2,Ni-doped TiO2 nanotubes were fabricated through anodic oxidation in this work.Effect of the fabrication process on nanostructures and PEC properties of Ni-doped TiO2 nanotube arrays were investigated.In order to further improve the photocatalytic performance,surface modification and metal reduction methods were employed to modify Ni-doped TiO2 nanotubes.Main research results are as follows:(1)Pulse anodic oxidation method was used to prepare Ni-doped TiO2 nanotube arrays in the electrolyte with a mixture of ethylene glycol and glycerol.Fluoride ion concentration in the electrolyte and the pulse voltage affected the macrostructure of the Ni doped TiO2nanotubes.When the anodic oxidation voltage was 40 V and fluorine ion concentration was0.2 M,the Ni-doped TiO2 nanotubes had uniform morphology.Phase transformation of the Ni-doped TiO2 nanotubes were investigated through Raman spectroscopy.It indicated that the phase of Ni-doped TiO2 material changed more easily compared to undoped TiO2.When the Ni-doped TiO2 nanotubes were annealed at 650°C,it transformed into rutile phase completely.When the heat-treatment temperature was 600°C,the Ni-doped TiO2 nanotube photoanodes exhibited better PEC performance.N/Ni-codoped TiO2 photoanode material was prepared by impregnation method.N/Ni-codoping effectively widened TiO2 material response range for sunlight and improved its PEC performance.(2)The electronic structure of the Ni-doped,N/Ni-codoped TiO2 was investigated by the First Principles simulation.It revealed that Ni element doping could produce new impurity level in the band gap of TiO2.With the increase of Ni doping amount in the TiO2,Ni,Ti and O elements enhanced hybridization,leading to the shrinkage of forbidden band width of TiO2and accompanied by the valence band and conduction band width.Due to the synergy effect between the elements,N/Ni-codoping could generate the impurity band at the top of the valence band of TiO2,which were composed of N 2p and Ni 3d state.(3)Cobalt phosphate/Ni-doped TiO2 nanotubes composite photoanodes was fabricated by electrodeposition method.The modification by cobalt phosphate can enhance the water oxidation capbility of TiO2,which effectively promoted the separation of electrons and holes,and thus increased the effective use of the sunlight.The amount of cobalt phosphate on the surface of the Ni-doped TiO2 nanotubes affected its PEC performance.When the deposition time was 400s,the PEC performance of the cobalt phosphate/Ni-doped TiO2 nanotubes composite photoanode was better,with a photocurrent density of 1.75 mA/cm2 at 0 V vs.Ag/AgCl.Compared to that of the undoped TiO2 nanotubes,the onset voltage of the composite photoanodes decreased 90 mV.(4)Reduced Ni-doped TiO2 photoanodes was fabricated through metal reduction method.Sn reduction modification can induce lattice distortion in crystalline Ni-doped TiO2nanotubes.The PEC performance of the Ni-doped TiO2 photoanodes significantly increased after metal reduction,which was 7.5 times as high as that of undoped TiO2.The reduced Ni-doped TiO2 photoanodes fabricated through Sn reduction amorphous Ni-doped TiO2exhihited more excellent photocatalysis performance than that of the reduced Ni-doped TiO2fabricated through Sn reduction of crystalline state Ni doped TiO2 nanotubes.The reduced Ni-doped TiO2 showed a better photocatalytic performance when the heat-treatment temperature of the amorphous Ni-doped TiO2 was 700°C.The photocurrent density was 4.5mA/cm2,which was 9.6 times as high as that of undoped TiO2 nanotubes.(5)Based on the reduced Ni-doped TiO2 photoanode,In2O3-x nanorodes/reduced Ni-doped TiO2 composite photoanode was fabricated through electrodeposition method.In2O3-x nanorodes modification can promote the sunlight absorption of reduced Ni-doped TiO2.The PEC performance of the In2O3-x nanorodes/reduced Ni-doped TiO2 composite photoanodes was 11.3 times as high as that of the undoped TiO2 nanotubes.