Performance Study Of Modified TiO₂Nanotube Arrays Photoanode For Hydrogen Production By Water Splitting

Hydrogen is one of the clean energy in future. The hydrogen production from photoelectrocatalytic water spliting which transfers solar to hydrogen energy by photoelectrochemical cell is the cleanest, low energy and economical feasible technology, the preparation of catalyst photoelectrode is the key role of this work. At present, the shortcomings of catalyst for photoelectrocatalytic water spliting maily include two aspects. Firstly the low visible light response cannot effectively use the sunlight. Secondly the light electron from valence band exciting into the conduction band is not sufficient to split water molecular to generate hydrogen as the positive conduction band potential. So the catalyst needs modification to meet the above requirements, and increase photoconversion efficiency of hydrogen generation.In this work, the TiO2nanotube arrays were modified by an appropriate metal and its oxide as semiconductor to improve the performance in water splitting hydrogen production. According to our knowledge, such photoelectrode preparation in photoelectrocatalytic splitting water to hydrogen production investigation has not been reported. The surface morphology and crystal phase of the modified TiO2nanotube arrays were analysised by SEM, XRD and XPS. The modified substances effect on TiO2nanotube arrays photoresponse were studied by semiconductor photocurrent response and electronic structure analysis. And the semiconductor characteristics and charge transfer kinetics on the TiO2nanotube arrays semiconductor/solution interface vere investigated by capacitance analysis of Mott-Schottky curves and electrochemistry impedance spectrum(EIS).The optimal TiO2nanotube arrays(TiO2NTs) prepared by anodic oxidation6h in the glycol solution containing F", showed well-ordered surface and profile as anatase after calcination at450℃and indicated the highest photocurrent response. This sample absorbed light redshift and its photocurrent improved four times than that of TiO2thin film.Reductive Cerium and oxidative Cerium were deposited on the TiO2nanotube arrays by electrochemical cathodic reduction and then anodic oxidation. The sample(TiO2NTs-Ce-CeOx) prepared in the10mmol/L cerium nitrate solution suggested an optimal deposition quantity. In this condition, the reductive Cerium was in the form of elementary Cerium and Ce2O3nanofibers dispersed both on the surface and inside of TiO2nanotubes. Compared with TiO2nanotubes without modification, reductive Cerium modification lowered the original bandgap energy from3.15eV to2.88eV and enhanced photocurrent response in visible spectra rather than in UV spectra. The flat band potentials also moved to negative direction. After anodic oxidation, the oxidative Cerium was in the form of elementary Cerium, Ce2O3and CeO2dispersed both on the surface and inside of TiO2nanotube. Comparred with TiO2nanotube, the photocurrent response of the sample was enhanced in visible spectra and in UV spectra region and the flat potentials moved to negative direction. As the anodic oxidation in depth, the photocurrent responses increased in visible light. The study of photocurrent on photon energy indicated that there was a band gap energy of2.4eV as Ce2O3oxidation phase in the reductive Cerium and oxidative Cerium.The donor materials adding in the anode electrolyte significantly reduced the charge transfer impedance of TiO2nanotube to promote the photocatalytic spitting water for production hydrogen by EIS with adding organic glycol(C2H6O2) optimum. Although the oxidative Cerium modified TiO2nanotube arrays had a new capacitace arc by EIS as the TiO2nanotube arrays electrode surface increased the new interface, while reduced the space charge layer impedance of TiO2nanotube arrays. This indicated the elemental cerium and cerium oxide work to improve the electron transmission performance of TiO2nanotube arrays. The bias potential on working electrode reduces charge transfer impedance on TiO2nanotube layer. The mechanism of the Cerium and oxidative Cerium acting on TiO2nanotube arrays to improve photocurrent response and reduce charge transfer was discussed. The narrow bandgap semiconductor Ce2O3(Eg=2.4eV)and wide bandgap semiconductor CeO2(Eg=3.16eV) respectively enhanced the photocurrent responces of TiO2nanotube arrays in the visible and UV region. And two cerium oxides had negative conduction band potential than TiO2, which contributed that photoinduced electrons move to the conduction band of TiO2and then reach the counter electrode to split water for hydrogen production; On the one hand, elemental cerium as the impurity levels in the band gap of TiO2was conducive to the separation of photo-generated electrons and holes, and promote visible light photoresponce. On the other hand, elemental cerium was doped into TiO2crystal lattice to form a mixed conduction band with Ti3d. This new conduction band narrowed the energy bandgap to promote the absorption of visible light.Cerium and oxidative Cerium increased TiO2nanotube arrays photoconversion efficiency for splitting water. In water TiO2NTs-Ce-CeOx photoelectrode without bias resulted in stable and consistent hydrogen generation throughout the5h reaction and the average hydrogen generation rate was0.092mL/h·cm2, but TiO2NTs photoelectrode without hydrogen generation. After adding C2H6O2in anode electrolyte, this optimized TiO2NTs-Ce-CeOx photoelectrode was found to split water with maximum photoconversion efficiency of5.9%under white light illumination, which was1.76times that of TiO2NTs and the average hydrogen generation rate was2.38mL/h·cm2with0.4V bias. In above370nm wavelength light illumination, TiO2NTs-Ce-CeOx photoelectrode was found to split water with maximum photoconversion efficiency of4.43%, which was1.92times that of TiO2NTs and the average hydrogen generation rate was1.39mL/h·cm2with0.4V bias.Very experiments were taken out in different metal and its oxide as method discussed above. The flat band potentials of TiO2NTs modified by Lead(Pb) and its oxides moved to the negative direction while its photocurrent responses in visible spectra and UV spectra reduced by modification. As a contrast, the photocurrent responses of TiO2NTs modified by In or Cu, respectively, improved in visible spectra and UV spectra, while their flat band potentials moved to the positive direction, which played a negative effect in photoelectrocatalytic splitting water for hydrogen production.