Preparation Of Titania Nanotubes And Its Properties On Photocatalysts And Electrocatalysts

TiO2 nanomaterials cause much attention because of their special characteristics of optics, catalysis, and photoelectricity conversion. TiO2 nanotubes have the particular one-dimension structure of TiO2 nanomaterials. It is an important and valuable result to study the preparation of TiO2 nanotubes and their properties on photocatalysts and electrocatalysts. A simple, rapid and efficient method was developed to synthesize TiO2 nanotubes by microwave irradiation and the mechanism of formation of TiO2 nanotubes was discussed. The conditions of preparation of TiO2 nanotubes by microwave irradiation were optimized. On the basis of the synthesized TiO2 nanotubes, the photocatalysts and electrocatalysts were prepared. The performances of them were investigated by splitting pure water into H2 gas and O2 gas, and the electro-oxidation of methanol, respectively. It is an interesting phenomenon that TiO2 nanotubes modified with 0.5wt% Pt by microwave assisted polyol method not only showed high H2 evolution under the whole light regions, but also produced H2 under visible light, which wavelength is over 420 nm. We have obtained TiO2 nanotube photocatalysts sensitized by the photo-sensitizers of TMPP and CoTMPP. They all showed good activity to produce H2 gas under visible light (λ>420nm). After the photocatalytic reaction time over 16h, the photocatalysts still had the ability to generate H2 gas under visible light. When Pt electrocatalysts were doped with certain content of TiO2 nanotubes, they had more electrocatalytic activity for methanol electro-oxidation, especially the second transition metal, such Ni, added into the electrocatalysts. At the conditions, the electrocatalyst contained 5wt% Pt, 10wt% Ni and 10wt% TiO2 nanotubes showed better activities than any other catalysts for methanol electro-oxidation. The main contents of the paper were listed as follows:1. The anatase/rutile crystalline of TiO2 nanoparticles were synthesized by co-deposition and so on. Then TiO2 nanotubes were got by microwave irradiations, which had the inner diameter of about 3 nm, the outer diameter of about 12 nm, and the length from several hundred nm to severalμm. They had the hollow, end-opened, and multilayer structure. From the result of XRD and ICP, it confirmed that the nanotubes were TiO2 nanotube. The optimal craftworks of the synthesizing TiO2 nanotubes were that when the power of microwave irradiation was 195 W, the concentration of sodium hydroxide solution was 10, and the microwave reaction time was between 60~90 min. When the reaction time was 70 min, the concentration of sodium hydroxide solution was between 9~11 mol·L-1. The quantity of TiO2 nanotubes were increased by the prolonged reaction time when the concentration of sodium hydroxide solution was a little lower. The precursors had much effect on the morphology of TiO2 nanotubes. When anatase crystalline of TiO2 nanoparticles was selected as precursors, the morphology of TiO2 nanotubes was good and the diameters of them are even. When rutile crystalline of TiO2 nanoparticles was precursors, the TiO2 nanotubes had the parallel structure. The quality of nanotubes became worse along with the larger particle sizes of precursors. It was found that washing process had crucial effect on formation of TiO2 nanotubes during the post-treatment and different microwave irradiation time. With the reaction time prolonged, the nanoparticles began to dispel and formed the intermediate of nano-tiles. The larger ones curled to form tubule and others tiles had the interaction, such as absorption, integration, and overlapped layer by layer. So TiO2 nanotubes were formed.2. The novel photocatalysts were synthesized by precious metals modifying TiO2 nanotubes. The activities of the catalysts were investigated by photocatalytic splitting pure water for H2 gas evolution. It indicated that TiO2 nanotubes modified with 0.5wt% Pt by microwave assisted polyol method had more photocatalytic activity. It had better photocatalytic activity than that of TiO2 nanoparticles. The former also had the ability to produce H2 gas under visible light (λ>420nm). The content of Pt loading had an appropriate quantity, because a little content of Pt lead to a little reactive site, and a lot of Pt became the recombination of electron-hole pairs. Thus the photocatalytic activity was not high.3. We also investigated the performance of photocatalysts of TiO2 nanotubes sensitized by photosensitizers of TMPP and CoTMPP. All the photocatalysts had the ability to produce H2 gas under visible light. The photocatalyst of TiO2 nanotube sensitized by TMPP had better activity than that of CoTMPP. All of them excelled the TiO2 nanoparticles sensitized under the same conditions. After photocatalytic reaction time of 16 h, the photocatalyst of TiO2 nanotube sensitized by TMPP still had the ability for H2 evolution under visible light. After heat treatment, the performance of the catalysts descended. It might be attributed to the destroying the structure of porphrin. But after the heat treatment of the photocatalyst of TiO2 nanotubes sensitized by CoTMPP, the activity of it increased. It ascribed to the interaction between CoTMPP and TiO2 nanotube offsetting the destroying the structure of porphyrin. The structure of the photosensitizers were not broken by the analyzing the reacted products. It indicated that the photocatalysts had good stability.4. When TiO2 nanotubes were mixed with commercial Pt/C electrocatalysts, it showed good performance for methanol electro-oxidation. The electrocatalysts mixed with the content of 20% TiO2 nanotubes had the highest activity. Chronoampermetry showed that the catalysts had better stability.5. Under the conditions of the Vulcan carbon as support and H2PtCl6 as Pt precursors, the electrocatalysts doped with TiO2 nanotubes, which were synthesized by microwave assisted polyol method, had good electrocatalytic activity for methanol oxidation. The doped content of TiO2 nanotubes was 10%, the microwave reaction time was 10 min, and the pH value of the solution before reaction was 9. The electrocatalyst doped with TiO2 nanotubes had better activity than that of TiO2 nanoparticles. The Pt nanoparticles sizes became small when it was doped with TiO2 nanotubes. From the result of XRD, it showed that the particle size of Pt was about 2.3 nm. And. The results of HRTEM and EDX showed the Pt nanoparticles mainly dispersed on the surface of carbon support. Some of them located the surface of TiO2 nanotubes. It could also be seen that some TiO2 nanotubes dispersed on the carbon support. The performance of the electrocatalyst became worse when it was treated under higher temperature because the diameter of Pt particles became larger during the heat treatment, and some of them gathered. At the same time, we studied the influences of the different prepared ways. It showed that the electrocatalysts synthesized by microwave irradiation had the higher activity for methanol electro-oxidation. Then the BP carbon was selected as another support to prepare electrocatalysts. It had higher catalytic activity than that of Vulcan carbon, because the surface area of BP carbon is larger than Vulcan carbon. The electrocatalysts doped with 10% TiO2 nanotubes improved the performance for methanol electro-oxidation than doped with other content of TiO2 nanotubes. At last the performance of electrocatalysts with different supports, that is Vulcan carbon, BP carbon, and carbon nanotubes, were compared. It can be concluded that the performance of electrocatalysts supported on BP carbon had highest active, and the Vulcan carbon was better than that of carbon nanotubes.6. According to the results of the above, PtMTiO2 nanotube electrocatalysts were synthesized (M=Ni, Co). The results showed that when nitrate nickel was selected as the precursor of Ni, the electrocatalytic performance for methanol electro-oxidation was improved by doping with TiO2 nanotubes. The electrocatalysts contained 20% Pt, 10% Ni, and 10% TiO2 nanotubes had the highest activity among the electrocatalysts with different content of Pt or Ni. When there were 10% Ni and 10% TiO2 nanotubes in the electrocatalysts, the electrocatalytic performance increased with the increased content of Pt. Then the citrate nickel as precursor of Ni was added into the electrocatalysts to prepare low content of Pt catalysts. It was found that when the electrocatalysts contained 5wt% Pt, 10wt% Ni, and 10wt% TiO2 nanotube, the activity for methanol electro-oxidation had much better activity for methanol electro-oxidation. It showed that the size of nanoparticles of Pt became smaller after adding TiO2 nanotubes into the catalysts by the measurement of XRD and TEM. There were some interactions between Pt, Ni, and TiO2 nanotubes. So the electrocatalysts with low content of Pt had good activity for methanol electro-oxidation.