Synthesis And Modification Of Potassium Hexaniobate Nanotubes And Their Photocatalytic Hydrogen Evolution From Water Reduction

The topic of photocatalytic hydrogen evolution from water reduction has attracted increased attention of scientists, in which solar energy can be transformed into clean and renewable hydrogen energy by means of semiconductor photocatalysis. An experimental setup for judging the photocatalytic activity of a catalyst is the basis of this research, while the design and preparation of a high efficient and stable semiconductor photocatalyst is its key. Potassium hexaniobate nanotubes are a potential photocatalyst in hydrogen evolution from water due to their structural features, including abundant active sites, easier separation of photogenerated electrons-holes and facile modification.In this dissertation, potassium hexaniobate nanotubes were prepared by an improved method. The effects of preparation methods of precursors, morphologies of potassium hexaniobate, species of sacrificial agents and cocatalysts, and pH values on the hydrogen evolution activity of potassium hexaniobate nanotubes were investigated by means of a home-made device. The main results are as follows.The home-made setup for photocatalytic activity measurements has several features, i.e. convenient switch of light source, closed gas system, high hydrogen detection sensitivity and simple date processing.Under simple and mild conditions via the acid-exchanging and exfoliating, the potassium hexaniobate nanotubes having favorable tubular morphology and large aspect ratio were prepared using potassium hexaniobate particles as the precursor obtained by the high-temperature solid state reaction. The structures and optical properties of the products were characterized by XRD, TEM, BET and solid diffuse reflectance UV-vis spectra. Pt was loaded onto surface of the potassium hexaniobate nanotubes by photodeposition, and the effect of the loading amounts of Pt on the separation of photogenerated electrons-holes was explored by means of fluorescence spectra. The influences of morphology of potassium hexaniobate, species of sacrificial agents, pH values and the loading amounts of Pt on the photocatalytic hydrogen evolution from water reduction under ultraviolet irradiation were investigated. The results displayed that the potassium hexaniobate nanotubes with favorable tubular morphology and large aspect ratio were beneficial to hydrogen evolution from water reduction.18.77 mmol·g-1·h-1 of the highest hydrogen evolution rate was obtained when 1.5 wt% Pt was loaded on the nanotubes in pH=310 vol% methanol aqueous solution.In addition, potassium hexaniobate particles were obtained in a low-temperature hydrothermal process. Using it as the precursor, potassium hexaniobate nanotubes were prepared by a modified method. The structures and optical properties of the products were characterized by means of XRD, TEM, BET and solid diffuse reflectance UV-vis spectra. Effects of sacrificial agents and the metal ions present in the solution on the photocatalytic hydrogen evolution from water reduction were investigated. Consequently, the potassium hexaniobate nanotubes showed the highest activity of hydrogen evolution using methanol as the sacrificial agent. It was also found that there existed a photoinduction period in the initial reaction stage when the Cu2+ was presented in the solution. Furthermore, the photoinduction period was prolonged by increasing the amount of Cu2+. After the photoinduction period, the activity of photocatalytic hydrogen evolution of potassium hexaniobate nanotubes was sharply increased relative to that of no any Cu2+ in the solution. It was possible that the Cu2+ could capture photogenerated electrons in the photoinduction period, and then produced elementary Cu, which was deposited on the surface of potassium hexaniobate nanotubes and acted as a cocatalyst. The highest hydrogen evolution activity was obtained when the mass ratio of Cu/potassium hexaniobate nanotube was 3 wt%. The maximal hydrogen evolution rate was 21.90 mmol·g-1·h-1, which can compare with the hydrogen evolution rate of the potassium hexaniobate nanotubes loaded with noble metal Pt as a cocatalyst. When Fe3+ was added into the methanol aqueous solution, the nanotubes lost the activity of hydrogen evolution. Because of the mutual conversion between Fe3+ and Fe2+, the photogenerated electrons and holes were depleted, respectively, which made photogenerated electrons be unable to react with water in the oxidation-reduction reaction.