Microstructure Regulation Of Porous Ta₂O₅ For Photocatalytic Hydrogen Production

Since photocatalytic water splitting for hydrogen production can be exploited as a direct and green means for effective utilization of solar energy, it has attracted extensive attention of researchers. Ta2O5 has been recognized as a promising material for photocatalytic hydrogen production owing to its higher photochemical stability and a stronger reducing capability of photoexictied electrons. What's more, its special corner-sharing octahedral(TaO6) structure is benefit for the migration of photogenerated electrons and holes. Regulating the microstructure of Ta2O5 to prepare porous Ta2O5 with a high crystallinity and a large special surface area is good for enhancing its photocatalytic hydrogen production efficiency. Because the porous structure usually endow photocatalysts large special surface areas and thin porous walls, which can increase the active sites of photocatalysis and decrease the migration distance of photogenerated charges. Additionally, a high crystallinity can decrease the defects, thereby effectively suppress the photogenerated carriers recombination. However, for Ta2O5, it is difficult to obtain a porous structure with a high crystallinity due to its high crystallization temperature(above 700 oC) and a rapid hydrolysis rate of tantalum-based precursors. Thus, in this dissertation, we focus on the microstructure regulation of Ta2O5. Mesoporous single-crystal Ta2O5 microcubes have been prepared by introducing silica spheres template attached with Ta2O5 seeds into the quasi-equilibrium growth of the tantalum oxides. On the other hand, pot-like porous nanospheres of Ta2O5 doped with Cu2+ ions(Cu-Ta2O5 PPNs) have been prepared by the hydrolysis of tantalum glycolate in acetone-ethylene glycol(EG) solution containing Cu(NO3)2 electrolyte and the subsequent calcination. The results can effectively improve the crystallinity of mesoporous Ta2O5, increase the specific surface area of photocatalysts and reduce body defects, wich is benefit for the separation and migration of photogenerated charge carriers, and thus enhance the photocatalytic activity for hydrogen production. The main results are as follows:1. Mesoporous single-crystal Ta2O5 microcubes have been prepared by a multistep stategy. Firstly, the seeded template has been prepared by introducing Ta2O5 seeds into the assembled silica spheres template. Then, tantalum oxides microcubes are formed by controlling the hydrolysis of Ta Cl5 in the presence of the seeded template in the solvothermal treatment. At last, the as-prepared mesoporous single-crystal Ta2O5 microcubes are obtained by calcination and the subsequent removal of the template. The formation process has been clearly elucidated by studying the effect of the seeded template. Moreover, more single mesoporous and hierarchical porous single-crystal Ta2O5 microcubes with diverse porosities have been obtained by facily changing the particle size and assembly of silica spheres. The photocatalytic activities of the as-prepared samples have been tested in 20 % methanol aqueous solution under the simulant solar light irradiation and the hierarchical porous single-crystal Ta2O5 microcubes exhibit the highest photocatalytic activity.2. Pot-like porous nanospheres of Ta2O5 doped with Cu2+ ions(Cu-Ta2O5 PPNs) have been easily prepared by the hydrolysis of tantalum glycolate in acetone-ethylene glycol(EG) solution containing Cu(NO3)2 electrolyte and the subsequent calcination. The formation mechanism of pot-like porous structure is clarified by introducing different electrolytes. In our protocol, the incorporation of Cu(NO3)2 electrolyte into the hydrolysis system cannot only increase the surface charges and dispersability of the hydrolyzed colloidal particles, but also can introduce into the hydrolyzed colloidal particles a cupric complex with EG, which decomposes at above 700 oC with releasing a mass of gases and heat to shape the resultant Cu-Ta2O5 PPNs. The doping of Cu2+ ions induces a donor level in the forbidden band of Ta2O5. The photocatalytic hydrogen production activity of the as-prepared Cu-Ta2O5 PPNs at CCu2+ = 0.5 % reaches the maximal value of 737.4 ?mol·h-1·g-1, which is 6 times higher than that of Ta2O5 submicrospheres. The enhanced photocatalytic activity can be reasonably ascribed to the decreased particle size, the large specific surface area and the doping of Cu2+ ions.