Targeted Synthesis And Photocatalytic Performance Of Tantalates And Vanadates

Various kinds of photon energy conversion systems have been extensivelystudied for solar energy utilization. In photocatalytic system, H2can be obtained andorganic pollutants can be decomposed by using sunlight. How to achievehigh-efficiency and low-cost phtocatalysts has attracted wide attention. Due to theiroutstanding performance in water splitting and photodegradation, tantalates andvanadates are considered to be suitable for basic research and practical application.In our study, several tantalates and vanadates materials have been synthesized viaone-step method. Series of tantalates and vanadates with excellent photocatalyticmaterials are obtained by studying the effects of conditions to their properties.Bi_1.5ZnTa_1.5O₇（α-BZT） and Bi₂Zn_2/3Ta_4/3O₇（β-BZT） with pyrochlore structureare synthesized by the solid state method and the band structure are adjusted bycooper doping in B site. Under UV light irradiation, the photocatalytic activity of Cudopped α-BZT is about5times higher than that of α-BZT. Under visible lightirradiation, Cu dopping in α-BZT makes it caplable of hydrogen generation. Themechanism is that the impure level forms revealed by Cu3d by DFT calculation.Furthermore, same effect is found for β-BZT. The activity of β-BZT is about10times higher than that of β-BZT.NKT nanocrystal is synthesized by the molten salt method. The rate ofhydrogen generation reaches up to19.9mmol·h^-1in the presence of CH3OH whilethe rate is2.51mmol·h^-1for pure water splitting. The high activity is due to thecontrol of Ta-O-Ta bond by A site adjustment. However, it is found that oxygenevolution rate is low and the ratio of hydrogen to oxygen deviats far from thedesired value of2:1. To improve the stability of the catalyst, tetravalent cations ofZr4+and Hf4+are doped. The as-synthesized NKZT and NKHT nanocrystals aremonodispersed and the rates of hydrogen generation reach up to4.65mmol·h^-1and4.96mmol·h^-1for pure water splitting. Furthermore, the ratio of hydrogen to oxygenis close to2:1with improved photostability.SNT mesocrystal is synthesized by the molten salt method, and the surface areais as high as to40.6m²·g^-1. The rate of hydrogen generation reaches up to27.5mmol·h^-1in the presence of CH3OH while the rates of hydrogen and oxygen is4.89mmol·h^-1and1.25mmol·h^-1for pure water splitting. The outstanding activity ismainly due to the enhanced charge separation facilited by the formation ofnanosteps. At the edges of the reduction sites, water can be efficiently reduced to H2and the grooves of the nanostep structure provide the catalytic active sites for O2formation. m-BiVO₄hierarchical structure is synthesized via the hydrothermal method andthe optimized conditions are pH=3, T=180℃andt=6h. Hollow monoclinic scheeliteBiVO₄spheres are facile synthesized by using urea as guiding surfactant. Theforming process consists of incorporation of bubble guiding, oriented attachmentand Ostwald ripening. The hollow spheres are built up with truncated octahedrons asconfirmed though theoretical calculation. It is found out that the m-BiVO₄withhollow structure shows the optimalizing activity and the reaction rate constantreaches up to0.035min^-1without H2O2as hydroxyl radicals donor. The degradationof RhB is attributed to intrinsically strong photo-oxidation ability rather thanphotosensitization and the synthesized samples also show efficient photocatalyticactivity for the degradation of2-propanol.Samples sbow high stability and durability,after four runs of RhB photodegradation, the photocatalytic ability of as-synthesizedm-BiVO₄did not show any loss.BiVO₄and BiVO₄/BiOCl phtocatalysts are synthesized via the one-step moltensalt method. The optimized conditions for LiNO₃/NaNO₃are w=15:1and t=2h. Theas-synthesized BiVO₄shows sheet-like morphology. Without cocatalyts loading,90%of RhB is removed in50min, and the photodegradation process agrees with thefirst order kinetics equation, the value of k is0.050min^-1. BiVO₄/BiOClheterostructure is obtained by using LiCl/KCl as molten salt and the optimizedconditions are w=10:1and reaction for t=2h. BiVO₄/BiOCl heterostructure showsthe morphology of flower-like which is built up of nanosheets. The size of nanosheetis200-500nm in diameter and5nm in thickness. HRTEM and element mappingshow that the body of nanosheet is BiOCl and BiVO₄exists as nanocrystal.90%ofRhB can be removed in30min and the value of k is60%higher that of BiVO₄. Theenhanced photocatalytic activity is due to high charge separation effect inducedheterojunctions.