Hydrothermal Synthesis Of Tantalate Photocatalyst And Study On Photocatalytic Properties

Tantalate composite oxides show much higher photocatalytic activities for water splitting into H₂ due to their unique electronic structure and energy band structure. However, most of these multi metal oxides are synthesized by a conventional solid-state reaction and the method has lower surface area and crystal defect unable to overcome, all of which limited their photocatalytic activities for further enhancement. Reports on tantalate photocatalysts prepared by hydrothermal method are very few. In this paper, transition metal ions(M= Ni,Mn) doped NaTaO₃ and K₂Ta₂O₆ single phase with pyrochlore-like structure were synthesized by the hydrothermal method. The catalysts were characterized by X-ray diffraction(XRD), FT-IR,BET, UV-Vis spectroscopy(UV-Vis), and Scanning electron microscopy(SEM). Photocatalytic properties were also investigated.Transition metal ions(M=Ni,Mn) doped NaTaO₃ were prepared by the hydrothermal method, tantalum pentoxide(Ta₂O₅), M(AC)₂·4H₂O(M= Ni,Mn) and sodium hydroxide (NaOH) were used as the source materials. XRD patterns show that the as-obtained samples were all pure phase, and small-angle diffraction spectra show that with the increasing of the amount of dopant, diffraction peaks move to a high angle deviation, which indicated that M²⁺(M=Ni,Mn) ion have replaced part of Ta5+ ion. FT-IR spectra of NaTa_1-xM_xO₃(M=Ni,Mn) indicate that the samples show characteristic vibration band, which ascribe to TaO6 octahedron stretching vibration. SEM images show that the sample particle size grows bigger and the crystallinity becomes weaker with the increase of dopant content, resulting in the decrease of photocatalytic activity under UV light. UV-Vis diffuse reflectance spectra of NaTa_1-xM_xO₃(M= Ni,Mn) show that the absorption wavelength become red-shifted, and have obvious absorption in visible light region(>420nm) with increasing of the amount of dopant. The photocatalytic activity was increased as the amount of dopant increasing under visible light irradiation, which was in accordance with energy band analysis.K₂Ta₂O₆ single phase was synthesized by the hydrothermal method using tantalum pentoxide and potassium hydroxide as the starting materials. XRD results showed that K₂Ta₂O₆ photocatalyst crystallized in the pyrochlore-like structure, with space group Fd3m. SEM images show that K₂Ta₂O₆ grows up to octahadral shape of the single phase and have excellent crystallinity. The effects of reaction time and reaction temperature on the photocatalytic activity were also investigated. The results indicated that K₂Ta₂O₆ obtained at 240℃for 72h exhibited much higher activity (12.5 mmol·h^-1·gcat^-1) for water splitting than that of KTaO₃(1.04mmol·h^-1·gcat^-1), which was also prepared by the hydrothermal method. The difference in crystal structure and surface areas between K₂Ta₂O₆ and KTaO₃ may result in the different photocatalytic activity.