Effects of Ta5+ substitution on the structure and photocatalytic behavior of the Ca2Nb2O7 photocatalyst

Nanosized Ca2Nb2-xTaxO7 (0 <= x <= 2) photocatalysts were synthesized at 160 degrees C by hydrothermal methods. The absorption onset of Ca2Nb2-xTaxO7 (0 <= x <= 2) photocatalysts shifted to shorter wavelengths with an increase of tantalum molar ratio. On the basis of theoretical calculations, the conduction band level of Ca2Nb2-xTaxO7 became higher, and the curvature of the conduction band became larger with an increase of tantalum content due to the different electronegativities of Ta 5d and Nb 4d. The photocatalytic activity of Ca2Nb2O7 slightly decreased with a small molar ratio of tantalum substitution (x <= 0.8), while the activity can be effectively improved with a large molar ratio of tantalum substitution (x > 0.8). The increase of activity as x > 0.8 can be attributed to more effective photoelectron transfer in the conduction band with larger curvature, and the slight decrease of activity as x <= 0.8 is attributed to the decreased amount of absorbed photons and lattice distortion, which can act as recombination centers.