Effects of cerium dopant concentration on structural properties and photocatalytic activity of electrospun Ce-doped TiO₂ nanofibers

Abstract

Electrospun $\hbox {TiO}_2$ and Ce-doped $\hbox {TiO}_2$ nanofibers were prepared with 0.5, 2.0 and 8.0 % weight Ce. The structural properties and phase composition were characterized using high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction and X-ray absorption near edge spectroscopy (XANES) at the Ti K-edge. The undoped nanofibers are composed of an assembly of $\hbox {TiO}_2$ nanoparticles and their crystal structure is a mixture of anatase and rutile phases with an anatase:rutile volume ratio close to 3:1. As Ce is introduced, the nanoparticles decrease in size and the rutile phase volume decreases. Ce $\hbox {L}_3$-edge XANES probed the local structure of Ce dopants. At 0.5 % Ce, most Ce ions are incorporated in the $\hbox {Ce}^{3+}$ charge state but, at 2 % Ce, the majority are $\hbox {Ce}^{4+}$. Visible light absorption indicated that $\hbox {Ce}^{3+}$ act as shallow acceptors that only participate in absorption of wavelengths below 420 nm but $\hbox {Ce}^{4+}$ impurity states are associated with absorption of wavelengths up to 550 nm. Photocatalytic performance of the nanofibers was assessed by measuring the degradation of adsorbed Rhodamine B in aqueous solution under visible and ultraviolet light. The 0.5 % Ce-doped $\hbox {TiO}_2$ nanofiber showed the best visible-light photocatalytic activity, which is probably due to the majority presence of $\hbox {Ce}^{3+}$. At higher Ce concentration, the photocatalytic reaction rate was lower than undoped nanofibers, indicating that recombination at the $\hbox {Ce}^{4+}$ sites is rate limiting.