Influence of Mn, Cu, and Cd–doping for titanium oxide nanotubes on the photocatalytic activity toward water splitting under visible light irradiation

Abstract

The photocatalytic activity of titanium oxide nanotubes toward water splitting under the visible light irradiation was improved by doping the nanotubes by three transition metals. The pristine titanate nanotubes have been prepared by the hydrothermal process. Then, a simple sonication process was exploited to induce the doping reaction. Transition metals with different electron configurations in the outer shell (Mn (3d5 4s2 [Ar]), Cu (3d10 4s1 [Ar]), and Cd (4d10 5s2 [Kr])) were inserted in the titanate nanotubes to check their performance as dopants on the photocatalytic activity. TEM images showed the appearance of open-ends and multi-walled nanotubes. The XRD patterns proved the formation of titanate nanotubes. In addition, the XPS analysis confirmed doping the titanium oxide nanotubes by the selected metals. The UV–vis spectrum indicated that both Mn and Cu doping enhanced the reception of the visible light by 35 and 21.9%, respectively, while for Cd, the light absorption was slightly improved. The calculated band gap energies for the pristine, and Mn, Cu, and Cd-doped titanate were 3.54, 2.81, 3.2, and 3.54 eV, respectively. Interestingly, all the selected metals have distinctly improved the photocatalytic activity toward water splitting, however, with different mechanisms. Mn and Cu strongly increased the photon absorption by decreasing both of direct and indirect band gaps. On the other hand, Cd doping led to enhance the electrons lifetime. Moreover, the doped nanotubes revealed good stability, which was concluded by studying the cyclability of the Mn-doped titanate nanotube toward water splitting.