“…In order to enhance the photocatalytic activity of TiO 2 , Xu et al prepared a series of rare earth metals ions (La 3 þ , Ce 3 þ , Er 3 þ , Pr 3 þ , Gd 3 þ , Nd 3 þ and Sm 3 þ ) doped TiO 2 by sol-gel [6]. In our previous work [7], yttrium and aluminum co-doped ZnO synthesized by the sol-gel method showed high photocatalytic activity for hydrogen production (5.71 mmol g À 1 h À 1 ) in the water-lactic acid system under visible-light irradiation for the first time, exhibiting excellent stability and recyclability. SnO 2 is a semiconductor with a wide band gap (3.5 eV), which is chemically and thermally stable.…”