Hydrogen is considered as an ideal energy carrier in the future. Photocatalytic splitting of water to hydrogen and oxygen has been regarded as the most promising approach ever since Fujishima and Honda reported the photoelectrochemical water splitting by a TiO 2 electrode [1].
TiO 2 is a good photocatalyst due to its stability and high activity. However, its bandgap is so large (E g = 3.20 eV) to be only excited by ultraviolet light with a wavelength of no longer than 387.5 nm, which accounts for only 4% of the incoming solar energy. Thus, it is significant to develop visible-light driven photocatalyst with high activity. To make the best use of solar energy, there have been many studies on doped TiO 2 . Among them metal ion doping has extensively been studied, for example, transition-metal ions [2,3], rare-earth ions [4,5].
Recently, many researchers studied nonmetal element doped TiO 2 , such as N [6-8], S [9,10] and C [11,12], to extend the photocatalytic activity into the visible-light region. Asahi et al. [6] reported that nitrogen doped TiO 2 prepared in the atmosphere of N 2 / Ar extended its light absorption to visible-light region. Yuan et al. [7] prepared N-TiO 2 by heating the mixture of urea and new-made TiO 2 .
To enhance the visible-light activity of the N-doped TiO 2 , several groups studied co-doped materials such as C and N [13], F and N [14], Li and F [15], N and Fe [16], and B and N [17]. In several cases some authors underlined the synergistic effect of co-doping. However, to our knowledge, no studies on the effect of boron and nitrogen co-doping for TiO 2 on its photoactivity for hydrogen production have been reported.
B-doped TiO 2 showed high photocatalytic activity compared to pure TiO 2 under UV light irradiation [18]. The type of B-doping into TiO 2 strongly depends on the preparation method (synthesis route precursors, calcination conditions, . . .) [19]. While Chen et al. [18] observed a bandgap increase due to B-doping, attributed to quantum-size effect, Zhao et al. measured a red shift [20]. The apparent contradiction can be explained considering the different geometry and electronic structures of the B-doped TiO 2 [19].
In the present study, we prepared boron and nitrogen co-doped TiO 2 photocatalyst by sol-gel method, with the aim to explore possible synergistic advantages arising from the simultaneous presence of the dopants. Photocatalytic activity for hydrogen production under visible-light (l ! 420 nm) irradiation was investigated. The photocatalysts were characterized by various techniques, and the photocatalytic mechanism is discussed.