Photocatalytic hydrogen production over Aurivillius compound Bi3TiNbO9 and its modifications by Cr/Nb co-doping

In this work, we have applied Cr/Nb co-doping strategy to the wide band gap semiconductor Bi3TiNbO9 and have performed a detailed investigation on the structure, optical and photocatalytic properties of these modified Aurivillius compounds Bi3Ti1-2xCrxNb1+xO9 (x = 0, 0.1, 0.2, 0.3). Our results suggest that Cr/Nb doping slightly expand the unit cell of Bi3TiNbO9 with a doping limit around 20%. The involvement of Cr/Nb dopants in the crystal structure significantly reduces the band gap of Bi3TiNbO9 by nearly 1 eV. Photocatalytic experiments and photoelectrochemical measurements confirms the critical role of Cr/Nb dopants in improving photocatalytic hydrogen production and anodic photocurrent. More than two-fold enhancement in hydrogen production has been noticed for merely 10% Cr/Nb co-doping. The highest photocatalytic activity belongs to Bi3Ti0.8Cr0.1Nb1.1O9 (x = 0.1) for full range illumination and to Bi3Ti0.6Cr0.2Nb1.2O9 (x = 0.2) for visible light illumination, with apparent quantum efficiency (AQE) approaching 0.52% and 0.27%, respectively. DFT calculation discloses the role of Cr in forming a new valence band inside the band gap of Bi3TiNbO9. In addition, strong anisotropic phenomenon in charge transportation of Bi3TiNbO9 is also verified by DFT, as both conduction band minimum (CBM) and valence band maximum (VBM) are buried in the [BiTiNbO7]2− perovskite slabs and charges are only allowed to migrate within the slabs.