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

- Up to 20% Cr/Nb can be doped into Bi3TiNbO9 with a slight unit cell expansion.
- Cr/Nb co-doping improves visible light absorption and photocatalytic activity.
- A new valence band with Cr 3d character accounts for the visible light absorption.
- Bi3TiNbO9 has anisotropic charge transportation phenomenon.

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.

Aurivillius compound Bi3TiNbO9 exhibit interesting layered architectures that are promising for photocatalytic water splitting. Up to 20% Cr/Nb dopants can be incorporated into the crystal structure of Bi3TiNbO9 with laminated framework maintained. Co-doping Cr/Nb greatly extends visible light absorption of Bi3TiNbO9 and is responsible for the much improved photocatalytic activity. Theoretical calculation confirms the critical role of Cr for such modifications and reveals the anisotropic charge transportation phenomenon in Bi3TiNbO9: CBM and VBM located dominantly in [BiTiNbO7]2− perovskite slabs.131