Favourable band edge alignment and increased visible light absorption in β-MoO3/α-MoO3 oxide heterojunction for enhanced photoelectrochemical performance

Optimum band gap values, favourable band edge positions and stability in the electrolyte are critical parameters required for a semiconductor to have efficient photoelectrode properties. The present investigation carried out on the phase pure α & β MoO ₃ thin film shows that the low bandgap β-MoO ₃ possesses a mis-alignment with the water oxidation potential, while a more suitable band alignment is observed for the comparatively large bandgap α-MoO ₃. Both experimental and DFT calculations show that the valence edge of the orthorhombic (α-MoO ₃) phase is located at a higher energy (0.9 eV higher in VB-XPS and 1 eV higher in the DOS plots) than the monoclinic (β-MoO ₃) phase, while the conduction edge value is roughly at the same energy level (−2.5 eV) in both polymorphs. Based on the above investigations, an all oxide heterojunction comprising of β-MoO ₃/α-MoO ₃ is found to be suitable for improved PEC performance due to favourable energy band diagram and increased visible light absorption in β-MoO ₃. Significantly higher cathodic photocurrent is observed for the β-MoO ₃/α-MoO ₃ (1.6 mA/cm ² at applied bias of −0.3V _RHE under simulated 1 sun irradiation) as compared to the very low anodic response in β-MoO ₃ (∼1.0 nA/cm ²) and α-MoO ₃ (32 μA/cm ²).