Harvesting, Storing and Utilising Solar Energy using MoO3: Modulating Structural Distortion through pH Adjustment

Nanostructured molybdenum oxide (alpha-MoO3) thin film photoelectrodes were synthesised by anodisation. Upon band gapexcitation by light illumination, alpha-MoO3 is able to store a portion of the excited charges in its layered structure with the simultaneous intercalation of alkali cations. The stored electrons can be discharged from alpha-MoO3 for utilisation under dark conditions, and alpha-MoO3 is able to recharge itself with successive illuminations to behave as a 'self-photo-rechargeable' alkali-ion battery. The alteration of the anodisation pH allowed the crystal structure and oxygen vacancy concentrations of alpha-MoO3 to be modulated to achieve (i) a distorted MoO6 octahedra for enhanced charge separation and storage, (ii) a layered structure with a greater exposed (010) crystal face for rich and reversible ion intercalation and (iii) a highly crystalline thin film that suppresses electron-hole pair recombination. Overall, the larger MoO6 octahedral distortion in alpha-MoO3 at a higher pH favours charge storage, whereas smaller octahedral distortion at a lower pH leads to anodic photocurrent enhancement.