Harvesting, storing and utilising solar energy using MoO₃: modulating structural distortion through pH adjustment

Nanostructured molybdenum oxide (α-MoO₃) thin film photoelectrodes were synthesised by anodisation. Upon band gap-excitation by light illumination, α-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 α-MoO₃ for utilisation under dark conditions, and α-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 α-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 α-MoO₃ at a higher pH favours charge storage, whereas smaller octahedral distortion at a lower pH leads to anodic photocurrent enhancement.