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. Photogenic: Subjected to positive bias and illumination, α-MoO₃ demonstrates the capability to generate and store photoexcited charges in its layered structure through the intercalation of alkali cations. The electrolyte pH during anodization can be used to modulate the crystal structure of α-MoO₃, which in turn governs charge storage and mobility properties.