 Bandgap Engineering: Further reducing the bandgap of TiO₂ by optimizing dopant levels to enable absorption of visible light.  Surface Plasmon Resonance (SPR): Incorporating noble metals like Ag at specific concentrations to harness SPR effects for visible light activation.  Coupling with Other Semiconductors: Combining TiO₂ with narrow-bandgap semiconductors, such as CdS or ZnO, to form heterojunctions that enhance charge separation and broaden the light absorption range.