**6. Conclusion**

Over recent decades, the tremendous efforts have been paid to develop TiO<sup>2</sup> nanomaterials and large amount of information provided on their synthesis, modification, and applications. The one-dimensional TiO2 nanostructures have been comprehensively studied due to its distinctive advantages with regard to less charge carrier recombination rates and unique physical and chemical properties. In this chapter, we first explain the structural features of the three TiO<sup>2</sup> polymorphs that have been seen in TiO<sup>2</sup> nanostructures. Then, the various synthesis processes of one-dimensional TiO<sup>2</sup> nanostructures were discussed. Hydro/solvothermal method and the effect of the experimental parameters on the formation of various morphologies and properties of 1D-TiO<sup>2</sup> nanostructures were thoroughly reviewed. In addition, we studied some strategies on bandgap engineering of titania to improve optical properties and charge carrier separation and transfer to the surface of photocatalyst. Finally, the applications of 1D-TiO<sup>2</sup> nanostructures in photocatalytic water splitting and dye-sensitized solar cells have been reported with regard to their specific structure and properties. Therefore, these data favorable for further investigation on the development of sustainable environmental remediation and energy technologies based the photocatalytic process by driving solar light as renewable source of energy.
