Solar energy has been considered to be an alternative energy source to meet the exponential demand for renewable energy. To date, the use of nanomaterials has provided alternative and promising ways to improve solar device performance. In particular, nanoscale heterostructures hold great promise in the improvement of energy conversion efficiency, due to their advantages of possessing diverse functionalities in a single structure and their unique properties arising from strong interfacial interaction. Moreover, the safety risk associated with nanomaterials needs to be comprehensively assessed prior to their widespread use. In this dissertation, we have attempted to develop facile and reliable routes to two different categories of nanoscale heterostructures, namely (a) zero-dimensional (0D) - one-dimensional (1D) heterostructures and (b) 1D coaxial heterostructures, and to study their intriguing properties in applications associated with the energy conversion process. Specifically, the first part of the dissertation, including Chapter 2 and 3, will demonstrate the rational design of heterostructures consisting of carbon nanotubes and one or more types of nanocrystals, with precise control over the location and coverage of nanocrystals on the nanotube surface. The investigation of their photophysical properties reveals insight into the dynamics of charge carriers in the heterostructures. Moreover, the fabrication of SnO<sub>2</sub>/TiO<sub>2</sub> 1D radial heterostructures by coaxial electrospinning technique will be discussed as an example of creating discrete multifunctional 1D heterostructures. The applicability of such inorganic radial heterostructures in energy conversion applications is manifested by their photocatalytic activity in the degradation of organic dyes (Chapter 4). Finally, the morphology effect upon the toxicity of ZnO nanostructures towards marine diatoms has been studied, revealing the impact of nanomaterials on the environment (Chapter 5). Overall, these studies have provided valuable insight into the fabrication and utilization of nanostructures to resolve problems with global significance as well as to provide useful implications for evaluating the potential environmental risks of nanomaterials during the course of their life cycle.