In the semiconductor and/or metal-based heteronanostructures (HNSs) for solar energy conversion, “nanoscale band engineering” taking the size quantization and interfacial design into consideration is essential for achieving the long-range charge transfer to enhance the efficiency because the effective space charge layer is usually not formed. HNSs consisting of metal chalcogenide quantum dot and metal oxide semiconductor (MX QD/MO, X = S, Se, Te) represented by QD/TiO2 are highly promising as the photoelectrode for not only solar cells but also photoelectrochemical cells for hydrogen generation from water. In this case, rational hybridization of the QDs (MX-MX, MX-plasmonic metal, plasmonic metal-metal) with atomically commensurate junctions between the components can improve the cell performances by tailoring the optical and electrical properties. This talk focuses on the in-situ synthesis of hybrid QDs/MO with epitaxial (EPI) junctions on the TiO2 (or ZnO) scaffold surface, and the effects of the interfacial bond and state of junctions between the components on the photoinduced charge generation and separation, and the cell performance for solar-to-hydrogen conversion. Firstly, the basic structure of the QD-sensitized photoelectrochemical (QD-SPEC) cell and the design of the QD/TiO2 photoanode are presented. Secondly, the mechanism on the electron injection from QD to TiO2 is dealt with. Finally, the synthesis of TiO2 coupled with hybrid QDs with EPI junctions, and the state-of-the-art QD-SPEC cells using the hybrid QD/TiO2 as the photoelectrode are described.