Colloidal semiconductor nanocrystals (CS-NCs) possess compelling benefits of low-cost, large-scale solution processing, and tunable optoelectronic properties through controlled synthesis and surface chemistry engineering. Our research interest includes the controlled synthesis of CS-NCs in terms of crystalline structure, particle size, dominant exposed facet, and their surface passivation. This work specifically focuses on controlling the exposed facets and hence the dimensionality and shape of ZnO nanocrystals using a non-hydrolytic route. The morphologies and crystal facets of the ZnO nanocrystals were controlled by varying reaction temperature and the reactant ratio. Four distinct ZnO nanocrystal types were produced (nanocones, nanobullets, nanorods and nanoplates). The effects of reaction conditions on ZnO formation were investigated and possible self-assembly mechanisms were suggested.¹

Due to its high bulk electron mobility, ZnO is considered as a promising photoanode material for solar cell applications. Although various strategies have been taken to improve the efficiency of ZnO-based dye-sensitized solar cells (DSSCs), their performances still lag far behind TiO₂ counterparts. This is mainly attributed to the instability of ZnO, which leads to the formation of low conductivity Zn²⁺-dye complexes on ZnO surfaces. The etching of ZnO surfaces not only deteriorate physical and chemical properties of ZnO, but also decrease the effective adsorption of dyes and the electron transfer from dyes to ZnO. Here, ZnO nanocones with high-index {10-11} facets were first time employed in DSSCs. The devices were compared with DSSCs made from more commonly used ZnO nanorods where {10-10} facets are predominantly exposed. When ZnO nanocones were used, DSSCs sensitised with C218, N719 and D205 dyes universally displayed better power conversion efficiency, with the highest power conversion efficiency of 4.36% observed with the C218 dye. First-principles calculations indicated that the enhanced DSSCs performance could be attributed to the self-passivation of ZnO nanocones with 100% O-terminated surfaces, and the enhanced binding strength between dye molecules and reactive ZnO {10-11} facets. Our results suggest that better understanding of the crystal growth behaviour and proper control of nanocrystal surfaces are essential for improving solar cell performances.²