Ligand-directed rapid formation of ultralong ZnO nanowires by oriented attachment for UV photodetectors

Ultralong ZnO nanowires with lengths of 20-80 μm and aspect ratios of 200-500 were synthesized within 15 minutes via a low-temperature hydrothermal method. With the assistance of sodium dodecyl sulfonate (SDSN) as the capping ligand, ZnO nanowires were formed by the initial nucleation of nanocrystals followed by the ligand-directed oriented attachment. Head-to-head attachment, side-by-side coalescence and nanocrystals attached to the surfaces were observed at different growth stages. ZnO microrods (lengths: 2-10 μm, diameters: 0.5 to 5 μm) were formed in the absence of SDSN. FT-IR spectra, XPS analysis and molecular dynamics simulations revealed that SDSN molecules were preferentially adsorbed onto the (100) planes rather than polar (001) planes, with their sulfonate groups coordinating with the surface zinc ions and possibly forming Zn-SO₃ complexes. Such selective adsorption not only protected the initially nucleated ZnO nanocrystals from rapid aggregation, but also directed their subsequent self-assembly into highly-anisotropic nanowires. The as-prepared ZnO nanowires exhibited improved photoluminescence properties compared to the microrods. I-V characteristics indicated that the ZnO nanowires exhibited a much lower dark current, while an enhanced photocurrent upon 360 nm light illumination compared to the microrods. In addition, UV photodetectors using ZnO nanowires showed 27 times higher photo-sensitivity and 15.4/13.8 times higher rise/decay rates compared to those using ZnO microrods, which were attributed to the morphological effects in addition to the improved optical properties.