The critical role of Au catalysts in the synthesis has been discussed and demonstrated. By varying the reactant concentration and reaction time, ZnO NSSs with stems and branches of desirable dimensions can be readily obtained. The 1-D catalysts can be readily removed, leaving the ZnO NSSs intact. In this work, we report an innovative approach to synthesize comb-like ZnO NSSs in a one-pot and one-step manner assisted by 1-D catalysts with a hydrothermal method. 24 The ZnO nanoferns exhibit enhanced visible emission and photocatalysis as compared to as-prepared ZnO nanorods. Recently, Li and coworkers reported a metal hydrothermal oxidation approach to prepare ZnO nanoferns without catalysts. 23 However, such methods require high temperature and elaborate instrumentation, which are disadvantageous in cost-effective manufacturing. 13 The dendritic growth is a result of rapid crystallization with high-supersaturated reactants. Recently, direct synthesis of ZnO NSSs through thermal evaporation method without any catalysts was reported. It also introduces foreign particles, such as Au nanodots, which may impair the performance of nanodevices made of ZnO NSSs. This method requires a complex multi-step fabrication process. For instance, ZnO branches were grown on pre-fabricated ZnO nanowires via catalysts/seeds of gold (Au) or ZnO nanodots dispersed on the surface of the ZnO nanowires. Most thermal evaporation and hydrothermal methods are based on multi-step catalysts/seeds assisted reactions 2, 8, 19– 22. Various attempts have been made to synthesize ZnO NSSs, including hydrothermal and thermal evaporation methods. However, the applications of ZnO NSSs are still hampered by the difficulty of synthesizing ZnO NSSs in a facile, controllable and low-cost manner.
Highly branched ZnO NSSs also exhibit superior performance in various other devices such as solar-assisted water splitting electrodes 12, nanolasers 13, biosensors 14, optical gratings 15, 16/polarizers 16, field emission 17, and microwave absorption 18. It has been demonstrated that ZnO NSSs can improve the energy conversion efficiency (3.74% 8, 11) by 150%, when compared with simple one-dimensional (1-D) nanowire arrays (~1.5%) for DSSCs 7. Among all the ZnO nanostructures, highly branched ZnO nanosuperstructures (NSS) have generated an intense interest for enhanced performance in energy conversion devices, such as polymer-inorganic hybrid solar cells (PIHSCs) 6, dye sensitized solar cells (DSSCs) 7– 9, and piezoelectric nanogenerators 10.įor instance, the highly branched ZnO NSSs with multi-scale hierarchical configurations can substantially improve the energy conversion efficiency for PIHSCs and DSSCs due to the large surface area and enhanced charge transport properties.
As a result, the study and fabrication of ZnO nanomaterials, such as nanowires 2, nanodots 3, nanorings 4, and highly branched superstructures 5 have been at the frontline of recent research. These properties have found various applications in optics and energy conversion devices. Zinc Oxide (ZnO), a wide bandgap semiconductor material, exhibits unique electronic and optical properties when at least one dimension is constrained at the nanoscale 1.
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