Ammonia is a well-known additive to promote crystal growth in hydrothermal synthesis of ZnO nanowires. Although the effect of ammonia on the nanowire growth has been intensively investigated, its influence on the seed layer, which governs the initial nanowire growth, is rarely discussed. Here, we demonstrate that ammonia strongly affects the seed layer as well as the following nanowire growth. On increasing the ammonia concentration, the nanowire density first increases and then decreases, while the nanowire growth rate keeps increasing. Experimental results and thermodynamic calculations of the initial growth process reveal that the transformation of the seed layer induced by ammonia prior to nucleation critically determines the nanowire density and thus also influences the following nanowire growth. Present results highlight the critical importance of discussing the variation of seed layers in ammonia-involved hydrothermal synthesis and suggest a novel seed engineering approach for tailoring the ZnO nanowire growth.
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