From function-guided assembly of a lotus leaf-like ZnO nanostructure to a formaldehyde gas-sensing application

• A novel nanosheet-on-nanorod (i.e., lotus leaf-like) ZnO nanostructure is fabricated by solution-based route with extremely low supersaturation of growth units.
• Such a ZnO nanostructure displays superior response to formaldehyde even in the case of 1 ppm.
• The extremely thin nanosheets (leaves), matching some characteristic physical lengths, may be ideal for studying 2D quantum behavior and relevant properties and exploiting novel nanodevices.

A facile solution-based route is developed for the preparation of aligned lotus leaf-like ZnO nanostructure via an immersion of oriented ZnO nanorods in hexamethylenetetramine solution at a water-bath temperature of 90 °C. During the immersion, a morphology-evolution from nanorod to lotus leaf-like nanostructure and to nanosheet is obtained by a simple change of heat-time. From gas-sensing measurement, the as-prepared aligned lotus leaf-like ZnO nanostructure has a superior gas-sensing response to low-concentration formaldehyde. The superior response may attribute to large surface-to-volume ratio, complete depletion of carriers inside the leaves, and direct transport of carriers between the leaves and the conductive substrate. Based on the ideal crystal-growth mode proposed by periodic bond chain (PBC) theory, a preferential a- and b-axis growth mechanism is suggested to understand the formation of the lotus leaf-like nanostructure. Furthermore, the nanosheets and the leaves of the lotus leaf-like nanostructure have extremely thin thicknesses of ~2–4 nm matching to the mean free path of electrons, coherence length, and the screening length, which may be of great fundamental significance.

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