Sunflower arrays

ZnO is a promising material for the realization and future of nanotechnology. With its wide band-gap (3.37 eV), high excitonic binding energy, and high breakdown strength, ZnO can be utilized for electronic and photonic devices, as well as for high-frequency applications [1]. Moreover, one-dimensional ZnO nanostructures have attracted a great deal of attention over the past few years due to their novel properties and the potential applications in optoelectronics and device miniaturization. Up to now, ZnO nanostructures with various morphologies have been widely investigated [2], and structures of ZnO including prismatic, needle-like, ellipsoidal, tetrapod-like, nanorod, nanofiber, nanobelts, and nanotubes have been prepared by various physical and chemical methods. It is well known that the novel properties of nanostructured materials are dependent on the morphology; therefore, development of a morphologically controllable synthesis method of ZnO semiconductor is urgently needed to fully explore the potential applications of ZnO [3].

In recent years, control over the properties of ZnO through either chemical or physical routes has attracted increasing interest, and many methods for the synthesis of ZnO structures have been reported [4]. Among the numerous synthesis methods, the hydrothermal method is the most attractive, as it allows perfect control of morphology, purity, crystallinity, and composition, as well as low cost for large-scale production [5]. Until now, although various precursors have been adopted to synthesize ZnO nanostructures, the growth of different ZnO nanostructures with well controlled crystalline morphology is still to be well exploited. It is worth noting that in order to enhance the (multi-)functionality and tailor morphologies, the synthesis of complex nanostructures, especially nanoflowers, has attracted many researchers.

In our work, patterned sapphire with a cone shaped frustum was designed to grow ZnO nanoflower arrays. The ZnO nanostructures are grown in a two-step approach, including the synthesis of a homo-seed layer and the growth of ZnO nanostructures in aqueous solutions at low temperature. From the scanning electron microscope (SEM) image, the nanostructures show sunflower-like morphology and are distribute regularly along the patterned sapphire. Due to the geometry of the patterned sapphire, ZnO nanostructures are grown along the upper base and sidewall region on the cone shaped sapphire frustum, forming a sunflower-like morphology as a whole. Each flower is composed of nanorods along the top base and sidewall of the frustum. The nanorods are around 1 μm in length with a diameter of about 100–200 nm. Meanwhile, some larger nanorods with diameters of around 400 nm surround the edge of the top base formed by the self-organized growth. The patterned sapphire directs ZnO to form sunflower-like nanostructures due to the multiple nucleation on the frustum surface. It is known that the nucleation on bare and smooth substrates requires relatively higher energy (or supersaturation degree) than the crystal growth. The presence of a precoated ZnO seeding layer can eliminate this process, thus maintaining a higher supersaturation degree during the crystal growth.

The image featured on this issue's cover was obtained by field emission scanning electron microscopy (FE-SEM Hitachi S-4800). The ZnO sunflower arrays grown on patterned sapphire show high photocatalytic activity. This behavior is mainly due to the high surface-to-volume ratio and high content of oxygen vacancies, which is also confirmed by photoluminescence. The large-scale periodical sunflower-like characteristics make the material a potential candidate for applications in ultra-sensitive gas sensing and exciton based photonic devices.

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51102036),973 Program (Grant No. 2012CB626801), National Natural Science Foundation of China (Grant No.11274057), Fundamental Research Funds for the Central Universities DC12010207, Program for Liaoning Excellent Talents in University (No. LJQ2012116) and the Opening Project of Chongqing Key Laboratory of Micro/Nano Materials Engineering and Technology (No. KFJJ1205).

Further reading
[1] M.C. Newton et al. Mater. Today, 10 (5) (2007), p. 40
[2] M. Telford, Mater. Today, 8 (11) (2005), p. 10
[3] J.Y. Jung et al. Curr. Appl. Phys., 8 (6) (2008), p. 720
[4] H. Li et al. Mater. Lett., 65 (23–24) (2011), p. 3440
[5] K. Pal et al. J. Mol. Struct., 1027 (14) (2012), p. 36