ZnO cacti

Zinc oxide (ZnO) is a II–VI compound, n-type semiconductor with a direct wide-band gap (3.37 eV) and a large exciton binding energy (60 meV). It is one of the most important functional materials thanks to its unique properties, which include piezoelectricity, chemical stability, and its behavior as a catalyst. Furthermore, its biocompatibility and biosafety make it suitable for use in biomedical applications [1] and [2]. It possesses hexagonal wurtzite-type structure, with space group P63mc and lattice parameters a = 3.2 Å and c = 5.2 Å. The wurtzite structure of ZnO can be described as a series of alternating planes composed of tetrahedrally coordinated O2− and Zn2  ions stacked alternately along the c-axis. There is no center of inversion, which allows for anisotropic growth. ZnO exhibits positive (Zn2 ) and negative (O2−) polar facets, which produce a normal dipole moment and a spontaneous polarization along the c-axis, with a divergence in surface energy.

ZnO has an incredibly rich family of nanostructures due to the three fast growing directions: <20>, <010>, and <±0001>. The final structure can be determined by controlling the growth of these directions. The relative growth rate along these directions determines the surface morphology and the aspect ratio of the structure. A variety of nanostructures, such as nanowires, nanobelts, nanorods, pyramids, nanocages, dandelions, branched crystallites, organic capped nanocrystals, nanotubes, hollow beads, nanocones, nanoflowers, and nanostars have all been demonstrated. These structures have been produced using a number of different methods, including vapor–liquid–solid growth, chemical vapor deposition, pulse laser deposition, sputtering, electrodeposition, template based methods, and thermal evaporation.

ZnO is suitable for a wide range of applications, including room-temperature UV lasers, light-emitting diodes, chemical sensors, actuators, varistors, photodetectors, and photonic crystals [3], [4] and [5]. There is considerable interest in using ZnO in dye sensitized solar cells (DSSCs) due to its high electron mobility and stability compared to organic dyes. The properties of ZnO depend on the size, orientation, morphology, aspect ratio, and crystalline density of the microstructures. The surface area also plays a crucial role in many applications: three dimensional (3D) hierarchical branching nanowire structures offer better electron transport while maintaining a large specific surface area [6]. The addition of branching to arrays of nanorods improves dye loading by adding more material to absorb incident light. Moreover, hierarchical nanostructures scatter the light intensely, which increases the interaction between incident light and the material. Therefore, the localization of photons in the anode improves the light harvesting efficiency [7]. Hence 3D nanostructures are applicable for use efficient in solar cells. With this in mind, we have grown ZnO with a cactus-like morphology using a facile aqueous chemical method at relatively low temperatures.

Great care was taken to coat a uniform ZnO seed layer onto ultrasonically cleaned glass substrates. The seed solution was prepared in absolute ethanol with 0.05 M zinc acetate (Zn(CH3COO)2·2H2O) and 0.05 M diethanolamine (HN(CH2CH2OH)2, DEA). The clean glass substrate was dip coated for 10 seconds in a seed solution and then kept at room temperature over night for drying. The dried films were annealed at 400 °C for 5 minutes in air to yield a seed layer of ZnO on the substrate. This seed coating process was repeated twice to ensure the uniform coverage of ZnO seed on the substrate. The seeded substrates were placed vertically in 200 ml of growth solution containing equimolar (0.05 M) amounts of zinc acetate and hexamethylenetetramine (HMTA), and refluxed at 95 ± 3 °C for 5 hours. The resulting film exhibited a dense forest of vertically aligned ZnO nanorods. Various surfactants such as polyethylenimine (PEI), polyacrylic acid (PAA), and diaminopropane (DAP) were added to fine-tune the surface morphology.

Vertically aligned ZnO nanorods, faceted microrods, nanoneedles, and nanotowers were grown on glass substrates. We also studied the influence of pH on morphology. It was found that the nanodisks and nanoflowers were produced using acidic and basic baths, respectively. Finally, the nanoneedle film was re-seeded and re-refluxed at 90 °C for 5 hours in the presence of DAP (∼0.2 ml) in the growth solution.

This month's cover image shows the cactus-like morphology, formed by the combination of two types of needle that contain central primary microneedles, and secondary nanoneedles emanating from the primary. The primary needles are oriented along the c-axis with an average 2 μm tip diameter. The dense secondary nanoneedles possess an average diameter of 65 nm and a length of 9 μm.

Further Reading
[1] J. Qian et al. Adv Mater, 21 (2009), p. 3663
[2] K. Park et al. Adv Mater, 22 (2010), p. 2329
[3] Z.L. Wang, Mater Sci Eng R, 64 (2009), p. 33
[4] J.B. Baxter, E.S. Aydil Sol Energy Mater Sol Cells, 90 (2006), p. 607
[5] R.C. Pawar et al. Sensor Actuat B: Chem, 151 (2010), p. 212
[6] M.J. Bierman, S. Jin Energy Environ Sci, 2 (2009), p. 1050
[7] T.P. Chou et al. Adv Mater, 19 (2007), p. 2588