Cover Image: Issue 10, Materials Today.
Cover Image: Issue 10, Materials Today.

Zinc oxide (ZnO) is perhaps the most intensively investigated oxide material due to its wide range of technological applications owing to its interesting optical, electrical, thermal and catalytic properties. It has been successfully employed in a plethora of applications such as gas sensors, UV lasers (exciton binding energy ~ 60 meV), photocatalysis, dye sensitized solar cells and lightemitting diodes. Moreover ZnO exhibits visible transparency (optical band gap energy ~ 3.37 eV) and electrical conductivity; and so is useful for transparent conducting oxides. Recently its use as a piezoelectric component has also been demonstrated [1].

ZnO has been synthesized using a variety of simple and cost-effective solution routes such as hydrothermal, solvothermal, sol–gel, solid state reaction, electrodeposition and template assisted growth processes. Exotic nano-morphologies like nanorods, nanowires, nanoneedles, nanobelts, nanotubes, nanocables, nanoprisms and nanospheres have been reported. The morphogenesis shows the possibility of controlling the morphology of ZnO nanostructures by employing suitable synthetic process [2,3]. This month’s cover image shows beautiful cabbage like morphology of the spray pyrolyzed Sn–ZnO thin films grown by using heterogeneous solutions. The image was obtained by the scanning electron microscope (SEM) and has been colored.

Spray pyrolysis is a simple and cost-effective solution route to synthesize a wide variety of high quality oxide and chalcogenide thin films [4]. However, this method has been sparsely employed to synthesize ZnO nanostructures. Based on the inherent simplicity and compositional flexibility, spray pyrolysis processes have the potential to expand the availability and compositional range of nanoparticles. We have reported a variety of spray deposited morphologies of oxides, such as; wheels, doughnuts, wires, fibers, golf-ball-like, reticulated, etc. In the spray pyrolysis deposition process, a precursor solution is atomized into discrete droplets andn subsequently transported to heated substrates. At the proper temperature the droplets or solid particles are decomposed and then sintered or melted on the heated substrate to form a thin solid film. However, various preparative conditions affect the quality of the samples deposited, such as the nature of the precursor solutions (true solution, colloidal dispersions, emulsions and sols), atomization technique (pneumatic, ultrasonic, electrostatic, etc.), substrate temperature, solution concentration, nozzle to substrate distance, growth rate, etc. [5]. An ideal deposition condition is when the droplet approaches the substrate just as the solvent is completely removed. The morphology of the thin films depends on the processes of droplet landing, reaction and solvent evaporation, which are related to droplet size and momentum. By tuningthese processes various composite materials, magnetic materials and porous nanomaterials are made.

We have spray deposited pristine and doped (Ni, Co, Cu, Au, Ag, Sn) ZnO thin films with various morphologies [6–8]. Particularly for Sn–ZnO thin films the precursor solution containing zinc acetate and tin chloride was used in appropriate volumetric proportions. In a typical experiment, 0.1 Maqueous solution of tin chloride was added to 0.4 M zinc acetate solution. The solution for 20 at% tin chloride remained slightly colloidal in spite of the addition of acetic acid. Thus the solution sprayed was heterogeneous, containing zinc ions and tin hydroxide particles.The substrate temperature was kept constant at 450 ºC and the nozzle to substrate distance was 22 cm. The sprayed droplets experience an incremental temperature gradient and undergo various reactions, such as solvent evaporation, solute condensation and vaporization, and transport of vapors toward the heated substrates to form a thin film. Zinc decomposes effectively and forms nano-hexagonal sheets on the substrates, whereas tin hydroxide colloidal particles do not undergo complete thermal decomposition and hence get deposited as spherical particles and then decompose into tin oxide. The coalescence and agglomeration of these particles subsequently forms bigger spherical particles. This can occur as long as the precursor solution containing tin phase is present. During the formation of spherical particles the simultaneous deposition of zinc oxide nanosheets is continued. Hence a composite of bigger spherical balls with embedded and (002) oriented ZnO nanosheets is formed. This particular morphology may be useful in dye sensitized solar cells based on ZnO, wherein the effective light scattering via spherical particles and the subsequent increment in the optical path length can be utilized for maximum light harvesting.

This work is partially supported by the Human Resource Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 20124010203180) and the Core Technology Development Program for Next generation Solar Cells of Research Institute for Solar and Sustainable Energies (RISE), GIST.

Further reading
[1] Z.L. Wang, Mater. Today 10 (5) (2007) 20.
[2] Z.W. Pan, et al. Science 291 (2001) 1947.
[3] Z.L. Wang, Mater. Today 7 (6) (2004) 26.
[4] P.S. Patil, Mater. Chem. Phys. 59 (1999) 185.
[5] W.H. Suh, et al. JACS 127 (2005) 12007.
[6] N.L. Tarwal, et al. Appl. Phys. A 109 (2012) 591.
[7] N.L. Tarwal, et al. Electrochim. Acta 56 (2011) 6510.
[8] N.L. Tarwal, et al. Electrochim. Acta 72 (2012) 32.

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DOI: 10.1016/j.mattod.2013.09.006