Exotic fern-like morphologies

Semiconducting oxides have potential applications in a variety of technologically important devices such as solar cells, photocatalysts, supercapacitors, gas sensors, display devices, electrochromic smart windows, fuel cells and batteries. The exotic nano-morphologies of semiconducting oxides and sulphides have been reported extensively in the literature. Most strikingly, materials such as zinc oxide, titanium oxide, cadmium sulphide and lead sulphide exhibit interesting nano-morphogenesis. Size, shape, aspect ratio, inter-granular connectivity, porosity and surface area to volume ratio significantly affect the chemical and physical properties. The energetics and kinetics of materials can be significantly tailored through morphology to produce highly efficient, cost effective and lightweight materials for advanced applications.

Dendritic crystal growth patterns have been attracting the attention of many scientists due to the remarkable connectivity between the crystals, which enables the construction of high performance electrodes. The significant effect of such a morphology in gas sensors in terms of remarkable gas adsorption/desorption properties has been demonstrated for leaf-like CdS [1]. The dendritic growth patterns have been produced using facile techniques such as electrodeposition and the hydrothermal process. The growth mechanisms have been thoroughly discussed in the literature on the basis of mass transport and diffusion transport limited processes and reaction limited fractal aggregation reactions [2] and [3].

Semiconducting and amphoteric tin oxide (SnO₂) is a key functional material that has been used extensively for various applications. SnO₂ belongs to the important family of oxide materials that combine low electrical resistance with high optical transparency in the visible range and high reflectivity in the infrared range of the electromagnetic spectrum. The formation energy of oxygen vacancies and tin interstitials in SnO₂ is very low and thus these defects form readily, explaining the frequently observed high conductivity of pure but non-stoichiometric SnO₂. SnO₂ is also known as cassiterite. It possesses the rutile structure that has a tetragonal unit cell with space group symmetry P4₂/nmm. The lattice constants are a = b = 4.7374 Å and c = 3.1864 Å. In bulk, all the Sn atoms are sixfold coordinated to threefold coordinated oxygen atoms.

Such intensive study of SnO₂ has been carried out due to its technologically important applications. SnO₂ is regarded as one of the most promising metal oxide semiconductors for sensor applications. The catalysts based on SnO₂ exhibit good activity toward CO/O₂ and CO/NO reactions, and SnO₂ is also suitable for the detection of reducing gases such as H₂, H₂S, and CO, as the electrical conductance of SnO₂ changes in response to these gases at 300 °C [4].

Recently, various morphologies of SnO₂, such as nanowires, nanorods, nanotubes, diskettes, V-shapes, nanoflowers, nanoribbons, and hollow nanoarchitectures have been reported [5]. These nanostructures have been synthesized through various physical and chemical methods. However, a dendritic or fern-like morphology of SnO₂ is rarely created.

The image on this issue's cover of Materials Today shows a 3D fern-like morphology of hydrothermally grown SnO₂ thin films deposited directly onto glass substrates for gas sensor applications, using stannic chloride, sodium hydroxide and ethanol. Various preparative parameters such as deposition time, temperature, pressure and solution concentration were judiciously optimized so as to reproducibly yield the fern-like morphology.

The pH of the solution was adjusted to 12 and the clear solution was transferred to a 25 mL teflon-lined stainless steel autoclave up to 80% of the total volume. An ultrasonically cleaned glass substrate was immersed in the solution vertically and the autoclave was sealed and maintained in a furnace at 180 °C for 18 h. The surface morphology was examined via scanning electron microscopy (SEM model JEOL-JSM 6360), and the resulting film was shown to exhibit a dense forest of ferns/dendrites with a 3D architecture of SnO₂. The fern leaves emanated from a single nucleation center with nanoscale branches of length ~3–4 μm at an angle ∼120° growing in an outward direction from the midrib. The large-scale periodical forest of ferns with 3D architecture makes the material a potential candidate for applications in ultra-sensitive gas sensing and exciton based photonic devices.

This work is supported by the Defence Research and Development Organization (DRDO), New Delhi, project no. DRDO/ERIP/ER/0803719/M/01/1343 and Human Resources Development program (No.20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Trade, Industry and Energy.

Further reading

[1] X. Fu et al., J. Mater. Chem., 22 (2012), p. 17782
[2] K.-S. Choi, Dalton Trans., 40 (2008), p. 5389
[3] E.J.H. Lee et al., Chem. Phys., 328 (2006), p. 229
[4] M. Batzill, U. Diebold, Prog. Surf Sci., 79 (2005), p. 47
[5] Z.R. Dai et al., J. Am. Chem. Soc., 124 (2002), p. 8673