Nanostructural Growth Investigation of ZnO Nanorods Derived from Chemical Bath Deposition for Transparent Heater Application

One dimensional Zinc Oxide (ZnO) nanostructures in the forms of nanowire, nanorod, nanotube have been attracting scientific and technology interests in the last few years. This current study investigated the effects of chemical bath deposition (CBD) synthesis parameters i.e. seeding solution concentration and growing time on the nanostructural characteristics of ZnO nanorods and considering their potential application for transparent heater. Seed solutions were prepared by dissolving 1:1 equimolar zinc nitrate tetrahydrate and hexamethylenetetraamine in water at 0°C for 1 hour. Upon the synthesis, the seeding solution concentration was varied from 0.005, 0.025, 0.05 M. The formation of thin films containing ZnO nanoseeds was carried out by spin coating the precursors on the conducting indium tin oxide (ITO) glass substrates, followed with annealing at 200^oC for 5 minutes then further growing the ZnO nanorods at 90^oC for 3 hours. Another variation in this work was also carried out by selecting a different route upon CBD process, i.e. with a fixed 0.05 M seeding solution prepared from the same equimolar zinc nitrate tetrahydrate and hexamethylenetetraamine in water at 25^oC for 1 hour, followed with the same annealing stage but continued by variation in the growing stage at 90^oC for different times (3, 4 and 5 hours). ZnO nanorods were characterized using x-ray diffraction, field emission scanning electron microscopy and ultraviolet-visual spectroscopy. The results of these investigations demonstrated that with the increase in reaction time from 3 to 5 hours, the band gap energy, E_g of the nanorods decreased from 3.63 to 3.13 eV, a consequence of the increase in their diameter and crystallite size from 325 to 583 nm and 22.68 to 34.28 nm, respectively. The desired coverage of ZnO nanorods for transparent heater applications was obtained with a 0.05 M seeding solution and 5-hour reaction time.

Chemical bath deposition; Nanorods; Reaction time; Seeding concentration; Zinc oxide

The development of nanostructure materials has attracted scientific and technological interests in the last few decades. This is due to the high demand for these materials in various strategic applications owing to their unique properties at nanometer scale which are different to those of their bulk forms at the macro-and micro-scales. Therefore, one dimensional nanostructures such as nanowires, nanorods, or nanotubes have been seen as a breakthrough in the materials world. In this context, they can be expected to provide a much faster electron transfer via ballistic effect owing to their pipe-like structure.

Zinc oxide (ZnO) is one of inorganic semiconductor oxide materials which play important role in various strategic applications including sensors (Wan et al., 2004), optoelectronic (Law et al., 2005) and piezoelectric devices (Kadota & Miura, 2002). This can be attributed to its wide band gap energy, E_g of 3.07 eV and large exciton binding energy of 60 meV at room temperature (Jin et al., 2005). ZnO is also well-known as one of strategic materials for piezoelectric devices due to the asymmetry in its wurtzite structure. In addition, currently ZnO nanostructures are being considered as potential candidate to substitute for TiO₂ (titanium dioxide) in the dye sensitized solar cell (DSSC) application (Hamann et al., 2008). This is due to its electronic characteristic as direct band gap semiconductor that can be expected to be more responsive in the photon absorption process than TiO₂, which is an indirect band gap semiconductor. Moreover, reducing the size of ZnO structures down to the nanometer scale has been hailed as a breakthrough in enhancing the performance of DSSC devices. This is based on the fact that ZnO nanostructures have a much higher surface-to-volume ratio compared to its bulk form, resulting in a more pronounced interaction with the dye molecules.

In this study ZnO nanorods grown on glass substrates were successfully synthesized via CBD technique involving seeding and growing stages. It has been found that a seeding solution (0.05 M) prepared at 25^oC can provide more homogeneous coverage of ZnO nanorods on the substrate surface with a larger average nanorod diameter, in comparison to previous reactions at 0^oC. This resulted from the ripening mechanism in the seeding stage, which allowed the nuclei to grow more in the subsequent CBD process. Varying CBD growing time from 3 to 5 hours produced nanorods with average crystallite sizes and E_gs of 22.85 to 32.48 nm and 3.63 to 3.13 eV. Considering their nanostructural charateristics and optical properties, the resulting ZnO nanorod samples are promising for transparent heater applications. However, further optimization is still necessary to achieve the desired combination of high transparency and electrical conductivity.

This project was financially supported by the Directorate of Research and Community Services of Universitas Indonesia through the PITTA Research Grant of Universitas Indonesia, Year 2018, contract number 2370/UN2.R3.1/HKP.05.00/2018. 

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