Functionalizing nanostructure material is essential in future technology development.  ZnO nanosheet is one type of metal oxide nanomaterials with its unique structures utilized in many devices, such as dye sensitized solar cell^1,2,3, gas sensor⁴, photodetector^4,5, and piezoelectric power generator⁶. Controlling ZnO nanosheet morphology in terms of its height, density, porous structure, etc. thus is of importance matter to improve the performance of the devices. Deposition of ZnO nanosheet on conductive substrate by electrochemical method offers a simple way to get excellent material-to-substrate attachment which ensure good electron transport needed for electronic devices. Generally the electrochemical deposition have been conducted in the potentiostatic mode^1,2but not galvanostatic because it is considered that the potential decides the deposition reaction. However, for such an insulating nanostructure, it is difficult to directly relate the reaction to the electrode potential. On the other hand, the galvanostatic electrodeposition, that is convenient by using two electrodes system, can give better control over the constant crystal growth rate due to the external current responsible for the growth being always supplied in the constant manner throughout deposition, regardless the changes of the structure.

In this report, galvanostatic electrodeposition of ZnO nanosheet on FTO or ITO substrate is studied to reveal important parameters responsible for controlling its morphology. The Acetate ion was used as the additive in the Zinc-Nitrate-based electrolyte solution in this study. Several key parameters such as applied current density, deposition time and temperature were varied, and the effect to the crystal growth and morphological changes was observed. XRD, SEM and TG/DTA were used to analyze the ZnO crystal growth mechanism, morphological changes and phase transformation during annealing process, respectively.

It was found that both ZnO and Zn₅(CH₃COO)₂(OH)₈·2H₂O crystals were observed in the as-deposited samples, which were then completely transformed to ZnO after annealing at 400 ^oC for 1 hour. Zn₅(CH₃COO)₂(OH)₈·2H₂O is a side product produced due to the presence of Acetate ion, in which the anion’s main role is to inhibit ZnO preferential growth at (002) direction thus produce the desired nanosheet structure as the result. The effect of deposition time on the morphology was examined by varying the deposition time from 1, 5, 30 and 60 minutes with the current density and deposition temperature held constant at –1 mA/cm² and 60 ^oC, respectively. Increasing deposition time was found to increase in nanosheet’s height and density, but further prolonging will promote growth of dense layer at the bottom part of the nanosheet. To observe the effect of applied current density, deposition time and temperature were held constant at 5 minutes and 60 ^oC respectively, with the current density varied from –1, –3 and –5.3 mA/cm². Increasing applied current is found to be effective in increasing nanosheet’s height and density without producing the extra bottom layer. The deposition temperature was varied from 50, 60 and 70 ^oC under the same applied current density and deposition time, –1 mA/cm²and 5 minutes, respectively. The results indicate that the morphology is not sensitive to the temperature change in the range. In this report, optimization strategy to control the ZnO nanosheet morphology and complete explanation of the crystal growth mechanism will be presented and discussed.

References

1. Prog. Photovolt: Res. Appl., 2014, 22:440–451
2. ACS Appl. Mater. Interfaces, 2011, 3: 2358–2367
3. Energy Environ. Sci., 2011, 4: 3448-3455
4. CrystEngComm, 2012, 14: 4582–4588
5. Nature Communications, 2014, 5: 3813. DOI: 10.1038/ncomms4813
6. Scientific Reports, 2013, 3: 2017. DOI: 10.1038/srep02017

Figure 1. XRD of (top left) as-deposited, (bottom left) after annealing samples, and (right) the respective SEM pictures of the as-deposited samples.