Optimization of ZnO Seed Layer for Growth of Vertically Aligned ZnO Nanorods on Glass Surface

Vertically well aligned zinc oxide (ZnO) nanorods have been developed for the application to photoanode of dye-sensitized solar cells (DSCs). For the growth of the ZnO nanorods, a seed layer is necessary. Spin coating and chemical bath deposition methods are chosen to form the seed layer and the ZnO nanorods, respectively. The effects of speed and cycle number of spin coating are investigated. The most optimized nanorods was found at seed layer with growing conditions of 3000 RPM rotational speed repeated for 3 times. The SEM image shows the longest length at 1.5 – 2 m length while the XRD chart shows considerable peak at (002) crystallinity.


Introduction
Zinc oxide (ZnO) and titanium dioxide (TiO2) materials are widely used for dye sensitized solar cells (DSCs) applications.Although ZnO-based DSCs usually yield lower conversion efficiency than TiO2-based ones, it still remains as a possible alternatitive due to similar properties with TiO2 and higher electron mobility.One possible method to optimize the efficiency of ZnO-based DSCs is to fabricate vertically aligned nanorods to provide direct electron pathway [1,2].Such structure can potentially reduce the electron recombination rate in the DSCs.A seed layer must be formed first before the growth of vertically aligned nanorods [3,4].Spin coating technique is chosen as the formation method of the seed layer due to easiness of use.In this research, the number of spin coating cycles (number of repetition for the spin coating and heat treatment) and rotational speed of the spin coater influence the roughness and thickness of the seed layer [5,6].Other factors that might affect the diameter and length of the nanorods are the molar ratio of the starting solution, formation duration, formation temperature, refreshing the solution, seed layer thickness, annealing temperature, etc [7][8][9][10][11][12].

Experimental
Normal glass substrates were ultrasonically cleaned with commercially available surfactant [Extran MA 02, Merck], acetone, and ethanol for 5 min, respectively.A seed layer solution was made by mixing 0.7 g of zinc acetate dihydrate with 20 mL of 2-methoxyethanol.The resulting seed layer solution was then added with 10 drops of ethanolamine while stirring at 400 rpm and heated at 60°C for 20 min.The spin coating method was used to deposit the seed solution on the normal glass substrates and followed by thermal treatment at 100°C for 10 min.The spin speed (v) and the cycle number of spin coating followed by the heat treatment (n) were decided as the variables, namely v=3000, 4000 rpm and n=1, 3, 5 cycles, respectively.The final annealing was done at 350°C for 1 h.The growth solution for ZnO nanorods fabrication was made by mixing 30 mL of 0.03 M zinc acetate dihydrate with 30 mL of 0.03 M hexamethyline tetramine.The substrate coated by the seed layer was immersed in the growth solution vertically.The chemical bath deposition was done at 85°C for 2 h.After the nanorods were grown, the substrate was dried at room temperature.The scanning electron microscope (SEM) images were taken to analyze the ZnO seed layers and nanorods.X-ray Diffraction (XRD) profiles were performed to analyze the crystallinity of the seed layers and nanorods.

Results and Discussions
The effects of spin speed (v) and number of growth cycles (n) were analyzed.The effects of v and n can be seen in Fig. 1.When v increases, the surface of the seed layer will become smoother and the seed density could decrease.This result is caused by the increase in kinetic energy due to the increase in the rotational speed.As shown in Fig. 1 (d), the seed grains are more evenly spread compared to Fig. 1(a).The effect of n on the surface of the seed layer is deduced from the smoothness of the surface.When n increases, the seed layers' surface becomes smoother.Further analysis on the effect of v and n towards the growth of the ZnO nanorods need to be performed.The SEM images of the ZnO nanorods can be seen in Fig. 2 -7.The dimensional values of the nanorods' diameter (d) and length (L) can be seen on Table 1.On the case of v = 3000 RPM, d decreases with an increase of n as seen in Fig. 4. One factor that contribute such effect is the non-homogenous spread of the seed layer.This effect could also cause nanorods with differing diameters to grow.The opposite effect can be seen in the case of v = 4000 RPM where d increases as proportional to the increasing n.It can be said that the more homogenously spread of the seed layer in the case of v = 4000 RPM deliver the opposite effect.Moreover, the values of v and n have little to no correlation to the values of L. The crystallinity of the nanorods, which can be seen in Fig. 9, on the vertical direction provide the boost for the length of the nanorods.In the case of v = 3000 RPM and n = 5, the crystallinity along (100) direction obstruct the growth for the vertical direction.Fig. 8 and 9 give the X-ray Diffraction (XRD) profiles of the ZnO seed layers and nanorods, respectively.It can be seen that the XRD profiles of the two structures are considerably different.Such results are deduced as the impact from difference of the dimensional growth, 2-D for the seed layers and 1-D for the nanorods.Another factor to be considered is the growth temperature.As shown in Fig. 8(e), there are two peaks compared to the other XRD profiles.Such difference is attributed due to the difference in the grains' shape.In Fig. 9, the XRD profiles generally have two peaks, a small peak on the direction of (100) while another peak is considerably higher in the (002) direction.Such profiles mean that the nanorods can be considered to be grown vertically aligned.In the case of n = 5, there is a considerable increase in the (100) direction.This result is attributed by the more homogenously spread of the seed layer which decreases the (002) growth.

Conclusions
The seed layer is a major property to grow vertically aligned ZnO nanorods with high surface area.The smoothness and seed density of the seed layer are very important to grow vertically aligned nanorods.The rotational speed (v) and the number of spin cycles (n) have the ability to control the smoothness and seed density of the seed layer.Surface area of the nanorods becomes higher for the thinner nanorods array which was grown on the seed layer with high density.The length of the nanorods is affected by the growth on (002) direction.The vertical growth obstruction is deduced as the result of the growth on (100) crystallinity.The most optimized nanorod is grown on v = 3000 RPM and n = 3 cycles.

Fig. 2 .
Fig. 2. The SEM images of ZnO nanorods for v = 3000 RPM and n = 1 cycle.(a) is the planar view of the ZnO nanorods; (b) is the cross sectional view of the ZnO nanorods.

Table 1 .
The average values of nanorods' diameter (d) and length (L) on the cases of various rotational speed (v) and number of coating cycles (n)