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Optimization of experimental parameters based on the Taguchi robust design for the formation of zinc oxide nanocrystals by solvothermal method
Materials Research Bulletin 46 (2011) 639–642
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optimization of experimental parameters based on the Taguchi robust design for the formation of zinc oxide nanocrystals by solvothermal method Doungporn Yiamsawas, Kanittha Boonpavanitchakul, Wiyong Kangwansupamonkon * National Nanotechnology Center, National Science and Technology Development Agency, 111 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 May 2010 Received in revised form 22 December 2010 Accepted 1 February 2011 Available online 1 March 2011
Zinc oxide (ZnO) nanoparticles and nanorods were successfully synthesized by a solvothermal process. Taguchi robust design was applied to study the factors which result in stronger ZnO nanocrystal growth. The factors which have been studied are molar concentration ratio of sodium hydroxide and zinc acetate, amount of polymer templates and molecular weight of polymer templates. Transmission electron microscopy and X-ray diffraction technique were used to analyze the experiment results. The results show that the concentration ratio of sodium hydroxide and zinc acetate ratio has the greatest effect on ZnO nanocrystal growth. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis D. Crystal structure
1. Introduction ZnO nanocrystals are an interesting material because of their wide band gap energy (3.37 eV) and large exciting binding energy (60 meV), and therefore can be used as a photocatalyst [1]. They have a large potential for industrial applications such as sensors [2–4], solar cells [5,6], dyes [7,8], etc. Most applications require controlling the shape of the nanocrystal. Our group has investigated applications of the ZnO nanostructure and also analyzed the effect of some parameters on ZnO growth by solvothermal process [9,10]. Previously, Zhang et al. [11] and Yang et al. [12] noted the influence of important factors such as surfactant and molar ratio of Zn(OAc)2 and NaOH on the size and shape of ZnO nanocrystal growth via a surfactant-assisted alcohol thermal process at a low temperature. In contrast, other reports have not detected effects due to these factors [13]. This discrepancy may be due to the fact that analysis using conventional methods to optimize each factor such as analyzing one factor at a time (OFAT) or full factorial analysis is complicated and inefficient [14]. Taguchi is an engineering method for product or process design that focuses on minimizing variation or sensitivity to noise. Taguchi designs are balanced, that is, no factor is weighted to a greater or lesser degree in an experiment, thus allowing each factor to be analyzed independently of the other factors. The major concepts in Taguchi experimental design are; (1) factor: studied variable affecting the
* Corresponding author. Tel.: +66 2 0564 7100; fax: +66 2 0564 6981. E-mail address: [email protected] (W. Kangwansupamonkon). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.02.004
response of an experiment; (2) level: values of studied factors in an experiment; (3) orthogonal array (OA): in experimental design, there is different set of OA, for example, L9 array is used for 3 factors each in 3 levels; (4) optimum condition: optimum conditions to be found involve three major categories (a) the maximum value is the best, (b) the minimum value is the best and (c) typical value is optimum one. After performing experiments, analysis would be done according to the OA. Analysis is performed in order to achieve one or more of the following goals: (1) determination of the optimum conditions for achieving the optimum value. (2) Determination of the contribution received from each factor. (3) Prediction of the response at optimized conditions [15,16,12,17]. In this study, the preparation of ZnO nanocrystals via solvothermal process was studied by using the Taguchi method to determine the principal influence of three factors each in three levels on ZnO nanocrystal growth. 2. Experiment 2.1. Synthesis of ZnO nanocrystals Synthesis of ZnO was carried out by solvothermal process at 80 8C [13]. Poly (vinyl pyrrolidone) (PVP, Aldrich) was dissolved in absolute ethanol with stirring at room temperature. Then, zinc acetate dihydrate (Zn(CH3COO)22H2O, Fluka) was slowly added to the solution. Subsequently, the solid sodium hydroxide (NaOH, Aldrich) was added to the reaction mixture. The resulting solution was stirred for several minutes. The solution was then transferred to a glass vessel sealed with gas impermeable film and heated at
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
640 Table 1 Factors and levels used in this experiment.
2.2. Characterization of synthesized ZnO nanocrystals
Parameters
Levels
(A) Molar concentration ratio of [NaOH]/[Zn2+] (B) Amount of polymer template (g) (C) Molecular weight of polymer template
1
2
3
1 0.2 K15
5 0.6 K30
10 0.8 K90
Solvent = EtOH 30 ml, temperature = 80 8C, time = 24 h.
Table 2 L9 (33) orthogonal array. Experiment no.
A
B
C
Error
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
80 8C for 24 h. After cooling to room temperature, the white powder was precipitated and then washed with absolute ethanol several times to dissolve other impurities. Finally, the powder was dried at 60 8C overnight and its crystals classified in terms of their structure and morphology.
The crystalline structures of the products were analyzed by Xray diffraction (XRD) analysis using a JOEL JDX-3530 diffractometer. The X-ray diffraction (XRD) patterns were recorded from 208 to 808 in 2u with a scanning rate of 0.28/s. The size and morphology of the products were observed by transmission electron microscopy (TEM) which was taken using a JOEL JEM-2010 electron microscope with an accelerating voltage of 200 kV in bright field. The average aspect ratio (length/width) of ZnO nanostructures and its standard deviation of the prepared ZnO nanostructures were examined through TEM images taken at 100,000 magnification and were estimated by counting 50 particles from representative TEM images of samples.
3. Experimental design 3.1. Selection of factors and their levels Three controlling factors, which were the molar concentration of [NaOH]/[Zn2+], amount of polymer templates (PVP) and molecular weight of polymer templates (PVP), were considered regarding their effect on the aspect ratio of ZnO nanocrystals. The three factors that could affect the particle size were assigned three levels which are summarized in Table 1.
Fig. 1. TEM images of ZnO nanocrystal from experiment nos. 1–9.
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
641
Table 4 Response table for signal to noise ratio of each factor at each level.
3.2. Determination of orthogonal arrays The number of factors and levels of particular factors designates the smallest orthogonal array that is possible to use. Although this results in lowest resolution, it can save work operational time and costs. Larger orthogonal array, which, of course, provides a higher resolution potential, can give more accurate result but it requires a lot of efforts. Taguchi robust design method with L9 (33: 3 factors at 3 levels each) orthogonal array is the smallest orthogonal array of three factors at three levels which was selected to optimize in some previous experiments [18,19]. In this experiment, the orthogonal array of L9 was also determined by Taguchi design as shown in Table 2. L9 means 9 experiments reduced from full factorial 33 = 27 runs. The last column of orthogonal array was defined as an error term to increase the accuracy of the analysis.
Parameter
A B C
Mean S/N ratio for aspect ratio
Mean S/N ratio for SD
Level 1
Level 2
Level 3
Level 1
Level 2
Level 3
1.17 6.53 8.49
7.71 7.41 5.77
13.91 8.84 8.52
4.2 5.61 7.83
10.42 4.78 5.85
15.05 10.86 7.57
4. Results and discussion Fig. 1 shows TEM images of ZnO nanocrystals that were synthesized from 9 experiments. Spherical-like and rod-like shaped of ZnO nanocrystals can be seen. A report to describe the mechanism of ZnO nanocrystal growth has also been prepared [12]. It was proposed that some small clusters of ZnO are formed first. Then, smaller clusters bonded together to form a larger cluster. Because its side surfaces are tightly passivated by polymer templates, the growth velocity of (0 0 1) direction is larger than other directions. 4.1. Taguchi design method A Taguchi method design was applied to determine the most influential factor on ZnO nanocrystal growth by using the signal to noise (S/N) ratio for determining the quality characteristics deviating from the desired value. Since the larger aspect ratio of length to width (L/W) of ZnO has a stronger influence on ZnO growth, the characteristic of S/N ratio was chosen for problem of scale. In this case, S/N ratio was calculated by the following formula [16]: S=N ¼ 10 log
ð1=Y12 þ 1=Y22 þ 1=Y32 þ Þ n
where Yi is the aspect ratio and standard deviation of ZnO nanostructure for each experiment (determined from TEM), n is the replication number for each experiment (n = 50). The S/N ratio of aspect ratio and standard deviation of ZnO nanocrystal for 9 experiments are calculated as shown in Table 3. Then, the S/N ratio response to each factor at each level were obtained from the orthogonal array experiments which calculated from the average S/N ratio for each level of the factors as shown in Table 4. The response table and main effects plot (Fig. 2) were applied to
Fig. 2. Main effect plots for signal to noise ratio of aspect ratio.
compare the relative magnitude of the effect by calculating the highest average for each factor minus the lowest average for each factor. From that evaluation, among the three parameters that affected ZnO nanocrystal growth, molar concentration ratio of [NaOH]/[Zn2+] was determined to have the strongest effect on ZnO nanocrystal growth. 4.2. Predicting results and confirmation To predict the response (aspect ratio) at optimized condition, the optimized level of each factor was selected. In this case, since the optimum condition for ZnO nanocrystal growth were determined by increasing the S/N ratio, the larger contribution of one control factor at that level increased the aspect ratio of ZnO growth. Thus the resulting optimum factors for high aspect ratio ZnO nanocrystal were determined to be A3, B3 and C3. To reconfirm the S/N ratio and aspect ratio, the optimal level (A3B3C3) of the design factors were selected for preparation ZnO nanostructure. XRD pattern of synthesized ZnO samples from optimum condition is illustrated in Fig. 3. All peaks of the obtained results correspond to the hexagonal wurtzite structure of ZnO reported in JCDDS card (No. 36-1451, a = 3.249 A˚, c = 5.206 A˚). No
Table 3 Aspect ratios of ZnO nanocrystals and signal-to-noise ratios (S/N) of 9 runs. Experiment no.
1 2 3 4 5 6 7 8 9
Aspect ratio (L/W)
Standard deviation (SD) ()
Data
S/N
Data
S/N
1.13 1.05 1.11 2.94 2.79 3.91 5.79 5.30 5.19
0.99 0.30 2.21 3.69 8.45 11.00 14.92 13.48 13.32
0.13 0.08 0.28 0.62 0.62 1.29 1.07 1.44 1.53
5.21 8.92 1.45 9.13 8.34 13.79 12.91 14.91 17.33
Fig. 3. XRD pattern of zinc oxide nanocrystal from optimum condition.
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
642
rized in Table 5. Finally, the predicted S/N ratio and predicted aspect ratio were 16.14 and 6.41, respectively. However, the actual obtained result is shown in Table 5. The experiment aspect ratio for optimum condition was 6.39, which is slightly lower compared with the Taguchi method design. As expected with optimum conditions under Taguchi design, gain in S/N ratio and aspect ratio can be decreased and increased with 0.31 of predicted aspect ratio. This indicated that Taguchi experiment design has been successfully applied to the determination and prediction of the aspect ratio of prepared ZnO nanocrystal with polymer template in solvothermal method. 5. Conclusions The synthesis of ZnO nanostructures via solvothermal method has been investigated by Taguchi robust design. ZnO nanoparticles and nanorods have been obtained under three different parameters which are [NaOH]/[Zn2+] ratio, amount of PVP and molecular weight of PVP. From the Taguchi method analysis, the [NaOH]/ [Zn2+] ratio plays the most important factor on the aspect ratio of prepared ZnO. The results also indicated that the Taguchi method can provide an advantage for design the size and shape of ZnO nanostructure. Fig. 4. TEM image of ZnO nanocrystal from optimum condition.
Acknowledgements Table 5 Prediction and confirmation experimental for ZnO nanocrystal growth.
The authors are grateful for research funding from National Nanotechnology Center and the Institute of Solar Energy Technology Development under National Science and Technology Development Agency, Thailand (grant number: B22 AR010110RDAR01).
Optimal level A3B3C3
Aspect ratio
Standard deviation ()
S/N
Data
S/N
Data
Prediction Confirmation
16.14 15.80
6.41 6.39
10.11 13.50
0.31 1.09
characteristic peaks were observed for impurities. TEM image as shown in Fig. 4 revealed the nanorod structure of the synthesized ZnO. These results confirmed that the ZnO nanostructure was successfully synthesized by the solvothermal reaction and also proved that the ZnO nanostructure could crystallize well at low temperature which was consistent with the previous studies [15]. The predicted S/N of aspect ratio and standard deviation by using the optimal level of each factor is calculated from following equations: S=Npredicted ¼ S=Nm þ ðS=NA3 S=Nm Þ þ ðS=NB3 S=Nm Þ þ ðS=NC3 S=Nm Þ and S=Npredicted ¼ 10 log 1=Y 2 where S/Nm is the mean total S/N ratio and S/NA3, S/NB3, S/NC3 are the mean S/N ratio at the optimal level. Then Y, which is the predicted aspect ratio and standard deviation at the optimal level, was calculated by using above equation. The results are summa-
References [1] M.D. Driessen, T.M. Miller, V.H. Grassian, J. Mol. Catal. A: Chem. 131 (1998) 149– 156. [2] P. Mitra, A.P. Chatterjee, H.S. Maiti, Mater. Lett. 35 (1998) 33–38. [3] H. Tang, Y. Li, C. Zheng, J. Ye, X. Hou, Y. Lv, Talanta 72 (2007) 1593–1597. [4] N. Hongsith, C. Viriyaworasakul, P. Mangkorntong, N. Mangkorntong, S. Choopun, Ceram. Int. 34 (2008) 823–826. [5] J.B. Baxter, E.S. Aydil, Sol. Energy Mater. Sol. Cells 90 (2006) 607–622. [6] A.E. Suliman, Y. Tang, L. Xu, Sol. Energy Mater. Sol. Cells 91 (2007) 1658–1662. [7] N. Daneshvar, D. Salari, A.R. Khataee, J. Photochem. Photobiol. A: Chem. 162 (2004) 317–322. [8] H. Wang, C. Xie, W. Zhang, S. Cai, Z. Yang, Y. Gui, J. Hazard. Mater. 141 (2007) 645– 652. [9] D. Yiamsawas, K. Boonpavanitchakul, W. Kangwansupamonkon, Proceeding of the 2008 International Conference on Nanoscience and Nanotechnology (ICONN2008), Melbourne, Victoria, Australia, February 25–29, 2008. [10] D. Yiamsawas, K. Boonpavanitchakul, W. Kangwansupamonkon, J. Microsc. Soc. Thailand 23 (2009) 75–78. [11] H. Zhang, J. Feng, J. Wang, M. Zhang, Mater. Lett. 61 (2007) 5202–5205. [12] C. Yang, Y. Li, G. Xu, X. Ma, J. Mater. Sci. Technol. 23 (2007) 583–586. [13] C. Wang, E. Shen, E. Wang, L. Gao, Z. Kang, C. Tian, Y. Lan, C. Zhang, Mater. Lett. 59 (2005) 2867–2871. [14] P. Ali, A.-F. Mohammad Sadegh, Polym. Int. 56 (2007) 283–289. [15] W.L. Liu, S.H. Hsieh, W.J. Chen, J.H. Lee, Surf. Coat. Technol. 201 (2007) 9238–9242. [16] G.S. Peas, Taguchi Methods, Addison-Wesley, Singapore, 1992. [17] C.C. Chen, C.C. Tsao, Y.C. Lin, C.Y. Hsu, Ceram. Int. 36 (2010) 979–988. [18] K.D. Kim, D.W. Choi, Y.H. Choa, H.T. Kim, J. Mater. Process. Technol. 202 (2008) 569–573. [19] M. Barmala, A. Moheb, R. Emadi, J. Alloys Compd. 485 (2009) 778–782.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optimization of experimental parameters based on the Taguchi robust design for the formation of zinc oxide nanocrystals by solvothermal method Doungporn Yiamsawas, Kanittha Boonpavanitchakul, Wiyong Kangwansupamonkon * National Nanotechnology Center, National Science and Technology Development Agency, 111 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 May 2010 Received in revised form 22 December 2010 Accepted 1 February 2011 Available online 1 March 2011
Zinc oxide (ZnO) nanoparticles and nanorods were successfully synthesized by a solvothermal process. Taguchi robust design was applied to study the factors which result in stronger ZnO nanocrystal growth. The factors which have been studied are molar concentration ratio of sodium hydroxide and zinc acetate, amount of polymer templates and molecular weight of polymer templates. Transmission electron microscopy and X-ray diffraction technique were used to analyze the experiment results. The results show that the concentration ratio of sodium hydroxide and zinc acetate ratio has the greatest effect on ZnO nanocrystal growth. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis D. Crystal structure
1. Introduction ZnO nanocrystals are an interesting material because of their wide band gap energy (3.37 eV) and large exciting binding energy (60 meV), and therefore can be used as a photocatalyst [1]. They have a large potential for industrial applications such as sensors [2–4], solar cells [5,6], dyes [7,8], etc. Most applications require controlling the shape of the nanocrystal. Our group has investigated applications of the ZnO nanostructure and also analyzed the effect of some parameters on ZnO growth by solvothermal process [9,10]. Previously, Zhang et al. [11] and Yang et al. [12] noted the influence of important factors such as surfactant and molar ratio of Zn(OAc)2 and NaOH on the size and shape of ZnO nanocrystal growth via a surfactant-assisted alcohol thermal process at a low temperature. In contrast, other reports have not detected effects due to these factors [13]. This discrepancy may be due to the fact that analysis using conventional methods to optimize each factor such as analyzing one factor at a time (OFAT) or full factorial analysis is complicated and inefficient [14]. Taguchi is an engineering method for product or process design that focuses on minimizing variation or sensitivity to noise. Taguchi designs are balanced, that is, no factor is weighted to a greater or lesser degree in an experiment, thus allowing each factor to be analyzed independently of the other factors. The major concepts in Taguchi experimental design are; (1) factor: studied variable affecting the
* Corresponding author. Tel.: +66 2 0564 7100; fax: +66 2 0564 6981. E-mail address: [email protected] (W. Kangwansupamonkon). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.02.004
response of an experiment; (2) level: values of studied factors in an experiment; (3) orthogonal array (OA): in experimental design, there is different set of OA, for example, L9 array is used for 3 factors each in 3 levels; (4) optimum condition: optimum conditions to be found involve three major categories (a) the maximum value is the best, (b) the minimum value is the best and (c) typical value is optimum one. After performing experiments, analysis would be done according to the OA. Analysis is performed in order to achieve one or more of the following goals: (1) determination of the optimum conditions for achieving the optimum value. (2) Determination of the contribution received from each factor. (3) Prediction of the response at optimized conditions [15,16,12,17]. In this study, the preparation of ZnO nanocrystals via solvothermal process was studied by using the Taguchi method to determine the principal influence of three factors each in three levels on ZnO nanocrystal growth. 2. Experiment 2.1. Synthesis of ZnO nanocrystals Synthesis of ZnO was carried out by solvothermal process at 80 8C [13]. Poly (vinyl pyrrolidone) (PVP, Aldrich) was dissolved in absolute ethanol with stirring at room temperature. Then, zinc acetate dihydrate (Zn(CH3COO)22H2O, Fluka) was slowly added to the solution. Subsequently, the solid sodium hydroxide (NaOH, Aldrich) was added to the reaction mixture. The resulting solution was stirred for several minutes. The solution was then transferred to a glass vessel sealed with gas impermeable film and heated at
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
640 Table 1 Factors and levels used in this experiment.
2.2. Characterization of synthesized ZnO nanocrystals
Parameters
Levels
(A) Molar concentration ratio of [NaOH]/[Zn2+] (B) Amount of polymer template (g) (C) Molecular weight of polymer template
1
2
3
1 0.2 K15
5 0.6 K30
10 0.8 K90
Solvent = EtOH 30 ml, temperature = 80 8C, time = 24 h.
Table 2 L9 (33) orthogonal array. Experiment no.
A
B
C
Error
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
80 8C for 24 h. After cooling to room temperature, the white powder was precipitated and then washed with absolute ethanol several times to dissolve other impurities. Finally, the powder was dried at 60 8C overnight and its crystals classified in terms of their structure and morphology.
The crystalline structures of the products were analyzed by Xray diffraction (XRD) analysis using a JOEL JDX-3530 diffractometer. The X-ray diffraction (XRD) patterns were recorded from 208 to 808 in 2u with a scanning rate of 0.28/s. The size and morphology of the products were observed by transmission electron microscopy (TEM) which was taken using a JOEL JEM-2010 electron microscope with an accelerating voltage of 200 kV in bright field. The average aspect ratio (length/width) of ZnO nanostructures and its standard deviation of the prepared ZnO nanostructures were examined through TEM images taken at 100,000 magnification and were estimated by counting 50 particles from representative TEM images of samples.
3. Experimental design 3.1. Selection of factors and their levels Three controlling factors, which were the molar concentration of [NaOH]/[Zn2+], amount of polymer templates (PVP) and molecular weight of polymer templates (PVP), were considered regarding their effect on the aspect ratio of ZnO nanocrystals. The three factors that could affect the particle size were assigned three levels which are summarized in Table 1.
Fig. 1. TEM images of ZnO nanocrystal from experiment nos. 1–9.
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
641
Table 4 Response table for signal to noise ratio of each factor at each level.
3.2. Determination of orthogonal arrays The number of factors and levels of particular factors designates the smallest orthogonal array that is possible to use. Although this results in lowest resolution, it can save work operational time and costs. Larger orthogonal array, which, of course, provides a higher resolution potential, can give more accurate result but it requires a lot of efforts. Taguchi robust design method with L9 (33: 3 factors at 3 levels each) orthogonal array is the smallest orthogonal array of three factors at three levels which was selected to optimize in some previous experiments [18,19]. In this experiment, the orthogonal array of L9 was also determined by Taguchi design as shown in Table 2. L9 means 9 experiments reduced from full factorial 33 = 27 runs. The last column of orthogonal array was defined as an error term to increase the accuracy of the analysis.
Parameter
A B C
Mean S/N ratio for aspect ratio
Mean S/N ratio for SD
Level 1
Level 2
Level 3
Level 1
Level 2
Level 3
1.17 6.53 8.49
7.71 7.41 5.77
13.91 8.84 8.52
4.2 5.61 7.83
10.42 4.78 5.85
15.05 10.86 7.57
4. Results and discussion Fig. 1 shows TEM images of ZnO nanocrystals that were synthesized from 9 experiments. Spherical-like and rod-like shaped of ZnO nanocrystals can be seen. A report to describe the mechanism of ZnO nanocrystal growth has also been prepared [12]. It was proposed that some small clusters of ZnO are formed first. Then, smaller clusters bonded together to form a larger cluster. Because its side surfaces are tightly passivated by polymer templates, the growth velocity of (0 0 1) direction is larger than other directions. 4.1. Taguchi design method A Taguchi method design was applied to determine the most influential factor on ZnO nanocrystal growth by using the signal to noise (S/N) ratio for determining the quality characteristics deviating from the desired value. Since the larger aspect ratio of length to width (L/W) of ZnO has a stronger influence on ZnO growth, the characteristic of S/N ratio was chosen for problem of scale. In this case, S/N ratio was calculated by the following formula [16]: S=N ¼ 10 log
ð1=Y12 þ 1=Y22 þ 1=Y32 þ Þ n
where Yi is the aspect ratio and standard deviation of ZnO nanostructure for each experiment (determined from TEM), n is the replication number for each experiment (n = 50). The S/N ratio of aspect ratio and standard deviation of ZnO nanocrystal for 9 experiments are calculated as shown in Table 3. Then, the S/N ratio response to each factor at each level were obtained from the orthogonal array experiments which calculated from the average S/N ratio for each level of the factors as shown in Table 4. The response table and main effects plot (Fig. 2) were applied to
Fig. 2. Main effect plots for signal to noise ratio of aspect ratio.
compare the relative magnitude of the effect by calculating the highest average for each factor minus the lowest average for each factor. From that evaluation, among the three parameters that affected ZnO nanocrystal growth, molar concentration ratio of [NaOH]/[Zn2+] was determined to have the strongest effect on ZnO nanocrystal growth. 4.2. Predicting results and confirmation To predict the response (aspect ratio) at optimized condition, the optimized level of each factor was selected. In this case, since the optimum condition for ZnO nanocrystal growth were determined by increasing the S/N ratio, the larger contribution of one control factor at that level increased the aspect ratio of ZnO growth. Thus the resulting optimum factors for high aspect ratio ZnO nanocrystal were determined to be A3, B3 and C3. To reconfirm the S/N ratio and aspect ratio, the optimal level (A3B3C3) of the design factors were selected for preparation ZnO nanostructure. XRD pattern of synthesized ZnO samples from optimum condition is illustrated in Fig. 3. All peaks of the obtained results correspond to the hexagonal wurtzite structure of ZnO reported in JCDDS card (No. 36-1451, a = 3.249 A˚, c = 5.206 A˚). No
Table 3 Aspect ratios of ZnO nanocrystals and signal-to-noise ratios (S/N) of 9 runs. Experiment no.
1 2 3 4 5 6 7 8 9
Aspect ratio (L/W)
Standard deviation (SD) ()
Data
S/N
Data
S/N
1.13 1.05 1.11 2.94 2.79 3.91 5.79 5.30 5.19
0.99 0.30 2.21 3.69 8.45 11.00 14.92 13.48 13.32
0.13 0.08 0.28 0.62 0.62 1.29 1.07 1.44 1.53
5.21 8.92 1.45 9.13 8.34 13.79 12.91 14.91 17.33
Fig. 3. XRD pattern of zinc oxide nanocrystal from optimum condition.
D. Yiamsawas et al. / Materials Research Bulletin 46 (2011) 639–642
642
rized in Table 5. Finally, the predicted S/N ratio and predicted aspect ratio were 16.14 and 6.41, respectively. However, the actual obtained result is shown in Table 5. The experiment aspect ratio for optimum condition was 6.39, which is slightly lower compared with the Taguchi method design. As expected with optimum conditions under Taguchi design, gain in S/N ratio and aspect ratio can be decreased and increased with 0.31 of predicted aspect ratio. This indicated that Taguchi experiment design has been successfully applied to the determination and prediction of the aspect ratio of prepared ZnO nanocrystal with polymer template in solvothermal method. 5. Conclusions The synthesis of ZnO nanostructures via solvothermal method has been investigated by Taguchi robust design. ZnO nanoparticles and nanorods have been obtained under three different parameters which are [NaOH]/[Zn2+] ratio, amount of PVP and molecular weight of PVP. From the Taguchi method analysis, the [NaOH]/ [Zn2+] ratio plays the most important factor on the aspect ratio of prepared ZnO. The results also indicated that the Taguchi method can provide an advantage for design the size and shape of ZnO nanostructure. Fig. 4. TEM image of ZnO nanocrystal from optimum condition.
Acknowledgements Table 5 Prediction and confirmation experimental for ZnO nanocrystal growth.
The authors are grateful for research funding from National Nanotechnology Center and the Institute of Solar Energy Technology Development under National Science and Technology Development Agency, Thailand (grant number: B22 AR010110RDAR01).
Optimal level A3B3C3
Aspect ratio
Standard deviation ()
S/N
Data
S/N
Data
Prediction Confirmation
16.14 15.80
6.41 6.39
10.11 13.50
0.31 1.09
characteristic peaks were observed for impurities. TEM image as shown in Fig. 4 revealed the nanorod structure of the synthesized ZnO. These results confirmed that the ZnO nanostructure was successfully synthesized by the solvothermal reaction and also proved that the ZnO nanostructure could crystallize well at low temperature which was consistent with the previous studies [15]. The predicted S/N of aspect ratio and standard deviation by using the optimal level of each factor is calculated from following equations: S=Npredicted ¼ S=Nm þ ðS=NA3 S=Nm Þ þ ðS=NB3 S=Nm Þ þ ðS=NC3 S=Nm Þ and S=Npredicted ¼ 10 log 1=Y 2 where S/Nm is the mean total S/N ratio and S/NA3, S/NB3, S/NC3 are the mean S/N ratio at the optimal level. Then Y, which is the predicted aspect ratio and standard deviation at the optimal level, was calculated by using above equation. The results are summa-
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