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properties of chemical bath synthesized ZnO nanowires
SCT-21055; No of Pages 6 Surface & Coatings Technology xxx (2016) xxx–xxx
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Correlation between seed layer characteristics and structures/properties of chemical bath synthesized ZnO nanowires Tzu-Ling Chen, Jyh-Ming Ting ⁎ Department of Material Science and Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 18 November 2015 Revised 27 March 2016 Accepted in revised form 28 March 2016 Available online xxxx Keywords: ZnO Nanowires Seed layer
a b s t r a c t A two-step process including the deposition of a ZnO seed layer on a glass substrate and the subsequent growth of ZnO nanowires in a chemical bath was developed. ZnO seed layers were prepared with various precursor solutions using a spin coating technique. The use of various precursor solutions allows the resulting seed layers to exhibit many different characteristics. The effects of the precursor type and solution concentration on the growth and characteristics of the obtained ZnO nanowires are presented and discussed. In particular, we report an unusual growth kinetics in which the lengthening of the nanowires is proportional to t1.5. A growth model is therefore proposed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a unique material that exhibits a variety of nanostructures, including nanowires, nanorods, and nanowalls [1–8], and attracts numerous attentions and interests. Although different synthesis methods have been reported, the use of a ZnO seed layer for the growth of ZnO nanostructures through a solution route has shown to be appealing because of its low growth temperature and potential for scale-up. This technique involves two steps: seed layer deposition and growth of ZnO on the seed layer using a solution process. A seed layer can be prepared using different methods and precursors, for example, zinc acetate dehydrate (ZAD)s [9]. There are also different solution processes available for the synthesis of ZnO nanostructures. Furthermore, factors influencing the characteristics of ZnO nanostructures have been intensively studied. For example, the pH value of a solution has been shown to be crucial in determining the morphology of the ZnO nanostructure [10,11]. The dimensions of ZnO nanowires were found to vary with the composition of the chemical bath [12]. The growth temperature also affects the diameter and length of the ZnO nanowire arrays synthesized using a hydrothermal method [13]. While the average diameter of ZnO nanowires was found to increase with the growth time [9], it was also reported that the increase of diameter occurred at a much slower rate than the increase of length [14]. While attentions have been paid to the investigation of the effects of solution conditions on the obtained ZnO nanostructures, less work has been done to explore the effect of seed layer preparation. A seed layer can be a film prepared using atomic layer or sputter deposition [15, 16], or consisting of nanoparticles prepared from a mixture of lithium ⁎ Corresponding author. E-mail address: [email protected] (J.-M. Ting).
hydroxide and ZAD dissolved in ethanol [9]. The effect of the concentration of ZnO colloid, prepared by mixing ZAD with monoethanolamine (MEA) stabilizer and 2-methoxyethanol solvent, was studied [17,18]. It was found that the concentration is a very strong factor in controlling the ZnO nanorod array (ZNA) density. Heat treatment of ZnO seed layer also influences the density of the ZNA [19]. The density of the ZNAs reduces with the heat treatment temperature and the time. In this study, ZnO seed layers were prepared using various precursor solutions through a spin coating technique. The characteristics of the resulting seed layers depend on the precursor solution conditions. Therefore, the use of various precursor solutions allows the resulting seed layers to exhibit many different characteristics. More importantly, these characteristics affect the growth and characteristics of the obtained ZnO nanowires. The effects are presented and discussed. 2. Experimental ZnO seed layers were first deposited on glass substrates using a spin coating technique followed by heat treatment at 400 °C for 1 h. The solution for the spin coating process was prepared by mixing a Zn precursor and a chemical in a solvent at 60 °C under agitation. Four different solutions were prepared. The first kind is designated as ZADN that consists of ZAD, NaOH(aq), and methanol. The molar concentrations of ZAD were 0.01, 0.02, 0.04, and 0.75 M, and the molar ratio of ZAD/NaOH was fixed at 1/3. The solution was then spin-coated on the substrate at 800 rpm for 20 s, followed by drying at 90 °C. The second kind is designated as Z that consists of Zn powders, 6.0 × 10−6 M poly (acrylic acid) (PAA), and DI water. The amount of Zn powders in the suspension fluid is equivalent to 0.75 M. The solution was then spin-coated on the substrate at 1000 rpm for 10 s and then 2000 rpm for 20 s, followed by drying at 90 °C. The third kind is designated as ZADP that consists of ZAD,
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Fig. 1. (a) XRD diffraction pattern of a 1-h, 400 °C heat treated seed layer prepared using Zn powders as the precursor. (b) Seed layer (ZADN) thickness increases linearly with the number of spin coatings. (c) TEM image showing the uniformity in the thickness. The inset is a diffraction pattern of the film.
Fig. 2. (a) AFM image of a ZADP seed layer. The variations of (b) surface roughness and (c) (0002) grain size with layer thickness.
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Fig. 3. XRD analysis of nanowires grown on seed layers Z, ZADP, and ZNH. Fig. 5. Nanowire area density is affected by the surface roughness of the seed layer.
polyvinyl alcohol (PVA), and de-ionized (DI) water. The molar concentrations of ZAD were 0.11 and 0.22 M, and the molar concentration of PVA was fixed 0.002 M. The solution was then spin-coated on the substrate at 1000 rpm for 10 s and then 2000 rpm for 20 s, followed by drying at 90 °C. The last kind is designated as ZNH that consists of 0.04 M zinc nitrate hexahydrate (ZNH), 0.04 M hexamethylenetetramine (HMT), and DI water. The solution was spin-coated on the substrate at 800 rpm for 20 s, followed by drying at 90 °C. These as-spun coatings were subjected to heat treatment at 400 °C for 1 h to obtain seed layers. High-density ZnO nanowires were then grown on the seeded glass substrates in an aqueous solution consisting of 0.02 M ZNH and 0.02 M HMT at 90 °C using various growth time. No heat treatment was applied to the obtained ZnO nanowires. The crystalline structures of the ZnO seed layer and ZnO nanowire were examined using X-ray diffraction (XRD). The surface and cross-sectional morphologies were characterized using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The ZnO nanowires were also examined using transmission electron microscopy (TEM). 3. Results and discussion Each seed layer consists of multiple spun layers. After the heat treatment, the as-spun amorphous seed layer became ZnO polycrystalline with no preferred orientation, as shown in Fig. 1(a) for a seed layer obtained using Zn powders as the precursor. It was found that the ZnO seed layer thickness increases linearly with the spin coating times as shown in Fig. 1(b) for the seed layers prepared using ZAD as the precursor. Depending on the precursor concentration, the slope (see Fig. 1(b))
or thickness/coating ratio varies. For example, the slopes are 3.5, 7.5, 11.1, and 88 nm/coatings for seed layers obtained from precursors having ZAD concentrations of 0.01, 0.02, 0.04, and 0.75 M. The coating efficiency, represented by the slope, increases linearly with the precursor concentration. In other words, the seed layer thickness increases with the precursor concentration. The seed layer thickness was also found to depend on the type of the precursor. Under the same spin-coating and heat treatment conditions, Z seed layer is the thickest, followed by ZADP, ZADP, ZADN, and ZNH seed layers. The thicknesses of the obtained ZnO seed layers are all very uniform as shown in Fig. 1(c) for a heat treated ZADP seed layer. The inset is a diffraction pattern of the seed layer, showing polycrystalline ZnO. Depending on the preparation condition, the surface roughness and the crystalline size of the ZnO seed layer vary. The surface roughness was determined using AFM image, as shown in Fig. 2(a) for a ZADP seed layer. The result indicates that, regardless of the precursor type, the roughness increases with the layer thickness as shown in Fig. 2(b). The grain size, determined by the Sherrer Eq. using the (0002) peak in the XRD pattern, also depends on the layer thickness, as shown in Fig. 2(c). As shown in Fig. 1(c), all the heat treated seed layers exhibit a columnar structure. In a columnarstructured coating, the grain grows with the coating thickness. However, such grain growth is also limited by the thermal energy, which is provided by the seed layer heat treatment temperature. As a result, the grain size increases initially with the thickness of the layer and then saturates, indicating the saturation of the grain growth under the isothermal condition used. The obtained seed layers were then used to grow nanowires. XRD analysis indicates that all nanowires are ZnO as shown in Fig. 3 for
Fig. 4. ZnO nanowires grown on a seed layer Z: (a) cross sectional and (b) top views.
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Fig. 6. Nanowire diameter increases with the seed layer grain size.
that grown on seed layers Zn, ZADP, and ZNH. The (0002) preferred orientation is used as an indicator of the alignment of the nanowires. The (0002) preferred orientation was determined by the (0002) peak intensity percentage calculated using the following formula: (I(0002) / Iall) × 100, where I(0002) is the (0002) peak intensity and Iall is the intensity sum of all the peaks found in the XRD spectrum. It was found that the (0002) intensity percentage ranges from 61 to 65%. Therefore, in
general all the ZnO nanowires exhibit similar degrees of alignment. The semi-alignment is seen in Fig. 4(a) for ZnO nanowires grown on seed layer Zn. In general, the area density is high as shown in Fig. 4(b). However ZnO nanowires obtained without the use of a seed layer show a much lower density of nucleation. The area density is also affected by the surface roughness of the seed layer as shown in Fig. 5. Considering that the roughness range (~5 to ~160 nm), the variation of the area density is believed not to be random. Instead, the trend is that the area density in general decreases with the surface roughness. As shown later, during the growth of nanowires, the growth species also arrive at the seed layer, making the seed layer grows too. The growth of nanowires represents the dominant anisotropic growth as oppose to the much weak isotropic growth of the seed layer. As the surface roughness increases the surface energy increases too. This allows rapid nucleation of ZnO such that isotropic growth becomes more favorable due to the resulting high nucleation density on the surface. Therefore, the anisotropic or nanowire growth reduces with the surface roughness. The nanowire diameter was found to increase linearly with the grain size of the seed layer as shown in Fig. 6, suggesting each grain serves as a seed for the growth of a nanowire. When the growth time increases from 2 to 10 h, the nanowire diameter varies from 83.3 to 761.2 nm and the length varies from 0.24 to 6.62 μm. The increase of length is apparently much faster than that of diameter as expected. Fig. 7(a) shows the variation of nanowire length with the growth time. It was found that the length L is proportional to t1.5. Also, the length increases with the concentration of ZADN linearly, as shown in Fig. 7(b). Such growth kinetics can be explained by considering the lengthening to be a one-dimensional precipitation [20]. A short time solution to the one-dimensional precipitation is pffiffi C−C0 ¼ k1 t
ð1Þ
where C and C0 are the concentrations at time t and 0, and k1 is a constant. As shown in Fig. 7(b), the growth rate can be expressed as dL
. dt
¼ k2 ðC−C0 Þ
ð2Þ
where k2 is a constant. Therefore, dL
. dt
pffiffi ¼ k3 t
ð3Þ
where k3 is a constant. The growth kinetics shown in Fig. 7(a) is thus explained. Fig. 8(a) shows a TEM cross sectional view of a sample obtained using seed layer ZADP (thickness 50 nm and grain size 5.3 nm). An enlarged view of the small circle in Fig. 8(a) is given in Fig. 8(b), showing the seed layer area right below the nanowires. The seed layer thickness increase slightly to 58 nm. Also, unlike the seed layer before the growth, Fig. 8(c) shows a highly crystallized layer, indicating the growth of the ZnO NRs from the grains in the seed layer as mentioned above. The semi-aligned ZnO nanowires are also shown in Fig. 8(a). Diffraction pattern (Fig. 8(d)) shows that the nanowires are single crystal ZnO with a (0002) growth direction and the lattice spacing is 0.52 nm (Fig. 8(e)). Fig. 8(a) and (e) also indicates that the nanowire lengthen direction is perpendicular to the (0002) plane. 4. Conclusions
Fig. 7. Variations of nanowire diameter with (a) growth time and (b) concentration.
ZnO nanowires were grown on glass substrates that are predeposited with a ZnO seed layer. The seed layer was prepared using various precursor solutions through a spin coating technique and the nanowires were grown using a chemical bath method. The seed layer coating efficiency and the grain size depend on both the precursor type and concentration. In other words, the seed layer thickness increases with the
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Fig. 8. (a) TEM analysis of a sample obtained using Seed layer ZADP, (b) The seed layer area below nanowires (an enlarged view the small circle in panels (a), (c) Diffraction pattern of seed layer, (d) Diffraction pattern of ZnO nanowires, and (e) HR-TEM of ZnO nanowires.
precursor concentration. The seed layer thickness was also found to depend on the precursor type. The seed layer surface roughness increases with the layer thickness, regardless of the precursor type. The nanowire area density in general decreases with the seed layer surface roughness. The nanowire diameter increases linearly with the grain size of the seed layer, suggesting a grain serves as a nucleation site for the growth of a nanowire. Although the nanowire length increases with the precursor concentration, as expected, it was found that the lengthening is proportional to t1.5. Such growth kinetics has been explained by considering the lengthening a one-dimensional precipitation and a growth model is therefore proposed.
Acknowledgement This research was supported by Ministry of Science and Technology of Taiwan under Grant no. 103-2221-E-006 -051 -MY3.
References [1] K.C. Chiang, W.-Y. Wu, J.-M. Ting, Enhanced lateral growth of zinc oxide nanowires on sensor chips, J. Am. Ceram. Soc. 94 (2011) 713–716. [2] W.-Y. Wu, T.-L. Chen, J.-M. Ting, Effects of seed layer precursor type on the synthesis of ZnO nanowires using chemical bath deposition, J. Electrochem. Soc. 157 (2010) K177. [3] S.P. Sharma, J.-M. Ting, Z.-K. Chang, Growth of ZnO nanowires on carbon fibers by RF magnetron sputtering technique, J. Electrochem. Soc. 9 (2011) 74–79. [4] T.L. Chou, W.Y. Wu, J.M. Ting, Sputter deposited ZnO nanowires/thin film structures on glass substrate, Thin Solid Films 518 (2009) 1553–1556. [5] W.-Y. Wu, C.-C. Yeh, J.-M. Ting, Effects of seed layer characteristics on the synthesis of ZnO nanowires, J. Am. Ceram. Soc. 92 (2009) 2718–2723. [6] W.Y. Wu, J.M. Ting, P.J. Huang, Electrospun ZnO nanowires as gas sensors for ethanol detection, Nanoscale Res. Lett. 4 (2009) 513–517. [7] W.-T. Chiou, W.-Y. Wu, J.-M. Ting, Effect of electroless copper on the growth of ZnO nanowires, J. Mater. Res. 20 (2005) 2348–2353. [8] W.-T. Chiou, W.-Y. Wu, J.-M. Ting, Growth of single crystal ZnO nanowires using sputter deposition, Diam. Relat. Mater. 12 (2003) 1841–1844. [9] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. 115 (2003) 3139–3142.
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[10] K. Govender, D.S. Boyle, P.B. Kenway, P. O'Brien, Understanding the factors that govern the deposition and morphology of thin films of ZnO from aqueous solution, J. Mater. Chem. 14 (2004) 2575–2591. [11] H. Zhang, J. Feng, J. Wang, M. Zhang, Preparation of ZnO nanorods through wet chemical method, Mater. Lett. 61 (2007) 5202–5205. [12] C. Bekeny, T. Voss, H. Gafsi, J. Gutowski, B. Postels, M. Kreye, A. Waag, Origin of the near-band-edge photoluminescence emission in aqueous chemically grown ZnO nanorods, J. Appl. Phys. 100 (2006) 104317. [13] M. Guo, P. Diao, X. Wang, S. Cai, The effect of hydrothermal growth temperature on preparation and photoelectrochemical performance of ZnO nanorod array films, J. Solid State Chem. 178 (2005) 3210–3215. [14] J.B. Baxter, A.M. Walker, K. van Ommering, E.S. Aydil, Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells, Nanotechnology 17 (2006) S304–S312.
[15] Q. Li, V. Kumar, Y. Li, H. Zhang, T.J. Marks, R.P.H. Chang, Fabrication of ZnO nanorods and nanotubes in aqueous solutions, Chem. Mater. 17 (2005) 1001–1006. [16] R.B. Cross, M.M. Souza, E.M. Sankara Narayanan, A low temperature combination method for the production of ZnO nanowires, Nanotechnology 16 (2005) 2188–2192. [17] H. Li, J. Wang, H. Liu, C. Yang, H. Xu, X. Li, H. Cui, Sol–gel preparation of transparent zinc oxide films with highly preferential crystal orientation, Vacuum 77 (2004) 57–62. [18] Y.-S. Kim, W.-P. Tai, S.-J. Shu, Effect of preheating temperature on structural and optical properties of ZnO thin films by sol–gel process, Thin Solid Films 491 (2005) 153–160. [19] T. Ma, M. Guo, M. Zhang, Y. Zhang, X. Wang, Density-controlled hydrothermal growth of well-aligned ZnO nanorod arrays, Nanotechnology 18 (2007) 035605. [20] P. Shewmon, The Minerals, second ed. TMS, Metals & Materials Society, 1989.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Correlation between seed layer characteristics and structures/properties of chemical bath synthesized ZnO nanowires Tzu-Ling Chen, Jyh-Ming Ting ⁎ Department of Material Science and Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 18 November 2015 Revised 27 March 2016 Accepted in revised form 28 March 2016 Available online xxxx Keywords: ZnO Nanowires Seed layer
a b s t r a c t A two-step process including the deposition of a ZnO seed layer on a glass substrate and the subsequent growth of ZnO nanowires in a chemical bath was developed. ZnO seed layers were prepared with various precursor solutions using a spin coating technique. The use of various precursor solutions allows the resulting seed layers to exhibit many different characteristics. The effects of the precursor type and solution concentration on the growth and characteristics of the obtained ZnO nanowires are presented and discussed. In particular, we report an unusual growth kinetics in which the lengthening of the nanowires is proportional to t1.5. A growth model is therefore proposed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a unique material that exhibits a variety of nanostructures, including nanowires, nanorods, and nanowalls [1–8], and attracts numerous attentions and interests. Although different synthesis methods have been reported, the use of a ZnO seed layer for the growth of ZnO nanostructures through a solution route has shown to be appealing because of its low growth temperature and potential for scale-up. This technique involves two steps: seed layer deposition and growth of ZnO on the seed layer using a solution process. A seed layer can be prepared using different methods and precursors, for example, zinc acetate dehydrate (ZAD)s [9]. There are also different solution processes available for the synthesis of ZnO nanostructures. Furthermore, factors influencing the characteristics of ZnO nanostructures have been intensively studied. For example, the pH value of a solution has been shown to be crucial in determining the morphology of the ZnO nanostructure [10,11]. The dimensions of ZnO nanowires were found to vary with the composition of the chemical bath [12]. The growth temperature also affects the diameter and length of the ZnO nanowire arrays synthesized using a hydrothermal method [13]. While the average diameter of ZnO nanowires was found to increase with the growth time [9], it was also reported that the increase of diameter occurred at a much slower rate than the increase of length [14]. While attentions have been paid to the investigation of the effects of solution conditions on the obtained ZnO nanostructures, less work has been done to explore the effect of seed layer preparation. A seed layer can be a film prepared using atomic layer or sputter deposition [15, 16], or consisting of nanoparticles prepared from a mixture of lithium ⁎ Corresponding author. E-mail address: [email protected] (J.-M. Ting).
hydroxide and ZAD dissolved in ethanol [9]. The effect of the concentration of ZnO colloid, prepared by mixing ZAD with monoethanolamine (MEA) stabilizer and 2-methoxyethanol solvent, was studied [17,18]. It was found that the concentration is a very strong factor in controlling the ZnO nanorod array (ZNA) density. Heat treatment of ZnO seed layer also influences the density of the ZNA [19]. The density of the ZNAs reduces with the heat treatment temperature and the time. In this study, ZnO seed layers were prepared using various precursor solutions through a spin coating technique. The characteristics of the resulting seed layers depend on the precursor solution conditions. Therefore, the use of various precursor solutions allows the resulting seed layers to exhibit many different characteristics. More importantly, these characteristics affect the growth and characteristics of the obtained ZnO nanowires. The effects are presented and discussed. 2. Experimental ZnO seed layers were first deposited on glass substrates using a spin coating technique followed by heat treatment at 400 °C for 1 h. The solution for the spin coating process was prepared by mixing a Zn precursor and a chemical in a solvent at 60 °C under agitation. Four different solutions were prepared. The first kind is designated as ZADN that consists of ZAD, NaOH(aq), and methanol. The molar concentrations of ZAD were 0.01, 0.02, 0.04, and 0.75 M, and the molar ratio of ZAD/NaOH was fixed at 1/3. The solution was then spin-coated on the substrate at 800 rpm for 20 s, followed by drying at 90 °C. The second kind is designated as Z that consists of Zn powders, 6.0 × 10−6 M poly (acrylic acid) (PAA), and DI water. The amount of Zn powders in the suspension fluid is equivalent to 0.75 M. The solution was then spin-coated on the substrate at 1000 rpm for 10 s and then 2000 rpm for 20 s, followed by drying at 90 °C. The third kind is designated as ZADP that consists of ZAD,
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Fig. 1. (a) XRD diffraction pattern of a 1-h, 400 °C heat treated seed layer prepared using Zn powders as the precursor. (b) Seed layer (ZADN) thickness increases linearly with the number of spin coatings. (c) TEM image showing the uniformity in the thickness. The inset is a diffraction pattern of the film.
Fig. 2. (a) AFM image of a ZADP seed layer. The variations of (b) surface roughness and (c) (0002) grain size with layer thickness.
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Fig. 3. XRD analysis of nanowires grown on seed layers Z, ZADP, and ZNH. Fig. 5. Nanowire area density is affected by the surface roughness of the seed layer.
polyvinyl alcohol (PVA), and de-ionized (DI) water. The molar concentrations of ZAD were 0.11 and 0.22 M, and the molar concentration of PVA was fixed 0.002 M. The solution was then spin-coated on the substrate at 1000 rpm for 10 s and then 2000 rpm for 20 s, followed by drying at 90 °C. The last kind is designated as ZNH that consists of 0.04 M zinc nitrate hexahydrate (ZNH), 0.04 M hexamethylenetetramine (HMT), and DI water. The solution was spin-coated on the substrate at 800 rpm for 20 s, followed by drying at 90 °C. These as-spun coatings were subjected to heat treatment at 400 °C for 1 h to obtain seed layers. High-density ZnO nanowires were then grown on the seeded glass substrates in an aqueous solution consisting of 0.02 M ZNH and 0.02 M HMT at 90 °C using various growth time. No heat treatment was applied to the obtained ZnO nanowires. The crystalline structures of the ZnO seed layer and ZnO nanowire were examined using X-ray diffraction (XRD). The surface and cross-sectional morphologies were characterized using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The ZnO nanowires were also examined using transmission electron microscopy (TEM). 3. Results and discussion Each seed layer consists of multiple spun layers. After the heat treatment, the as-spun amorphous seed layer became ZnO polycrystalline with no preferred orientation, as shown in Fig. 1(a) for a seed layer obtained using Zn powders as the precursor. It was found that the ZnO seed layer thickness increases linearly with the spin coating times as shown in Fig. 1(b) for the seed layers prepared using ZAD as the precursor. Depending on the precursor concentration, the slope (see Fig. 1(b))
or thickness/coating ratio varies. For example, the slopes are 3.5, 7.5, 11.1, and 88 nm/coatings for seed layers obtained from precursors having ZAD concentrations of 0.01, 0.02, 0.04, and 0.75 M. The coating efficiency, represented by the slope, increases linearly with the precursor concentration. In other words, the seed layer thickness increases with the precursor concentration. The seed layer thickness was also found to depend on the type of the precursor. Under the same spin-coating and heat treatment conditions, Z seed layer is the thickest, followed by ZADP, ZADP, ZADN, and ZNH seed layers. The thicknesses of the obtained ZnO seed layers are all very uniform as shown in Fig. 1(c) for a heat treated ZADP seed layer. The inset is a diffraction pattern of the seed layer, showing polycrystalline ZnO. Depending on the preparation condition, the surface roughness and the crystalline size of the ZnO seed layer vary. The surface roughness was determined using AFM image, as shown in Fig. 2(a) for a ZADP seed layer. The result indicates that, regardless of the precursor type, the roughness increases with the layer thickness as shown in Fig. 2(b). The grain size, determined by the Sherrer Eq. using the (0002) peak in the XRD pattern, also depends on the layer thickness, as shown in Fig. 2(c). As shown in Fig. 1(c), all the heat treated seed layers exhibit a columnar structure. In a columnarstructured coating, the grain grows with the coating thickness. However, such grain growth is also limited by the thermal energy, which is provided by the seed layer heat treatment temperature. As a result, the grain size increases initially with the thickness of the layer and then saturates, indicating the saturation of the grain growth under the isothermal condition used. The obtained seed layers were then used to grow nanowires. XRD analysis indicates that all nanowires are ZnO as shown in Fig. 3 for
Fig. 4. ZnO nanowires grown on a seed layer Z: (a) cross sectional and (b) top views.
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Fig. 6. Nanowire diameter increases with the seed layer grain size.
that grown on seed layers Zn, ZADP, and ZNH. The (0002) preferred orientation is used as an indicator of the alignment of the nanowires. The (0002) preferred orientation was determined by the (0002) peak intensity percentage calculated using the following formula: (I(0002) / Iall) × 100, where I(0002) is the (0002) peak intensity and Iall is the intensity sum of all the peaks found in the XRD spectrum. It was found that the (0002) intensity percentage ranges from 61 to 65%. Therefore, in
general all the ZnO nanowires exhibit similar degrees of alignment. The semi-alignment is seen in Fig. 4(a) for ZnO nanowires grown on seed layer Zn. In general, the area density is high as shown in Fig. 4(b). However ZnO nanowires obtained without the use of a seed layer show a much lower density of nucleation. The area density is also affected by the surface roughness of the seed layer as shown in Fig. 5. Considering that the roughness range (~5 to ~160 nm), the variation of the area density is believed not to be random. Instead, the trend is that the area density in general decreases with the surface roughness. As shown later, during the growth of nanowires, the growth species also arrive at the seed layer, making the seed layer grows too. The growth of nanowires represents the dominant anisotropic growth as oppose to the much weak isotropic growth of the seed layer. As the surface roughness increases the surface energy increases too. This allows rapid nucleation of ZnO such that isotropic growth becomes more favorable due to the resulting high nucleation density on the surface. Therefore, the anisotropic or nanowire growth reduces with the surface roughness. The nanowire diameter was found to increase linearly with the grain size of the seed layer as shown in Fig. 6, suggesting each grain serves as a seed for the growth of a nanowire. When the growth time increases from 2 to 10 h, the nanowire diameter varies from 83.3 to 761.2 nm and the length varies from 0.24 to 6.62 μm. The increase of length is apparently much faster than that of diameter as expected. Fig. 7(a) shows the variation of nanowire length with the growth time. It was found that the length L is proportional to t1.5. Also, the length increases with the concentration of ZADN linearly, as shown in Fig. 7(b). Such growth kinetics can be explained by considering the lengthening to be a one-dimensional precipitation [20]. A short time solution to the one-dimensional precipitation is pffiffi C−C0 ¼ k1 t
ð1Þ
where C and C0 are the concentrations at time t and 0, and k1 is a constant. As shown in Fig. 7(b), the growth rate can be expressed as dL
. dt
¼ k2 ðC−C0 Þ
ð2Þ
where k2 is a constant. Therefore, dL
. dt
pffiffi ¼ k3 t
ð3Þ
where k3 is a constant. The growth kinetics shown in Fig. 7(a) is thus explained. Fig. 8(a) shows a TEM cross sectional view of a sample obtained using seed layer ZADP (thickness 50 nm and grain size 5.3 nm). An enlarged view of the small circle in Fig. 8(a) is given in Fig. 8(b), showing the seed layer area right below the nanowires. The seed layer thickness increase slightly to 58 nm. Also, unlike the seed layer before the growth, Fig. 8(c) shows a highly crystallized layer, indicating the growth of the ZnO NRs from the grains in the seed layer as mentioned above. The semi-aligned ZnO nanowires are also shown in Fig. 8(a). Diffraction pattern (Fig. 8(d)) shows that the nanowires are single crystal ZnO with a (0002) growth direction and the lattice spacing is 0.52 nm (Fig. 8(e)). Fig. 8(a) and (e) also indicates that the nanowire lengthen direction is perpendicular to the (0002) plane. 4. Conclusions
Fig. 7. Variations of nanowire diameter with (a) growth time and (b) concentration.
ZnO nanowires were grown on glass substrates that are predeposited with a ZnO seed layer. The seed layer was prepared using various precursor solutions through a spin coating technique and the nanowires were grown using a chemical bath method. The seed layer coating efficiency and the grain size depend on both the precursor type and concentration. In other words, the seed layer thickness increases with the
Please cite this article as: T.-L. Chen, J.-M. Ting, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.03.082
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Fig. 8. (a) TEM analysis of a sample obtained using Seed layer ZADP, (b) The seed layer area below nanowires (an enlarged view the small circle in panels (a), (c) Diffraction pattern of seed layer, (d) Diffraction pattern of ZnO nanowires, and (e) HR-TEM of ZnO nanowires.
precursor concentration. The seed layer thickness was also found to depend on the precursor type. The seed layer surface roughness increases with the layer thickness, regardless of the precursor type. The nanowire area density in general decreases with the seed layer surface roughness. The nanowire diameter increases linearly with the grain size of the seed layer, suggesting a grain serves as a nucleation site for the growth of a nanowire. Although the nanowire length increases with the precursor concentration, as expected, it was found that the lengthening is proportional to t1.5. Such growth kinetics has been explained by considering the lengthening a one-dimensional precipitation and a growth model is therefore proposed.
Acknowledgement This research was supported by Ministry of Science and Technology of Taiwan under Grant no. 103-2221-E-006 -051 -MY3.
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