- Email: [email protected]
On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb Iman Safaee, Mahmood Kazemzadn, Mehdi Alizadeh, Mohammad Reza Rahimipour Materials and Energy Research Center, P.O. Box 14155/4777, Tehran, Iran Received 17 May 2015; received in revised form 16 June 2015; accepted 16 June 2015
Abstract In this study, ZnO nanorod arrays were grown on a monolithic cordierite honeycomb surface by the application of a hydrothermal method. The parameters affecting the nanorod formation were systematically investigated under different scenarios by adjusting the steps of hydrothermal growth after seeding and the concentration of precursors. Analyzing scanning electron microscopy (SEM) images and the X-ray diffraction patterns revealed the alignment, compactness, uniformity and diameters of the prepared nanorods as well as the texture factor in different directions. According to the results, although there was no critical change of the morphology by altering the precursor concentrations using a twostep hydrothermal growth method, some redundant ZnO morphologies were formed beside the aligned nanorods. More interestingly, it was possible to obtain different morphologies under one-step hydrothermal growth, such as combination of the rods and polygons, the aligned rods or widely scattered unaligned rods depending on the concentration of the precursors. The superior alignment, compactness, uniformity as well as the high texture factor and low rod diameter (d o 140 nm) could be obtained by optimizing the one-step hydrothermal growth at the concentration of 25 mM of the precursors (zinc nitrate hexahydrate and hexamethylenetetramine). The results indicated that at optimized conditions, one-step hydrothermal growth is preferred to the two-step method as an efficient and facile technique for ZnO nanorod fabrication. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: ZnO; Nanorods; Monolithic cordierite honeycomb; Texture factor; Alignment
1. Introduction One-dimensional metal oxide nanostructures have attracted widespread attention due to their unique physical and chemical properties. Among metal oxides, ZnO which is the representative of semiconducting and piezoelectric materials with a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature is regarded as a leading candidate for many scientific and technological applications [1]. Owing to their low cost, abundance, morphological and chemical flexibilities, one-dimensional ZnO nanostructure arrays can play a vital role in a wide range of applications including catalysts [2], solar cells [3], gas sensors [4], biosensors [5], photocatalysts [6], light-emitting diodes [7], and field-emission [8]. One-dimensional ZnO nanostructures n
Corresponding author. Tel.: þ98 26 36280040; fax: þ 98 26 36201888. E-mail address: [email protected] (M. Kazemzad).
have been synthesized by a wide variety of techniques, including wet chemical methods [9], metal–organic chemical vapor deposition (MOCVD) [10], physical vapor deposition [11], pulsed laser deposition [12], molecular beam epitaxy (MBE) [13], flux methods [14], electrospinning [15], top-down approaches by etching [16] and hydrothermal synthesis [17]. Among these methods, hydrothermal technique is a promising low-cost, green and scalable method for fabrication of onedimensional ZnO nanostructures. In this regard, alignment of ZnO nanorods is an important demand in its various applications. The mass transfer and electrical properties could be improved in nanosized ZnO rods with high alignments and uniformity [18,19]. In a typical hydrothermal synthesis, the desirable morphology (high alignments and uniformity) could be obtained by varying synthesis conditions during the nucleation and crystal growth. The two-step hydrothermal growth (also known as base-growth hydrothermal synthesis) and one-step hydrothermal growth methods are two main
http://dx.doi.org/10.1016/j.ceramint.2015.06.077 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
2
procedures suggested in the literature [18]. In the former one, the procedure is completed in two steps: (1) growth at a high concentration for a short period of time (nucleation of ZnO is prominent in this step), and (2) growth at a low concentration for a long time (growth of ZnO rods is the prominent phenomenon in the second step) [18]. Moreover, preparation of the ZnO nanorod arrays on monolithic substrate is one of the appealing methods for fabrication of the new generation catalysts of car exhaust [20]. Although fabrication of onedimensional ZnO nanorods on monolithic substrate has previously been reported by some researchers [2], there is a lack of prospective systematic study of the parameters affecting both alignment and the size of hydrothermally grown nanorod arrays, especially in case of commercial monolithic honeycombs. The present study has investigated different parameters associated with the growth of aligned ZnO nanorod arrays on cordierite honeycombs by application of the SEM technique in addition to texture factor calculation using XRD patterns.
with deionized water and sonicated in the ethanol bath for 15 min, followed by drying the samples at 80 1C for several hours. In the two-step hydrothermal synthesis (also known as base-growth), the samples were immersed in the concentrated solution (50, 25 mM) at the first step and heated for an hour. In the second step, the samples were treated at a lower concentration (25, 12.5, 6.25 mM) and heated at 80 1C for 4 h. Then, the prepared samples were washed with water and ethanol as mentioned previously. A hotplate-stirrer (Heidolph, MR-Hei Standard) was used for the hydrothermal growth with a heating range of 0–300 1C and a stirring range of 0–1400 rpm. The X-ray diffraction analyses (XRD) were carried out on a Philips X'pert (3710) diffractometer with Cu kα operating at 40 kV and 30 mA. A Cambridge S360 scanning electron microscope (SEM) was utilized to acquire the micrographs of the as-grown ZnO nanorod arrays on the monolithic cordierite substrates.
2. Experimental
3.1. Analysis of the sample after the first step of base growth method
The ZnO nanorod arrays were prepared within the pores of the commercially available monolithic cordierite samples by the method adopted from the literature [18], as follows. The monolithic cordierite samples were purchased from Iran Delco Company having 1 mm 1 mm square channels and a wall thickness of 180 mm. The monoliths were cut into equal cube pieces sized 1 cm 1 cm 1 cm and then rinsed with acetone, ethanol and deionized water to remove all contaminants. The ZnO seeding procedure was performed by dipping the substrates in the 20 mM zinc acetate solution (Zn(CH3COO)2 2H2O) in ethanol. Then the samples were baked at 150 1C for 10 min. The seeding process was repeated for several times until a smooth layer of seeds stuck to the substrates. Then, the monolithic cordierite substrates were heated at 350 1C for 5 h. In the one-step hydrothermal synthesis, 100 mL aqueous solution was prepared containing different concentrations (50, 25, 12.5, 6.25 mM) of zinc nitrate hexahydrate (Zn (NO3)2 6H2O) in addition to equimolar amounts of hexamethylenetetramine (HMT: C6H12N4). Then, the substrates were put in the 120 mL autoclaves containing the above solutions and heated at 80 1C for 5 h while being stirred. Finally, the as-grown ZnO nanorod arrays within the monolithic cordierite were cleansed
3. Results and discussion
Fig. 1 shows the SEM micrographs with different magnifications for the two samples prepared during the first step of the base growth hydrothermal reaction using 25 and 50 mM of both zinc nitrate and HMT solutions as the precursors. According to the results, the sparsely scattered primary ZnO nanorods obtained in the 25 mM sample, illustrated in Fig. 1a, was not suitable for the second step of the growth. However, the ZnO nanorods with high compactness obtained in the 50 mM sample (Fig. 1b) were appropriate for the subsequent procedure. The results are in agreement with the classical crystallization mechanism indicating that at higher supersaturation levels, nucleation dominates crystal growth which eventually leads to the formation of a large number of particles [21]. 3.2. Analysis of the sample after the second step of base growth method Fig. 2a–f exhibits SEM micrographs of the ZnO nanorod arrays synthesized within the monolithic cordierite honeycomb after the second step with a precursor concentration varying from 6.25 to 25 mM. As observed from Fig. 2, the average
Fig. 1. SEM micrographs of primary ZnO nanorods within honeycomb monolithic cordierite from first step of base growth hydrothermal synthesis for 1 h: (a) 25 mM and (b) 50 mM. Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 2. SEM micrographs with two magnifications for second step of synthesis of ZnO nanorod arrays within honeycomb monolithic cordierite for 4 h at precursor concentrations of: (a, b) 25 mM, (c, d) 12.5 mM, and (e, f) 6.25 mM.
diameter of the nanorods descended from 250 to 175 nm as the precursor concentration decreased from 25 to 6.25 mM while other conditions such as the concentration at the first step and the temperature were constant. In the second step of the hydrothermal synthesis, there was a continued growth of the densely packed primary ZnO nanorods (formed at the first step) in all samples and as a result, the precursor concentration was of a low effect on the ordering and morphology of the ZnO nanorod arrays at second step of the synthesis with base growth. It can also be seen that although high alignment and compactness were obtained in all samples by the two-step hydrothermal growth, some undesirable redundant ZnO rods were formed which disturbed the uniformity of the structure (Fig. 2). Considering the obtained results, hydrothermal synthesis of ZnO nanorod arrays with base growth is not only an expensive and time consuming process but also suffers from lack of uniformity. Lack of uniformity which is the result of abnormally grown ZnO rods may restrict the use of these samples in different applications. To mitigate these problems, the simple and cost-effective one-step hydrothermal synthesis of ZnO nanorod arrays within honeycomb
monolithic was investigated. As mentioned in the experimental section, four samples were prepared having different precursor concentrations, i.e. 50, 25, 12.5, 6.25 mM (Fig. 3). Fig. 3a depicts the SEM micrographs of the sample grown in the 50 mM precursor solution with two different magnifications. Rods and polygons are two morphologies obtained at this concentration. Considering Fig. 3b, it is clear that only one sample represented ZnO nanorod arrays with favorable alignment and compactness which was grown at 25 mM concentration. More reduction of ZnO concentration (12.5 and 6.25 mM) resulted in the sparsely scattered rods on the monolith which is unfavorable. Additionally, with an increase in the precursor concentration, the average diameter of the ZnO nanorods was raised from 80 nm at 6.25 mM to 320 nm at 50 mM. Comparing the one-step hydrothermal synthesis with the base growth hydrothermal synthesis, precursor concentration was the key factor to attain high alignment and desired morphology. The XRD patterns of the prepared samples are depicted in Figs. 4 and 5. One of the important factors which may be
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
4
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 3. SEM micrographs of hydrothermally synthesized samples without base growth for 5 h with precursor concentration of (a) 50 mM, (b) 25 mM, (c) 12.5 mM, and (d) 6.25 mM.
utilized to compare different samples is the texture factor. It shows the preferred orientation in a specific direction and is directly proportional with the preferred growth in that direction. Texture factors of different directions could be calculated form XRD patterns applying Eq. (1) [22] which in fact compares the ratio of various diffraction lines of a sample with the standard sample [23]. In fact, texture factor (Thkl) quantitatively covers alignment topic [24]. P I hkl I o T h ¼ o P hkl ð1Þ I hkl I hkl Where Thkl is texture factor, Ihkl and I ohkl are intensity of diffraction line with Miller indices of hkl related to the tested sample and the standard sample (without any preferred orientation), respectively. Considering Thkl ¼ 1 as a random orientation, definite crystallographic lattice planes with probabilities greater than other planes (preferred orientation) results in Thkl to be larger than 1. Table 1 tabulates the texture factors of (1 0 0), (0 0 2) and (1 0 1) plains calculated from the XRD patterns. Fig. 4 demonstrates the XRD patterns of the samples hydrothermally synthesized with base growth. All the diffraction peaks were related to ZnO and cordierite. Considering the texture factors reported in Table 1, all samples (in the first and second steps) had a preferred orientation along with (0 0 2) direction. In the second step, by increasing the precursor concentration from 6.25 to 25 mM, the T100, T101 are decreased from 0.684, 0.723 to 0.596, 0.589 respectively; Moreover, the enhancement of the T002 plane from 2.824 to 3.984 verifies the improvement of alignment and compactness. Fig. 5 illustrates the XRD patterns of the samples prepared in the one-step hydrothermal growth method. ZnO phase was only detected in the 25 and 50 mM samples indicating a higher
Fig. 4. XRD patterns of hydrothermally synthesized samples with base growth.
alignment and compactness for the former and a greater ZnO content for the latter. In the samples prepared at 12.5 and 6.25 mM concentrations, ZnO phase could not be detected due to the low quantity and poor alignment and compactness of the ZnO nanorods in these samples. T100, T002 and T101 of the 25 mM sample were calculated to be 0.719, 3.110 and 0.610 respectively and that of the 50 mM sample were 0.971, 1.588 and 0.838 (Table 1). Compared to the two-step hydrothermal growth, the 25 mM sample prepared with the one-step hydrothermal growth not only depicted an acceptable alignment and compactness (considering T002 of this sample) but also showed a good uniformity (Fig. 3b). The Thkl calculated for various
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
5
high alignment, compactness and low diameter but also showed a good uniformity in comparison with the samples prepared in the two-step method. The texture factor of the samples calculated by using the XRD patterns also indicated that the growth was prominent along c axis and 〈0 0 2〉 direction for the aligned samples. Acknowledgment We would like to thank Dr. Yadolah Ganjkhanlou, Miss. Sharifmehr, and Mr. Siamak Noraei at the Materials and Energy Research Center for their assistance in conducting the research and preparing the paper. References Fig. 5. XRD patterns of hydrothermally synthesized samples without base growth. Table1 Texture factor (Thkl) calculated using XRD data and statistical results for diameter of ZnO nanorods. Sample
Th (1 0 0)
Th (0 0 2)
Th (1 0 1)
Diameter of rods (nm)
25 mM 1 h 50 mM 1 h 50 mM 1 h–6.25 mM 4h 50 mM 1 h–12.5 mM 4h 50 mM 1 h–25 mM 4 h 6.25 mM 5 h 12.5 mM 5 h 25 mM 5 h 50 mM 5 h
– 0.99 0.68
– 1.88 2.82
– 0.80 0.72
80 710 170 715 175 715
0.60
3.64
0.68
200 715
0.59 – – 0.71 0.97
3.98 – – 3.11 1.58
0.58 – – 0.61 0.83
250 720 85 710 130 715 140 715 320 725
directions was almost similar to each other for the 50 mM sample indicating a limited amount of the preferred growth. Regarding this sample, the same conclusion was confirmed by the SEM images (Fig. 3a) showing the formation of some polygons with a less preferred growth than rods.
4. Conclusion In summary, ZnO nanorod arrays were successfully prepared by one or two step hydrothermal growth within a monolithic cordierite honeycomb. In the first step of the hydrothermal synthesis with a two-step growth, the precursor concentration was optimized for the next step. The sample prepared using 50 mM of the precursor for 1 h had a satisfactory alignment and compactness for the second step of the synthesis. All the samples obtained from the hydrothermal synthesis with a twostep growth had a high alignment and compactness but they suffered from the lack of uniformity because of the ZnO rods abnormally grown on the surface. While the ZnO nanorod arrays were not obtained in all of the samples of the one-step hydrothermal growth but the 25 mM sample not only depicted a
[1] D.C. Look, Recent advances in ZnO materials and devices, Mater. Sci. Eng. B 80 (2001) 383–387. [2] Y. Guo, Z. Ren, W. Xiao, C. Liu, H. Sharma, H. Gao, A. Mhadeshwar, P. X. Gao, Robust 3-D configurated metal oxide nano array based monolithic catalysts with ultra high materials usage efficiency and catalytic performance tenability, Nano Energy 2 (2013) 873–881. [3] C. Zhu, X. Pan, C. Ye, L. Wang, Z. Ye, J. Huang, Effect of CdSe quantum dots on the performance of hybrid solar cells based on ZnO nanorod arrays, Ceram. Int. 39 (2013) 2975–2980. [4] Y.T. Lim, J.Y. Son, J.S. Rhee, Vertical ZnO nanorod array as an effective hydrogen gas sensor, Ceram. Int. 39 (2013) 887–890. [5] T. Kong, Y. Chen, Y. Ye, K. Zhang, Z. Wang, X. Wang, An amperometric glucose biosensor based on the immobilization of glucose oxidase on the ZnO nanotubes, Sens. Actuators B: Chem. 138 (2009) 344–350. [6] D. Chu, Y. Masuda, T. Ohji, K. Kato, Formation and photocatalytic application of ZnO nanotubes using aqueous solution, Langmuir 26 (2010) 2811–2815. [7] O. Lupan, T. Pauporté, T.L. Bahers, B. Viana, I. Ciofini, Wavelengthemission tuning of ZnO nanowire-based light-emitting diodes by Cu doping: experimental and computational insights, Adv. Funct. Mater. 21 (2011) 3564–3572. [8] B.H. Zeng, X. Xu, Y. Bando, U.K. Gautam, T. Zhai, X. Fang, B. Liu, D. Golberg, Template deformation-tailored ZnO nanorod/nanowire arrays: full growth control and optimization of field-emission, Adv. Funct. Mater. 19 (2009) 3165–3172. [9] L. Vayssieres, K. Keis, S.E. Lindquist, A. Hagfeldt, Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO, J. Phys. Chem. B 105 (2001) 3350–3352. [10] H. Yuan, Y. Zhang, Preparation of well-aligned ZnO whiskers on glass substrate by atmospheric MOCVD, J. Cryst. Growth 263 (2004) 119–124. [11] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [12] J.I. Hong, J. Bae, Z.L. Wang, R.L. Snyder, Room-temperature, texturecontrolled growth of ZnO thin films and their application for growing aligned ZnO nanowire arrays, Nanotechnology 20 (2009) 085609. [13] Y.W. Heo, V. Varadarajan, M. Kaufman, K. Kim, D.P. Norton, F. Ren, P.H. Fleming, Site-specific growth of Zno nanorods using catalysisdriven molecular beam epitaxy, Appl. Phys. Lett. 81 (2002) 3046–3048. [14] C.K. Xu, G.D. Xu, Y.K. Liu, G.H. Wang, A simple and novel route for the preparation of ZnO nanorods, Solid State Commun. 122 (2002) 175–179. [15] D. Lin, W. Pan, H. Wu, Morphological control of centimeter long aluminum-doped zinc oxide nanofibers prepared by electrospinning, J. Am. Ceram. Soc. 90 (2007) 71–76. [16] J.J. Wu, H.I. Wen, C.H. Tseng, S.C. Liu, Well-aligned ZnO nanorods via hydrogen treatment of ZnO films, Adv. Funct. Mater. 14 (2004) 806–810.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
6
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
[17] C.H. Lu, C.H. Yeh, Infuence of hydrothermal conditions on the morphology and particle size of zinc oxide powder, Ceram. Int. 26 (2000) 351–357. [18] W. Xiao, Y. Guo, Z. Ren, G. Wrobel, Z. Ren, T. Lu, P.X. Gao, Mechanical-agitation-assisted growth of large-scale and uniform ZnO nanorod arrays within 3D multichannel monolithic substrates, Cryst. Growth Des. 13 (2013) 3657–3664. [19] I.G. Valls, M.L. Cantu, Dye sensitized solar cells based on verticallyaligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime, Energy Environ. Sci. 3 (2013) 789–795. [20] P.-X. Gao, Y. Guo, Z. Ren and Z. Zhang, Metal oxide nanorod arrays on monolithic substrates, patent application PCT/US12/ 57974, 2012.
[21] J.W. Mullin, in: Crystallization, fourth ed., Butterworths: Heinemann, Oxford, 2011, p. 181–214. [22] M. Birkholz, P.F. Fewster, C. Genzel, Thin Film Analysis by X-Ray Scattering, Wiley-VCH, Weinheim, 2006, p. 183–220. [23] N.M. Torkaman, Y. Ganjkhanlou, M. Kazemzad, H.H. Dabaghi, M. Keyanpour-Rad, Crystallographic parameters and electro-optical constants in ITO thin films, Mater. Charact. 61 (2010) 362–370. [24] S.N. Heo, F. Ahmed, B.H. Koo, Growth temperature dependent properties of ZnO nanorod arrays on glass substrate prepared by wet chemical method, Ceram. Int. 40 (2014) 5467–5471.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb Iman Safaee, Mahmood Kazemzadn, Mehdi Alizadeh, Mohammad Reza Rahimipour Materials and Energy Research Center, P.O. Box 14155/4777, Tehran, Iran Received 17 May 2015; received in revised form 16 June 2015; accepted 16 June 2015
Abstract In this study, ZnO nanorod arrays were grown on a monolithic cordierite honeycomb surface by the application of a hydrothermal method. The parameters affecting the nanorod formation were systematically investigated under different scenarios by adjusting the steps of hydrothermal growth after seeding and the concentration of precursors. Analyzing scanning electron microscopy (SEM) images and the X-ray diffraction patterns revealed the alignment, compactness, uniformity and diameters of the prepared nanorods as well as the texture factor in different directions. According to the results, although there was no critical change of the morphology by altering the precursor concentrations using a twostep hydrothermal growth method, some redundant ZnO morphologies were formed beside the aligned nanorods. More interestingly, it was possible to obtain different morphologies under one-step hydrothermal growth, such as combination of the rods and polygons, the aligned rods or widely scattered unaligned rods depending on the concentration of the precursors. The superior alignment, compactness, uniformity as well as the high texture factor and low rod diameter (d o 140 nm) could be obtained by optimizing the one-step hydrothermal growth at the concentration of 25 mM of the precursors (zinc nitrate hexahydrate and hexamethylenetetramine). The results indicated that at optimized conditions, one-step hydrothermal growth is preferred to the two-step method as an efficient and facile technique for ZnO nanorod fabrication. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: ZnO; Nanorods; Monolithic cordierite honeycomb; Texture factor; Alignment
1. Introduction One-dimensional metal oxide nanostructures have attracted widespread attention due to their unique physical and chemical properties. Among metal oxides, ZnO which is the representative of semiconducting and piezoelectric materials with a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature is regarded as a leading candidate for many scientific and technological applications [1]. Owing to their low cost, abundance, morphological and chemical flexibilities, one-dimensional ZnO nanostructure arrays can play a vital role in a wide range of applications including catalysts [2], solar cells [3], gas sensors [4], biosensors [5], photocatalysts [6], light-emitting diodes [7], and field-emission [8]. One-dimensional ZnO nanostructures n
Corresponding author. Tel.: þ98 26 36280040; fax: þ 98 26 36201888. E-mail address: [email protected] (M. Kazemzad).
have been synthesized by a wide variety of techniques, including wet chemical methods [9], metal–organic chemical vapor deposition (MOCVD) [10], physical vapor deposition [11], pulsed laser deposition [12], molecular beam epitaxy (MBE) [13], flux methods [14], electrospinning [15], top-down approaches by etching [16] and hydrothermal synthesis [17]. Among these methods, hydrothermal technique is a promising low-cost, green and scalable method for fabrication of onedimensional ZnO nanostructures. In this regard, alignment of ZnO nanorods is an important demand in its various applications. The mass transfer and electrical properties could be improved in nanosized ZnO rods with high alignments and uniformity [18,19]. In a typical hydrothermal synthesis, the desirable morphology (high alignments and uniformity) could be obtained by varying synthesis conditions during the nucleation and crystal growth. The two-step hydrothermal growth (also known as base-growth hydrothermal synthesis) and one-step hydrothermal growth methods are two main
http://dx.doi.org/10.1016/j.ceramint.2015.06.077 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
2
procedures suggested in the literature [18]. In the former one, the procedure is completed in two steps: (1) growth at a high concentration for a short period of time (nucleation of ZnO is prominent in this step), and (2) growth at a low concentration for a long time (growth of ZnO rods is the prominent phenomenon in the second step) [18]. Moreover, preparation of the ZnO nanorod arrays on monolithic substrate is one of the appealing methods for fabrication of the new generation catalysts of car exhaust [20]. Although fabrication of onedimensional ZnO nanorods on monolithic substrate has previously been reported by some researchers [2], there is a lack of prospective systematic study of the parameters affecting both alignment and the size of hydrothermally grown nanorod arrays, especially in case of commercial monolithic honeycombs. The present study has investigated different parameters associated with the growth of aligned ZnO nanorod arrays on cordierite honeycombs by application of the SEM technique in addition to texture factor calculation using XRD patterns.
with deionized water and sonicated in the ethanol bath for 15 min, followed by drying the samples at 80 1C for several hours. In the two-step hydrothermal synthesis (also known as base-growth), the samples were immersed in the concentrated solution (50, 25 mM) at the first step and heated for an hour. In the second step, the samples were treated at a lower concentration (25, 12.5, 6.25 mM) and heated at 80 1C for 4 h. Then, the prepared samples were washed with water and ethanol as mentioned previously. A hotplate-stirrer (Heidolph, MR-Hei Standard) was used for the hydrothermal growth with a heating range of 0–300 1C and a stirring range of 0–1400 rpm. The X-ray diffraction analyses (XRD) were carried out on a Philips X'pert (3710) diffractometer with Cu kα operating at 40 kV and 30 mA. A Cambridge S360 scanning electron microscope (SEM) was utilized to acquire the micrographs of the as-grown ZnO nanorod arrays on the monolithic cordierite substrates.
2. Experimental
3.1. Analysis of the sample after the first step of base growth method
The ZnO nanorod arrays were prepared within the pores of the commercially available monolithic cordierite samples by the method adopted from the literature [18], as follows. The monolithic cordierite samples were purchased from Iran Delco Company having 1 mm 1 mm square channels and a wall thickness of 180 mm. The monoliths were cut into equal cube pieces sized 1 cm 1 cm 1 cm and then rinsed with acetone, ethanol and deionized water to remove all contaminants. The ZnO seeding procedure was performed by dipping the substrates in the 20 mM zinc acetate solution (Zn(CH3COO)2 2H2O) in ethanol. Then the samples were baked at 150 1C for 10 min. The seeding process was repeated for several times until a smooth layer of seeds stuck to the substrates. Then, the monolithic cordierite substrates were heated at 350 1C for 5 h. In the one-step hydrothermal synthesis, 100 mL aqueous solution was prepared containing different concentrations (50, 25, 12.5, 6.25 mM) of zinc nitrate hexahydrate (Zn (NO3)2 6H2O) in addition to equimolar amounts of hexamethylenetetramine (HMT: C6H12N4). Then, the substrates were put in the 120 mL autoclaves containing the above solutions and heated at 80 1C for 5 h while being stirred. Finally, the as-grown ZnO nanorod arrays within the monolithic cordierite were cleansed
3. Results and discussion
Fig. 1 shows the SEM micrographs with different magnifications for the two samples prepared during the first step of the base growth hydrothermal reaction using 25 and 50 mM of both zinc nitrate and HMT solutions as the precursors. According to the results, the sparsely scattered primary ZnO nanorods obtained in the 25 mM sample, illustrated in Fig. 1a, was not suitable for the second step of the growth. However, the ZnO nanorods with high compactness obtained in the 50 mM sample (Fig. 1b) were appropriate for the subsequent procedure. The results are in agreement with the classical crystallization mechanism indicating that at higher supersaturation levels, nucleation dominates crystal growth which eventually leads to the formation of a large number of particles [21]. 3.2. Analysis of the sample after the second step of base growth method Fig. 2a–f exhibits SEM micrographs of the ZnO nanorod arrays synthesized within the monolithic cordierite honeycomb after the second step with a precursor concentration varying from 6.25 to 25 mM. As observed from Fig. 2, the average
Fig. 1. SEM micrographs of primary ZnO nanorods within honeycomb monolithic cordierite from first step of base growth hydrothermal synthesis for 1 h: (a) 25 mM and (b) 50 mM. Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 2. SEM micrographs with two magnifications for second step of synthesis of ZnO nanorod arrays within honeycomb monolithic cordierite for 4 h at precursor concentrations of: (a, b) 25 mM, (c, d) 12.5 mM, and (e, f) 6.25 mM.
diameter of the nanorods descended from 250 to 175 nm as the precursor concentration decreased from 25 to 6.25 mM while other conditions such as the concentration at the first step and the temperature were constant. In the second step of the hydrothermal synthesis, there was a continued growth of the densely packed primary ZnO nanorods (formed at the first step) in all samples and as a result, the precursor concentration was of a low effect on the ordering and morphology of the ZnO nanorod arrays at second step of the synthesis with base growth. It can also be seen that although high alignment and compactness were obtained in all samples by the two-step hydrothermal growth, some undesirable redundant ZnO rods were formed which disturbed the uniformity of the structure (Fig. 2). Considering the obtained results, hydrothermal synthesis of ZnO nanorod arrays with base growth is not only an expensive and time consuming process but also suffers from lack of uniformity. Lack of uniformity which is the result of abnormally grown ZnO rods may restrict the use of these samples in different applications. To mitigate these problems, the simple and cost-effective one-step hydrothermal synthesis of ZnO nanorod arrays within honeycomb
monolithic was investigated. As mentioned in the experimental section, four samples were prepared having different precursor concentrations, i.e. 50, 25, 12.5, 6.25 mM (Fig. 3). Fig. 3a depicts the SEM micrographs of the sample grown in the 50 mM precursor solution with two different magnifications. Rods and polygons are two morphologies obtained at this concentration. Considering Fig. 3b, it is clear that only one sample represented ZnO nanorod arrays with favorable alignment and compactness which was grown at 25 mM concentration. More reduction of ZnO concentration (12.5 and 6.25 mM) resulted in the sparsely scattered rods on the monolith which is unfavorable. Additionally, with an increase in the precursor concentration, the average diameter of the ZnO nanorods was raised from 80 nm at 6.25 mM to 320 nm at 50 mM. Comparing the one-step hydrothermal synthesis with the base growth hydrothermal synthesis, precursor concentration was the key factor to attain high alignment and desired morphology. The XRD patterns of the prepared samples are depicted in Figs. 4 and 5. One of the important factors which may be
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
4
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 3. SEM micrographs of hydrothermally synthesized samples without base growth for 5 h with precursor concentration of (a) 50 mM, (b) 25 mM, (c) 12.5 mM, and (d) 6.25 mM.
utilized to compare different samples is the texture factor. It shows the preferred orientation in a specific direction and is directly proportional with the preferred growth in that direction. Texture factors of different directions could be calculated form XRD patterns applying Eq. (1) [22] which in fact compares the ratio of various diffraction lines of a sample with the standard sample [23]. In fact, texture factor (Thkl) quantitatively covers alignment topic [24]. P I hkl I o T h ¼ o P hkl ð1Þ I hkl I hkl Where Thkl is texture factor, Ihkl and I ohkl are intensity of diffraction line with Miller indices of hkl related to the tested sample and the standard sample (without any preferred orientation), respectively. Considering Thkl ¼ 1 as a random orientation, definite crystallographic lattice planes with probabilities greater than other planes (preferred orientation) results in Thkl to be larger than 1. Table 1 tabulates the texture factors of (1 0 0), (0 0 2) and (1 0 1) plains calculated from the XRD patterns. Fig. 4 demonstrates the XRD patterns of the samples hydrothermally synthesized with base growth. All the diffraction peaks were related to ZnO and cordierite. Considering the texture factors reported in Table 1, all samples (in the first and second steps) had a preferred orientation along with (0 0 2) direction. In the second step, by increasing the precursor concentration from 6.25 to 25 mM, the T100, T101 are decreased from 0.684, 0.723 to 0.596, 0.589 respectively; Moreover, the enhancement of the T002 plane from 2.824 to 3.984 verifies the improvement of alignment and compactness. Fig. 5 illustrates the XRD patterns of the samples prepared in the one-step hydrothermal growth method. ZnO phase was only detected in the 25 and 50 mM samples indicating a higher
Fig. 4. XRD patterns of hydrothermally synthesized samples with base growth.
alignment and compactness for the former and a greater ZnO content for the latter. In the samples prepared at 12.5 and 6.25 mM concentrations, ZnO phase could not be detected due to the low quantity and poor alignment and compactness of the ZnO nanorods in these samples. T100, T002 and T101 of the 25 mM sample were calculated to be 0.719, 3.110 and 0.610 respectively and that of the 50 mM sample were 0.971, 1.588 and 0.838 (Table 1). Compared to the two-step hydrothermal growth, the 25 mM sample prepared with the one-step hydrothermal growth not only depicted an acceptable alignment and compactness (considering T002 of this sample) but also showed a good uniformity (Fig. 3b). The Thkl calculated for various
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
5
high alignment, compactness and low diameter but also showed a good uniformity in comparison with the samples prepared in the two-step method. The texture factor of the samples calculated by using the XRD patterns also indicated that the growth was prominent along c axis and 〈0 0 2〉 direction for the aligned samples. Acknowledgment We would like to thank Dr. Yadolah Ganjkhanlou, Miss. Sharifmehr, and Mr. Siamak Noraei at the Materials and Energy Research Center for their assistance in conducting the research and preparing the paper. References Fig. 5. XRD patterns of hydrothermally synthesized samples without base growth. Table1 Texture factor (Thkl) calculated using XRD data and statistical results for diameter of ZnO nanorods. Sample
Th (1 0 0)
Th (0 0 2)
Th (1 0 1)
Diameter of rods (nm)
25 mM 1 h 50 mM 1 h 50 mM 1 h–6.25 mM 4h 50 mM 1 h–12.5 mM 4h 50 mM 1 h–25 mM 4 h 6.25 mM 5 h 12.5 mM 5 h 25 mM 5 h 50 mM 5 h
– 0.99 0.68
– 1.88 2.82
– 0.80 0.72
80 710 170 715 175 715
0.60
3.64
0.68
200 715
0.59 – – 0.71 0.97
3.98 – – 3.11 1.58
0.58 – – 0.61 0.83
250 720 85 710 130 715 140 715 320 725
directions was almost similar to each other for the 50 mM sample indicating a limited amount of the preferred growth. Regarding this sample, the same conclusion was confirmed by the SEM images (Fig. 3a) showing the formation of some polygons with a less preferred growth than rods.
4. Conclusion In summary, ZnO nanorod arrays were successfully prepared by one or two step hydrothermal growth within a monolithic cordierite honeycomb. In the first step of the hydrothermal synthesis with a two-step growth, the precursor concentration was optimized for the next step. The sample prepared using 50 mM of the precursor for 1 h had a satisfactory alignment and compactness for the second step of the synthesis. All the samples obtained from the hydrothermal synthesis with a twostep growth had a high alignment and compactness but they suffered from the lack of uniformity because of the ZnO rods abnormally grown on the surface. While the ZnO nanorod arrays were not obtained in all of the samples of the one-step hydrothermal growth but the 25 mM sample not only depicted a
[1] D.C. Look, Recent advances in ZnO materials and devices, Mater. Sci. Eng. B 80 (2001) 383–387. [2] Y. Guo, Z. Ren, W. Xiao, C. Liu, H. Sharma, H. Gao, A. Mhadeshwar, P. X. Gao, Robust 3-D configurated metal oxide nano array based monolithic catalysts with ultra high materials usage efficiency and catalytic performance tenability, Nano Energy 2 (2013) 873–881. [3] C. Zhu, X. Pan, C. Ye, L. Wang, Z. Ye, J. Huang, Effect of CdSe quantum dots on the performance of hybrid solar cells based on ZnO nanorod arrays, Ceram. Int. 39 (2013) 2975–2980. [4] Y.T. Lim, J.Y. Son, J.S. Rhee, Vertical ZnO nanorod array as an effective hydrogen gas sensor, Ceram. Int. 39 (2013) 887–890. [5] T. Kong, Y. Chen, Y. Ye, K. Zhang, Z. Wang, X. Wang, An amperometric glucose biosensor based on the immobilization of glucose oxidase on the ZnO nanotubes, Sens. Actuators B: Chem. 138 (2009) 344–350. [6] D. Chu, Y. Masuda, T. Ohji, K. Kato, Formation and photocatalytic application of ZnO nanotubes using aqueous solution, Langmuir 26 (2010) 2811–2815. [7] O. Lupan, T. Pauporté, T.L. Bahers, B. Viana, I. Ciofini, Wavelengthemission tuning of ZnO nanowire-based light-emitting diodes by Cu doping: experimental and computational insights, Adv. Funct. Mater. 21 (2011) 3564–3572. [8] B.H. Zeng, X. Xu, Y. Bando, U.K. Gautam, T. Zhai, X. Fang, B. Liu, D. Golberg, Template deformation-tailored ZnO nanorod/nanowire arrays: full growth control and optimization of field-emission, Adv. Funct. Mater. 19 (2009) 3165–3172. [9] L. Vayssieres, K. Keis, S.E. Lindquist, A. Hagfeldt, Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO, J. Phys. Chem. B 105 (2001) 3350–3352. [10] H. Yuan, Y. Zhang, Preparation of well-aligned ZnO whiskers on glass substrate by atmospheric MOCVD, J. Cryst. Growth 263 (2004) 119–124. [11] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [12] J.I. Hong, J. Bae, Z.L. Wang, R.L. Snyder, Room-temperature, texturecontrolled growth of ZnO thin films and their application for growing aligned ZnO nanowire arrays, Nanotechnology 20 (2009) 085609. [13] Y.W. Heo, V. Varadarajan, M. Kaufman, K. Kim, D.P. Norton, F. Ren, P.H. Fleming, Site-specific growth of Zno nanorods using catalysisdriven molecular beam epitaxy, Appl. Phys. Lett. 81 (2002) 3046–3048. [14] C.K. Xu, G.D. Xu, Y.K. Liu, G.H. Wang, A simple and novel route for the preparation of ZnO nanorods, Solid State Commun. 122 (2002) 175–179. [15] D. Lin, W. Pan, H. Wu, Morphological control of centimeter long aluminum-doped zinc oxide nanofibers prepared by electrospinning, J. Am. Ceram. Soc. 90 (2007) 71–76. [16] J.J. Wu, H.I. Wen, C.H. Tseng, S.C. Liu, Well-aligned ZnO nanorods via hydrogen treatment of ZnO films, Adv. Funct. Mater. 14 (2004) 806–810.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077
6
I. Safaee et al. / Ceramics International ] (]]]]) ]]]–]]]
[17] C.H. Lu, C.H. Yeh, Infuence of hydrothermal conditions on the morphology and particle size of zinc oxide powder, Ceram. Int. 26 (2000) 351–357. [18] W. Xiao, Y. Guo, Z. Ren, G. Wrobel, Z. Ren, T. Lu, P.X. Gao, Mechanical-agitation-assisted growth of large-scale and uniform ZnO nanorod arrays within 3D multichannel monolithic substrates, Cryst. Growth Des. 13 (2013) 3657–3664. [19] I.G. Valls, M.L. Cantu, Dye sensitized solar cells based on verticallyaligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime, Energy Environ. Sci. 3 (2013) 789–795. [20] P.-X. Gao, Y. Guo, Z. Ren and Z. Zhang, Metal oxide nanorod arrays on monolithic substrates, patent application PCT/US12/ 57974, 2012.
[21] J.W. Mullin, in: Crystallization, fourth ed., Butterworths: Heinemann, Oxford, 2011, p. 181–214. [22] M. Birkholz, P.F. Fewster, C. Genzel, Thin Film Analysis by X-Ray Scattering, Wiley-VCH, Weinheim, 2006, p. 183–220. [23] N.M. Torkaman, Y. Ganjkhanlou, M. Kazemzad, H.H. Dabaghi, M. Keyanpour-Rad, Crystallographic parameters and electro-optical constants in ITO thin films, Mater. Charact. 61 (2010) 362–370. [24] S.N. Heo, F. Ahmed, B.H. Koo, Growth temperature dependent properties of ZnO nanorod arrays on glass substrate prepared by wet chemical method, Ceram. Int. 40 (2014) 5467–5471.
Please cite this article as: I. Safaee, et al., On finding of the optimized condition for preparation of aligned ZnO nanorod arrays on monolithic cordierite honeycomb, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.077