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Functional properties of amine-passivated ZnO nanostructures and dye-sensitized solar cell characteristics
Chemical Engineering Journal 213 (2012) 70–77
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Functional properties of amine-passivated ZnO nanostructures and dye-sensitized solar cell characteristics M. Navaneethan a,⇑, J. Archana a, M. Arivanandhan b, Y. Hayakawa a,b a b
Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
h i g h l i g h t s " Shape controlled and monodispersed synthesis of ZnO nanosheets. " HMTA act as an efficient surface passivating agent to limit the particle size. " Functional properties of ZnO nanostructures have been enhanced by amine molecules. " ZnO based DSSC exhibits the efficiency of 3.21% using N719 sensitizer.
a r t i c l e
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Article history: Received 4 June 2012 Received in revised form 24 September 2012 Accepted 2 October 2012 Available online 12 October 2012 Keywords: ZnO nanostructures Wet chemical synthesis Surface passivation Optical properties Dye-sensitized solar cells
a b s t r a c t Monodispersed ZnO nanosheets have been synthesized by a facile wet chemical approach using hexamethylenetetramine (HMTA) as an organic ligand. The synthesized samples were characterized by Xray diffraction, field emission scanning and transmission electron microscopy, UV–visible absorption, photoluminescence spectrophotometry and X-ray photoelectron spectroscopy surface analysis. The role of HMTA concentration on the formation and functional properties of ZnO nanostructure were investigated. Particle agglomeration was restricted and the nanosheet size was limited to 20 nm by passivation of the amine molecule. The 0.05 M HMTA-capped ZnO nanosheets yield a high near band edge luminescence intensity. Dye sensitized solar cells were fabricated using synthesized ZnO nanostructures and a maximum efficiency of 3.21% was achieved. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures have been investigated extensively owing to their excellent optical and electrical properties [1–3]. ZnO is an important II-VI semiconductor since it has a direct band gap of 3.37 eV and high exciton binding energy of 60 meV. ZnO nanostructure applications have been realized in several fields such as solar cells, transparent electrodes, light emitting diodes, and sensors [4–9]. Recently, several reports have shown that nanostructured ZnO is a potential material for the preparation of a photoanode in the development of dye-sensitized solar cells (DSSCs) [10–15]. Several methods exist for the preparation of ZnO nanostructures such as vapor phase transport [16], thermal evaporation [17], electrodeposition [18], hydrothermal growth [19], microwave irradiation [20] and the wet chemical route [21]. Among the methods, the wet chemical route is simple and inex⇑ Corresponding author. Tel./fax: +81 53 4781338. E-mail address: [email protected] (M. Navaneethan). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.001
pensive and can be extended to large-scale production. In addition to that, monodispersed synthesis of nanostructures is very important for potential applications [22–25]. To investigate the functional properties and fabricate high efficiency DSSC devices, highly monodispersed nanostructures are required. Since the high reaction rate in solution route results in irregular morphology and Ostwald ripening leads to the formation of crystals of various sizes, the reaction system has to be controlled using additives such as organic ligands and surfactants. Several reports exist on the synthesis of ZnO nanostructures by the chemical route using organic ligands such as thiols and amines [26–29]. However, thiol molecules possess a sulfur group which yields a ZnO–ZnS core–shell structure. Amine molecules are the best choice in surface passivating ligands owing to their chemisorption nature which exists by virtue of their lone pair electrons on the nitrogen atoms. A variety of amine molecules have been realized as potential ligands for the surface passivation of semiconducting nanoparticles such as triethylamine, hexylamine, butylamine and ethylenediamine [30,31]. In our previous report, ethylenediaminetetraacetic acid was used as a
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capping ligand and thin hexagonal ZnO nano disks morphology with the size of 40 nm was obtained [32]. Umar et al. synthesized the mixed morphologies of spherical nanoparticles, triangle nanosheets and hexagonal shaped ZnO nanostructures with the size of 80 ± 20 nm using hexamethylenetetramine at 90 °C [33]. The molar amount of amine molecules used in the reaction is significant in achieving size confinement and to obtain excellent physicochemical properties. In this paper, the role of hexamethylenetetramine (HMTA) molar concentration on the formation and functional properties of ZnO nanoparticles has been investigated. The synthesized samples have been characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, UV–visible absorption spectrometry, photoluminescence spectrophotometry, Raman spectroscopy and X-ray photoelectron spectroscopy. Moreover, the characteristics of dye-sensitized solar cells using HMTA-capped ZnO nanostructures have been demonstrated. 2. Materials and methods 2.1. Synthesis of ZnO nanostructures All chemicals were purchased from Wako chemicals (Japan) and used without further purification. The synthesis of ZnO nanoparticles is as follows: 0.1 M of zinc acetate and 0.2 M sodium hydroxide was dissolved in 50 ml of deionized water whilst stirring at 480 rpm in three different beakers. The HMTA molar concentration in the three different beakers was 0.005, 0.05 and 0.075 M. The reaction was continued for 8 h at room temperature. Finally, the precipitates were washed with water several times and dried at 90 °C for 6 h. The three different molar concentrations of HMTAcapped ZnO nanoparticles were termed S1, S2 and S3 for the 0.005, 0.05 and 0.075 M HTMA concentrations, respectively. A similar reaction was carried out without HMTA to identify the effect of capping molecule and was termed uncapped ZnO. 2.2. Solar cell fabrication 1.5 g ZnO nanosheets and 50 ml ethanol were ground in a mortar for a few minutes to form colloidal suspensions. Thereafter, 3 drops of triton-X were added to the solution as an organic binder. Fluorine doped tin oxide (FTO) substrates were cleaned ultrasonically using a mixture of acetone and ethanol. The ZnO suspension in ethanolic solution was sprayed over the FTO substrate at a substrate temperature of 150 °C by spray deposition. ZnO-coated FTO substrates (photoanodes) were dried at 50 °C for 15 min and the photoanodes were then annealed at 475 °C for 2 h. Photoanodes were immersed in ethanolic solution with 0.03 M di-tetrabutylammonium cis–bis (isothiocyanato) bis (2,200 -bipyridyl-4,40 -dicarboxylato) ruthenium (II) (N-719). The dye-sensitized photoanode and Pt-coated counter electrode were clamped using clips. Finally, an iodide redox electrolyte was filled between the electrodes via capillary action.
a JEOL JSM 6320F field emission scanning electron microscope. TEM images were recorded using a JEOL JEM 2100F transmission electron microscope at an accelerating voltage of 200 kV. BET surface area analysis was obtained using Micromeritics ASAP 2020. I– V characteristics (1.5 AM 1000 W m2 simulated sunlight) were recorded with a calibrated solar-cell evaluation system (JASCO, CEP25BX). 3. Results and discussion XRD spectra of uncapped and S1, S2 and S3 samples are shown in Fig. 1. The diffraction peaks (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) were indexed to the wurtzite ZnO crystal structure and were well matched with standard JCPDS (card No: 89-0511) of the hexagonal ZnO phase [34]. No other reflections related to impurities were observed. Fig. 2a–d show FESEM images of uncapped, S1, S2 and S3 samples, respectively. Uncapped ZnO nanoparticles were agglomerated without any specific morphology. HMTA-capped ZnO nanoparticles (S1, S2 and S3) exhibited distinguished morphology with detail shown in the TEM images. Fig. 3a-1 and a-2 show TEM and high resolution TEM (HRTEM) images of the uncapped ZnO nanosheet with irregular morphology. Fig. 3b-1 and b-2 show the TEM and HRTEM images of S1. Triangleshaped thin nanosheets were formed. Nanosheet agglomeration was restricted effectively by surface passivation of the amine molecules. The nanosheets sizes were in the range of 20 ± 5 nm in length and 20 nm in base. The HRTEM image shows that the single crystalline nature of the ZnO triangle nanosheet and the spacing between the two fringes was 0.29 nm. The figure inset shows the corresponding FFT pattern of the single ZnO nanosheet. The spot pattern of the FFT confirmed the single crystallinity. The TEM image of S2 is shown in Fig. 3c-1 and exhibited a similar size and morphology to S1 whereas S3 possessed a dumbbell-like morphology with average length of 120 nm as shown in Fig. 3d-1. The molecular HMTA concentrations influenced the ZnO nanosheet morphology. Based on the morphological studies, the formation mechanism of a thin nanosheet is explained as follows: Zn2+ ions are released from zinc acetate and OH ions are released by sodium hydroxide in aqueous solution. This results in the formation of a zinc hydroxyl ion. Since the basic structure of this ion consists of a stacked layer of intercalated anions and a water layer, these layered structures tend to grow with sheet-like morphology [35,36]. The addition of HMTA molecules to the zinc hydroxyl ions restricts further growth
2.3. Measurement techniques XRD spectra were recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200) with Cu Ka radiation at 0.02°/s step interval. FTIR spectra were obtained from JEOL JIR-WINSPEC 50 spectrometer. UV–visible absorption analyses were performed using a Shimadzu (Japan) 3100 PC spectrophotometer with ethanol as dispersing medium. A photoluminescence spectrum was obtained using a xenon lamp source of 285 nm excitation wavelength. Raman spectra were obtained using a JASCO NR 1800 Raman spectrophotometer equipped with Nd:YAG laser. FESEM images were recorded using
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Fig. 1. XRD spectra of uncapped, S1, S2 and S3 samples.
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Fig. 2. FESEM images of (a) uncapped, (b) S1, (c) S2 and (d) S3 samples.
Fig. 3a. (a-1) TEM and (a-2) HRTEM images of uncapped ZnO.
Fig. 3b. (b-1) TEM and (b-2) HRTEM images of S1.
of the c-axis owing to the chemisorption or adsorption of amine molecules on the surface. Thus the adatoms of zinc hydroxyl ions cannot attach to the growing sheets because of steric hindrance by the six carbon chains and four nitrogen atoms. Finally, annealing at 90 °C leads to the formation of ZnO nanosheets. The chemical reaction to produce ZnO nanosheets is as follows:
ZnðCH3 COOÞ2 þ 2NaOH ! Zn2þ ðOHÞ2 þ 2CH3 COONa
ð1Þ
Zn2þ ðOHÞ2 þ C6 H12 N4 ! C6 H12 N4 :¼ Zn2þ ðOHÞ2
ð2Þ
C6 H12 N4 :¼ Zn2þ ðOHÞ2 ! C6 H12 N4 :¼ ZnO ðby annealingÞ
ð3Þ
Fig. 4. shows the FTIR spectra of uncapped and HMTA capped ZnO nanosheets. Uncapped ZnO nanoparticles exhibited the peaks below 1000 cm1, whereas S1, S2 and S3 samples clearly show the vibrational peaks from 1000 cm1 to 1500 cm1 which were considered
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Fig. 3c. (c-1) TEM and (c-2) HRTEM images of S2.
Fig. 3d. (d-1) TEM and (d-2) HRTEM images of S3.
Fig. 5. UV–visible absorption spectra of uncapped, S1, S2 and S3 samples. Fig. 4. FTIR spectra of uncapped, S1, S2 and S3 samples.
as the finger print region of HMTA molecule. The peak at 1040 cm1 attributed to N–C stretching vibration of HMTA. The peaks at 1388 and 1507 cm1 corresponded to CH2 wagging and scissoring vibrations of HMTA. The peak at 3415 cm1 corresponded to N–H stretching vibration of HMTA molecule [37]. It clearly demonstrated the surface passivation of HMTA on the surface of ZnO nanosheets. The UV–visible absorption spectra of uncapped and HMTAcapped ZnO dispersed in ethanol are illustrated in Fig. 5. The absorption edge of the uncapped and S1, S2 and S3 nanoparticles were 378, 375, 374 and 369 nm, respectively and exhibited a blue shift from 378 to 369 nm. The band gaps of the samples are calculated using Tauc’s plot and shown in Fig. 6a–d. The band gap of uncapped ZnO was found to be 3.35 eV, whereas S1, S2 and S3 samples exhibited the band gap of 3.4, 3.42 and 3.47 eV. HMTA
capped ZnO nanosheets show the blue shit in band gaps compared to that of uncapped ZnO. This may be due to the following reasons (i) the size reduction of nanosheets leads to the quantum confinement effect and (ii) the formation of anisotropic morphology which gives the edge dependent optical properties [38]. Photoluminescence spectra of uncapped and HMTA-capped ZnO nanosheets are shown in Fig. 7a–c indicate expanded spectra of near band edge (NBE) emission and trap level emission. HMTA-capped ZnO nanosheets exhibited enhanced NBE emission as compared with that of uncapped ZnO nanoparticles [39]. This indicated that the surface-related dangling bonds were effectively passivated by the HMTA molecules. However, 0.075 M capped ZnO nanosheets exhibited low intensity as compared with that of 0.005 and 0.05 M capped ZnO nanosheets. This demonstrated clearly that monodispersity and the avoidance of aggregation played an impor-
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Fig. 6. (a–d) Band gap plot of uncapped, S1, S2 and S3 samples.
tant role in the photoluminescence of ZnO nanostructures. The trap level green emission peaks seen at 570 nm in all sample resulted from intrinsic defects caused by oxygen vacancies [40]. Fig. 8 shows the relationship between the NBE emission and trap level emission as a function of HMTA concentration. This illustrates clearly the role of amine molecule concentration on NBE emission as well as trap level emission. When the amine concentrations were 0.005 and 0.05 M, the NBE emission was predominant over the trap level emission as compared with that of the uncapped and 0.075 M capped ZnO nanostructures. On the other hand, when the amine concentration was 0.075 M, it exhibited a strong trap level emission. This is due to the segregation of excess of HMTA molecule in ZnO nanostructures and more amount of organic ligand can quench the photoluminescence peak. This shows clearly that the molar concentrations of the organic ligand played a determinative role in the luminescence properties of ZnO. Laser Raman spectra of uncapped and HMTA-capped ZnO nanostructures are shown in Fig. 9. According to group theory, wurtzite ZnO belongs to the space group of C 46v with two formula units per primitive cell and all of the atoms occupying 2b sites of symmetry C3v. In all four samples, three peaks were observed clearly at 333, 436 and 580 cm1. Uncapped ZnO nanoparticles displayed a peak at 436 cm1 which corresponded to E2 (LO) phonon mode of the wurtzite crystal lattice of ZnO. However, E2 (LO) modes of the different concentrations of HMTA-capped ZnO nanostructures shifted towards higher energy of approximately 4–5 cm1 which indicated the influence of amine molecules on the lattice vibrations. The shift towards higher wave number was accounted for by oxygen vibrations. The broad peak at 333 cm1 was attributed to the second order Raman signals from zone boundary phonons E2high E2low. E1 (LO) mode was observed at 580 cm1 which corresponded to the oxygen deficiencies in the wurtzite crystal lattice of ZnO [41–43] and was evident from the defect level emission at 570 nm in the photoluminescence spectra of ZnO nanostructures.
Fig. 10a and b show the XPS spectra of the Zn 2p state and O 1s state of the samples. The XPS spectra of the Zn 2p state of the uncapped and HMTA-capped ZnO nanoparticles are symmetrical. The uncapped ZnO nanoparticles contain two peaks centered at 1025.67 and 1048.66 eV. The peak position at 1025 eV confirms that the Zn is coordinated or bonded with other elements (elemental Zn:1020 eV) [44,45]. The peak position of samples S1 and S2 were slightly shifted from 1025.67 to 1025. 37 eV when compared with that of the uncapped sample. This slight shift may be attributed to the Zn interaction with amine molecules. However, the peak position of S3 did not shift and it retains the same peak position as the uncapped ZnO. A possible reason may be that increased amounts of amine in solution hinder the reaction with Zn2+ ions. In addition, the reaction solution electro negativity was increased by the higher amine concentration. O1s spectra of all samples exhibit an asymmetrical shape and can be deconvoluted into two peaks. The main peak located at 530.27 eV is attributed to oxygen which is bonded with Zn in the ZnO lattices [46,47]; this peak is seen clearly in all samples. The peak located at 531.93 eV corresponds to surface adsorbed or chemisorbed oxygen in the sample. The dominance of the peak at 531.93 eV is slightly decreased for samples S1 and S2 compared with that at 530.27 eV. Furthermore, the dominance of surface adsorbed oxygen is higher than that of oxygen in the ZnO lattice of the S3 sample. It is evident that the high molar concentration of HMTA causes the oxygen-related defects in the ZnO as was confirmed by the photoluminescence results. The top and cross sectional FESEM views of the annealed photoanode of sample S2 are shown in Fig. 11. They show clearly that the pores adsorb enough dye molecules on the ZnO nanostructures. The surface areas of the samples were obtained by BET analysis. The uncapped, S1, S2 and S3 were 0.04, 2.33, 7.78 and 3.17 m2/g. HMTA capped samples S1, S2 and S3 exhibited enhanced surface area as compared to that of uncapped ZnO. It indicates the effective passivation of HMTA on surface of ZnO nanosheets. I–V curves of
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Fig. 7. (a) Photoluminescence spectra of uncapped, S1, S2 and S3 samples. Expanded (b) NBE emission and (c) trap level emission spectra of uncapped, S1, S2 and S3 samples.
Fig. 8. Relationship between NBE and trap level emission as a function of HMTA molar concentration.
Fig. 9. Raman spectra of uncapped, S1, S2 and S3 samples.
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Fig. 12. I–V curves of dye-sensitized solar cells.
Table 1 BET surface area and device parameters.
Fig. 10. (a) XPS spectra of Zn 2p state and (b) O 1s state of S1, S2 and S3 samples.
the devices are shown in Fig. 12 with parameters summarized in Table 1. The uncapped ZnO nanostructure-coated DSSC exhibits an efficiency of 0.20%, whereas the S1, S2 and S3 DSSCs exhibit a remarkably enhanced efficiency of 2.20, 3.21 and 2.80%, respectively. The short circuit current (Jsc) and open circuit voltage (Voc) of the devices increased gradually for the uncapped and S1 and S2 samples, whereas the Voc and FF values of the S3 sample are low as compared with those of the S1 and S2 samples. This is due to the decrease of surface area of S3 sample when compared to S2 sample as evidenced by BET analysis. In addition to that, the lower area of S3 sample limits the dye adsorption on the sur-
Sample name
BET surface area (m2/ g)
Jsc (mA cm2)
Voc (V)
FF
Uncapped S1 S2 S3
0.04 2.33 7.78 3.17
0.927 5.195 8.165 6.274
0.526 0.674 0.761 0.676
0.42 0.63 0.51 0.66
g (%) 0.20 2.20 3.21 2.80
face which leads to the decrease in Jsc of the DSSC. The overall efficiency of the 0.05 M HMTA-capped ZnO DSSC exhibits an enhanced efficiency compared with the uncapped ZnO DSSC. This clearly demonstrates monodispersity in size and morphology of ZnO nanostructures are important for the enhanced functional characteristics of nanostructures. Moreover, ZnO nanosheets coated DSSC shows a higher efficiency than the reported values for various ZnO nanostructures such as 1.9% of nanoflowers [48], 1.3% of nanowires [49], 0.3% of nanowires array [50]. 4. Conclusions The effect of amine molecule concentration on morphology, structural, optical and DSSC characteristics of ZnO nanostructures obtained by the wet chemical route was investigated. ZnO nanostructure-coated photoanodes were prepared by the spray deposition technique and the resultant DSSC characteristics were determined. ZnO nanosheets with an average size of 20 nm were
Fig. 11. (a) Top and (b) cross sectional view of annealed photoanode of sample S2.
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synthesized using HMTA. Monodispersity and a distinguished ZnO nanosheet morphology were obtained by HMTA addition. Enhanced photoluminescence intensity and DSSC efficiency were obtained for 0.05 M HMTA. This indicates that an optimal capping molecule concentration is important to obtain excellent functional properties. Acknowledgements The authors would like to thank Prof. K. Murakami, Prof. K. Hara and Prof. H. Kominami for their support in the I–V and photoluminescence measurements. M. Navaneethan thanks MEXT-Japan for the award of a research fellowship. References [1] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, ZnO nanowire UV photodetectors with high internal gain, Nano Lett. 7 (2007) 1003. [2] S.J. Chang, T.J. Hsueh, I.C. Chen, B.R. Huang, Highly sensitive ZnO nanowire CO sensors with the adsorption of Au nanoparticles, Nanotechnology 19 (2008) 175502. [3] M.H. Asif, A. Fulati, O. Nur, M. Willander, C. Brannmark, P. Stralfors, S.I. Borjesson, F. Elinder, Functionalized zinc oxide nanorod with ionophoremembrane coating as an intracellular Ca2+ selective sensor, Appl. Phys. Lett. 95 (2009) 023703. [4] S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang, Z.L. Wang, Self-powered nanowire devices, Nat. Nano 5 (2010) 366. [5] C. Klingshirn, ZnO: from basics towards applications, Phys. Status Solidi B 244 (2007) 3027. [6] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001) 1897. [7] J.S. Huang, C.Y. Chou, C.F. Lin, Enhancing performance of organic–inorganic hybrid solar cells using a fullerene interlayer from all-solution processing, Solar Energy Mater. Solar Cells 94 (2010) 182. [8] I. Gonzalez-Valls, M. Lira-Cantu, Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review, Energy Environ. Sci. 2 (2009) 19. [9] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455. [10] E. Puyoo, G. Rey, E. Appert, V. Consonni, D. Bellet, Efficient dye-sensitized solar cells made from ZnO nanostructure composites, J. Phys. Chem. C 116 (2012) 18117. [11] M. McCune, W. Zhang, Y. Deng, High efficiency dye-sensitized solar cells based on three-dimensional multilayered ZnO nanowire arrays with ‘‘caterpillarlikelike’’ structure, Nano Lett. 12 (2012) 3656. [12] Z. Dong, X. Lai, J.E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang, L. Jiang, Accurate control of multishelled ZnO hollow microspheres for dye-sensitized solar cells with high efficiency, Adv. Mater. 24 (2012) 1046. [13] X. Lai, J.E. Halpert, D. Wang, Recent advances in micro-/nano-structured hollow spheres for energy applications: from simple to complex systems, Energy Environ. Sci. 5 (2012) 5604. [14] S. Ameena, M.S. Akhtar, H.-K. Seoc, Y.S. Kim, H.S. Shina, Influence of Sn doping on ZnO nanostructures from nanoparticles to spindle shape and their photoelectrochemical properties for dye sensitized solar cells, Chem. Eng. J. 187 (2012) 351. [15] N. Mir, M. Salavati-Niasari, F. Davar, Preparation of ZnO nanoflowers and Zn glycerolate nanoplates using inorganic precursors via a convenient rout and application in dye sensitized solar cells, Chem. Eng. J. 181–182 (2012) 779. [16] J.S. Jeong, J.Y. Lee, J.H. Cho, C.J. Lee, S.J. An, G.C. Yi, R. Gronsky, Growth behaviour of well-aligned ZnO nanowires on a Si substrate at low temperature and their optical properties, Nanotechnology 16 (2005) 2455. [17] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Condens Phys. Matter 16 (2004) R829. [18] J. Yang, G. Liu, J. Lu, Y. Qiu, S. Yang, Electrochemical route to the synthesis of ultrathin ZnO nanorod/nanobelt arrays on zinc substrate, Appl. Phys. Lett. 90 (2007) 103109. [19] C.L. Kuo, T.J. Kuo, M.H. Huang, Hydrothermal synthesis of ZnO microspheres and hexagonal microrods with sheetlike and platelike nanostructures, J. Phys. Chem. B 109 (2005) 20115. [20] A. Irzh, N. Perkas, A. Gedanken, Microwave-assisted coating of PMMA beads by silver nanoparticles, Langmuir 23 (2007) 9891. [21] X. Tang, E.S. Guang Choo, L. Li, J. Ding, J. Xue, Synthesis of ZnO nanoparticles with tunable emission colors and their cell labeling applications, Chem. Mater. 22 (2010) 3383. [22] Y. Xia, T.D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S.C. Glotzer, N.A. Kotov, Self-assembly of self-limiting monodisperse supraparticles from polydisperse nanoparticles, Nat. Nanotechnol. 6 (2011) 580.
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Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Functional properties of amine-passivated ZnO nanostructures and dye-sensitized solar cell characteristics M. Navaneethan a,⇑, J. Archana a, M. Arivanandhan b, Y. Hayakawa a,b a b
Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
h i g h l i g h t s " Shape controlled and monodispersed synthesis of ZnO nanosheets. " HMTA act as an efficient surface passivating agent to limit the particle size. " Functional properties of ZnO nanostructures have been enhanced by amine molecules. " ZnO based DSSC exhibits the efficiency of 3.21% using N719 sensitizer.
a r t i c l e
i n f o
Article history: Received 4 June 2012 Received in revised form 24 September 2012 Accepted 2 October 2012 Available online 12 October 2012 Keywords: ZnO nanostructures Wet chemical synthesis Surface passivation Optical properties Dye-sensitized solar cells
a b s t r a c t Monodispersed ZnO nanosheets have been synthesized by a facile wet chemical approach using hexamethylenetetramine (HMTA) as an organic ligand. The synthesized samples were characterized by Xray diffraction, field emission scanning and transmission electron microscopy, UV–visible absorption, photoluminescence spectrophotometry and X-ray photoelectron spectroscopy surface analysis. The role of HMTA concentration on the formation and functional properties of ZnO nanostructure were investigated. Particle agglomeration was restricted and the nanosheet size was limited to 20 nm by passivation of the amine molecule. The 0.05 M HMTA-capped ZnO nanosheets yield a high near band edge luminescence intensity. Dye sensitized solar cells were fabricated using synthesized ZnO nanostructures and a maximum efficiency of 3.21% was achieved. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures have been investigated extensively owing to their excellent optical and electrical properties [1–3]. ZnO is an important II-VI semiconductor since it has a direct band gap of 3.37 eV and high exciton binding energy of 60 meV. ZnO nanostructure applications have been realized in several fields such as solar cells, transparent electrodes, light emitting diodes, and sensors [4–9]. Recently, several reports have shown that nanostructured ZnO is a potential material for the preparation of a photoanode in the development of dye-sensitized solar cells (DSSCs) [10–15]. Several methods exist for the preparation of ZnO nanostructures such as vapor phase transport [16], thermal evaporation [17], electrodeposition [18], hydrothermal growth [19], microwave irradiation [20] and the wet chemical route [21]. Among the methods, the wet chemical route is simple and inex⇑ Corresponding author. Tel./fax: +81 53 4781338. E-mail address: [email protected] (M. Navaneethan). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.001
pensive and can be extended to large-scale production. In addition to that, monodispersed synthesis of nanostructures is very important for potential applications [22–25]. To investigate the functional properties and fabricate high efficiency DSSC devices, highly monodispersed nanostructures are required. Since the high reaction rate in solution route results in irregular morphology and Ostwald ripening leads to the formation of crystals of various sizes, the reaction system has to be controlled using additives such as organic ligands and surfactants. Several reports exist on the synthesis of ZnO nanostructures by the chemical route using organic ligands such as thiols and amines [26–29]. However, thiol molecules possess a sulfur group which yields a ZnO–ZnS core–shell structure. Amine molecules are the best choice in surface passivating ligands owing to their chemisorption nature which exists by virtue of their lone pair electrons on the nitrogen atoms. A variety of amine molecules have been realized as potential ligands for the surface passivation of semiconducting nanoparticles such as triethylamine, hexylamine, butylamine and ethylenediamine [30,31]. In our previous report, ethylenediaminetetraacetic acid was used as a
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capping ligand and thin hexagonal ZnO nano disks morphology with the size of 40 nm was obtained [32]. Umar et al. synthesized the mixed morphologies of spherical nanoparticles, triangle nanosheets and hexagonal shaped ZnO nanostructures with the size of 80 ± 20 nm using hexamethylenetetramine at 90 °C [33]. The molar amount of amine molecules used in the reaction is significant in achieving size confinement and to obtain excellent physicochemical properties. In this paper, the role of hexamethylenetetramine (HMTA) molar concentration on the formation and functional properties of ZnO nanoparticles has been investigated. The synthesized samples have been characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, UV–visible absorption spectrometry, photoluminescence spectrophotometry, Raman spectroscopy and X-ray photoelectron spectroscopy. Moreover, the characteristics of dye-sensitized solar cells using HMTA-capped ZnO nanostructures have been demonstrated. 2. Materials and methods 2.1. Synthesis of ZnO nanostructures All chemicals were purchased from Wako chemicals (Japan) and used without further purification. The synthesis of ZnO nanoparticles is as follows: 0.1 M of zinc acetate and 0.2 M sodium hydroxide was dissolved in 50 ml of deionized water whilst stirring at 480 rpm in three different beakers. The HMTA molar concentration in the three different beakers was 0.005, 0.05 and 0.075 M. The reaction was continued for 8 h at room temperature. Finally, the precipitates were washed with water several times and dried at 90 °C for 6 h. The three different molar concentrations of HMTAcapped ZnO nanoparticles were termed S1, S2 and S3 for the 0.005, 0.05 and 0.075 M HTMA concentrations, respectively. A similar reaction was carried out without HMTA to identify the effect of capping molecule and was termed uncapped ZnO. 2.2. Solar cell fabrication 1.5 g ZnO nanosheets and 50 ml ethanol were ground in a mortar for a few minutes to form colloidal suspensions. Thereafter, 3 drops of triton-X were added to the solution as an organic binder. Fluorine doped tin oxide (FTO) substrates were cleaned ultrasonically using a mixture of acetone and ethanol. The ZnO suspension in ethanolic solution was sprayed over the FTO substrate at a substrate temperature of 150 °C by spray deposition. ZnO-coated FTO substrates (photoanodes) were dried at 50 °C for 15 min and the photoanodes were then annealed at 475 °C for 2 h. Photoanodes were immersed in ethanolic solution with 0.03 M di-tetrabutylammonium cis–bis (isothiocyanato) bis (2,200 -bipyridyl-4,40 -dicarboxylato) ruthenium (II) (N-719). The dye-sensitized photoanode and Pt-coated counter electrode were clamped using clips. Finally, an iodide redox electrolyte was filled between the electrodes via capillary action.
a JEOL JSM 6320F field emission scanning electron microscope. TEM images were recorded using a JEOL JEM 2100F transmission electron microscope at an accelerating voltage of 200 kV. BET surface area analysis was obtained using Micromeritics ASAP 2020. I– V characteristics (1.5 AM 1000 W m2 simulated sunlight) were recorded with a calibrated solar-cell evaluation system (JASCO, CEP25BX). 3. Results and discussion XRD spectra of uncapped and S1, S2 and S3 samples are shown in Fig. 1. The diffraction peaks (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) were indexed to the wurtzite ZnO crystal structure and were well matched with standard JCPDS (card No: 89-0511) of the hexagonal ZnO phase [34]. No other reflections related to impurities were observed. Fig. 2a–d show FESEM images of uncapped, S1, S2 and S3 samples, respectively. Uncapped ZnO nanoparticles were agglomerated without any specific morphology. HMTA-capped ZnO nanoparticles (S1, S2 and S3) exhibited distinguished morphology with detail shown in the TEM images. Fig. 3a-1 and a-2 show TEM and high resolution TEM (HRTEM) images of the uncapped ZnO nanosheet with irregular morphology. Fig. 3b-1 and b-2 show the TEM and HRTEM images of S1. Triangleshaped thin nanosheets were formed. Nanosheet agglomeration was restricted effectively by surface passivation of the amine molecules. The nanosheets sizes were in the range of 20 ± 5 nm in length and 20 nm in base. The HRTEM image shows that the single crystalline nature of the ZnO triangle nanosheet and the spacing between the two fringes was 0.29 nm. The figure inset shows the corresponding FFT pattern of the single ZnO nanosheet. The spot pattern of the FFT confirmed the single crystallinity. The TEM image of S2 is shown in Fig. 3c-1 and exhibited a similar size and morphology to S1 whereas S3 possessed a dumbbell-like morphology with average length of 120 nm as shown in Fig. 3d-1. The molecular HMTA concentrations influenced the ZnO nanosheet morphology. Based on the morphological studies, the formation mechanism of a thin nanosheet is explained as follows: Zn2+ ions are released from zinc acetate and OH ions are released by sodium hydroxide in aqueous solution. This results in the formation of a zinc hydroxyl ion. Since the basic structure of this ion consists of a stacked layer of intercalated anions and a water layer, these layered structures tend to grow with sheet-like morphology [35,36]. The addition of HMTA molecules to the zinc hydroxyl ions restricts further growth
2.3. Measurement techniques XRD spectra were recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200) with Cu Ka radiation at 0.02°/s step interval. FTIR spectra were obtained from JEOL JIR-WINSPEC 50 spectrometer. UV–visible absorption analyses were performed using a Shimadzu (Japan) 3100 PC spectrophotometer with ethanol as dispersing medium. A photoluminescence spectrum was obtained using a xenon lamp source of 285 nm excitation wavelength. Raman spectra were obtained using a JASCO NR 1800 Raman spectrophotometer equipped with Nd:YAG laser. FESEM images were recorded using
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Fig. 1. XRD spectra of uncapped, S1, S2 and S3 samples.
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Fig. 2. FESEM images of (a) uncapped, (b) S1, (c) S2 and (d) S3 samples.
Fig. 3a. (a-1) TEM and (a-2) HRTEM images of uncapped ZnO.
Fig. 3b. (b-1) TEM and (b-2) HRTEM images of S1.
of the c-axis owing to the chemisorption or adsorption of amine molecules on the surface. Thus the adatoms of zinc hydroxyl ions cannot attach to the growing sheets because of steric hindrance by the six carbon chains and four nitrogen atoms. Finally, annealing at 90 °C leads to the formation of ZnO nanosheets. The chemical reaction to produce ZnO nanosheets is as follows:
ZnðCH3 COOÞ2 þ 2NaOH ! Zn2þ ðOHÞ2 þ 2CH3 COONa
ð1Þ
Zn2þ ðOHÞ2 þ C6 H12 N4 ! C6 H12 N4 :¼ Zn2þ ðOHÞ2
ð2Þ
C6 H12 N4 :¼ Zn2þ ðOHÞ2 ! C6 H12 N4 :¼ ZnO ðby annealingÞ
ð3Þ
Fig. 4. shows the FTIR spectra of uncapped and HMTA capped ZnO nanosheets. Uncapped ZnO nanoparticles exhibited the peaks below 1000 cm1, whereas S1, S2 and S3 samples clearly show the vibrational peaks from 1000 cm1 to 1500 cm1 which were considered
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Fig. 3c. (c-1) TEM and (c-2) HRTEM images of S2.
Fig. 3d. (d-1) TEM and (d-2) HRTEM images of S3.
Fig. 5. UV–visible absorption spectra of uncapped, S1, S2 and S3 samples. Fig. 4. FTIR spectra of uncapped, S1, S2 and S3 samples.
as the finger print region of HMTA molecule. The peak at 1040 cm1 attributed to N–C stretching vibration of HMTA. The peaks at 1388 and 1507 cm1 corresponded to CH2 wagging and scissoring vibrations of HMTA. The peak at 3415 cm1 corresponded to N–H stretching vibration of HMTA molecule [37]. It clearly demonstrated the surface passivation of HMTA on the surface of ZnO nanosheets. The UV–visible absorption spectra of uncapped and HMTAcapped ZnO dispersed in ethanol are illustrated in Fig. 5. The absorption edge of the uncapped and S1, S2 and S3 nanoparticles were 378, 375, 374 and 369 nm, respectively and exhibited a blue shift from 378 to 369 nm. The band gaps of the samples are calculated using Tauc’s plot and shown in Fig. 6a–d. The band gap of uncapped ZnO was found to be 3.35 eV, whereas S1, S2 and S3 samples exhibited the band gap of 3.4, 3.42 and 3.47 eV. HMTA
capped ZnO nanosheets show the blue shit in band gaps compared to that of uncapped ZnO. This may be due to the following reasons (i) the size reduction of nanosheets leads to the quantum confinement effect and (ii) the formation of anisotropic morphology which gives the edge dependent optical properties [38]. Photoluminescence spectra of uncapped and HMTA-capped ZnO nanosheets are shown in Fig. 7a–c indicate expanded spectra of near band edge (NBE) emission and trap level emission. HMTA-capped ZnO nanosheets exhibited enhanced NBE emission as compared with that of uncapped ZnO nanoparticles [39]. This indicated that the surface-related dangling bonds were effectively passivated by the HMTA molecules. However, 0.075 M capped ZnO nanosheets exhibited low intensity as compared with that of 0.005 and 0.05 M capped ZnO nanosheets. This demonstrated clearly that monodispersity and the avoidance of aggregation played an impor-
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Fig. 6. (a–d) Band gap plot of uncapped, S1, S2 and S3 samples.
tant role in the photoluminescence of ZnO nanostructures. The trap level green emission peaks seen at 570 nm in all sample resulted from intrinsic defects caused by oxygen vacancies [40]. Fig. 8 shows the relationship between the NBE emission and trap level emission as a function of HMTA concentration. This illustrates clearly the role of amine molecule concentration on NBE emission as well as trap level emission. When the amine concentrations were 0.005 and 0.05 M, the NBE emission was predominant over the trap level emission as compared with that of the uncapped and 0.075 M capped ZnO nanostructures. On the other hand, when the amine concentration was 0.075 M, it exhibited a strong trap level emission. This is due to the segregation of excess of HMTA molecule in ZnO nanostructures and more amount of organic ligand can quench the photoluminescence peak. This shows clearly that the molar concentrations of the organic ligand played a determinative role in the luminescence properties of ZnO. Laser Raman spectra of uncapped and HMTA-capped ZnO nanostructures are shown in Fig. 9. According to group theory, wurtzite ZnO belongs to the space group of C 46v with two formula units per primitive cell and all of the atoms occupying 2b sites of symmetry C3v. In all four samples, three peaks were observed clearly at 333, 436 and 580 cm1. Uncapped ZnO nanoparticles displayed a peak at 436 cm1 which corresponded to E2 (LO) phonon mode of the wurtzite crystal lattice of ZnO. However, E2 (LO) modes of the different concentrations of HMTA-capped ZnO nanostructures shifted towards higher energy of approximately 4–5 cm1 which indicated the influence of amine molecules on the lattice vibrations. The shift towards higher wave number was accounted for by oxygen vibrations. The broad peak at 333 cm1 was attributed to the second order Raman signals from zone boundary phonons E2high E2low. E1 (LO) mode was observed at 580 cm1 which corresponded to the oxygen deficiencies in the wurtzite crystal lattice of ZnO [41–43] and was evident from the defect level emission at 570 nm in the photoluminescence spectra of ZnO nanostructures.
Fig. 10a and b show the XPS spectra of the Zn 2p state and O 1s state of the samples. The XPS spectra of the Zn 2p state of the uncapped and HMTA-capped ZnO nanoparticles are symmetrical. The uncapped ZnO nanoparticles contain two peaks centered at 1025.67 and 1048.66 eV. The peak position at 1025 eV confirms that the Zn is coordinated or bonded with other elements (elemental Zn:1020 eV) [44,45]. The peak position of samples S1 and S2 were slightly shifted from 1025.67 to 1025. 37 eV when compared with that of the uncapped sample. This slight shift may be attributed to the Zn interaction with amine molecules. However, the peak position of S3 did not shift and it retains the same peak position as the uncapped ZnO. A possible reason may be that increased amounts of amine in solution hinder the reaction with Zn2+ ions. In addition, the reaction solution electro negativity was increased by the higher amine concentration. O1s spectra of all samples exhibit an asymmetrical shape and can be deconvoluted into two peaks. The main peak located at 530.27 eV is attributed to oxygen which is bonded with Zn in the ZnO lattices [46,47]; this peak is seen clearly in all samples. The peak located at 531.93 eV corresponds to surface adsorbed or chemisorbed oxygen in the sample. The dominance of the peak at 531.93 eV is slightly decreased for samples S1 and S2 compared with that at 530.27 eV. Furthermore, the dominance of surface adsorbed oxygen is higher than that of oxygen in the ZnO lattice of the S3 sample. It is evident that the high molar concentration of HMTA causes the oxygen-related defects in the ZnO as was confirmed by the photoluminescence results. The top and cross sectional FESEM views of the annealed photoanode of sample S2 are shown in Fig. 11. They show clearly that the pores adsorb enough dye molecules on the ZnO nanostructures. The surface areas of the samples were obtained by BET analysis. The uncapped, S1, S2 and S3 were 0.04, 2.33, 7.78 and 3.17 m2/g. HMTA capped samples S1, S2 and S3 exhibited enhanced surface area as compared to that of uncapped ZnO. It indicates the effective passivation of HMTA on surface of ZnO nanosheets. I–V curves of
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Fig. 7. (a) Photoluminescence spectra of uncapped, S1, S2 and S3 samples. Expanded (b) NBE emission and (c) trap level emission spectra of uncapped, S1, S2 and S3 samples.
Fig. 8. Relationship between NBE and trap level emission as a function of HMTA molar concentration.
Fig. 9. Raman spectra of uncapped, S1, S2 and S3 samples.
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Fig. 12. I–V curves of dye-sensitized solar cells.
Table 1 BET surface area and device parameters.
Fig. 10. (a) XPS spectra of Zn 2p state and (b) O 1s state of S1, S2 and S3 samples.
the devices are shown in Fig. 12 with parameters summarized in Table 1. The uncapped ZnO nanostructure-coated DSSC exhibits an efficiency of 0.20%, whereas the S1, S2 and S3 DSSCs exhibit a remarkably enhanced efficiency of 2.20, 3.21 and 2.80%, respectively. The short circuit current (Jsc) and open circuit voltage (Voc) of the devices increased gradually for the uncapped and S1 and S2 samples, whereas the Voc and FF values of the S3 sample are low as compared with those of the S1 and S2 samples. This is due to the decrease of surface area of S3 sample when compared to S2 sample as evidenced by BET analysis. In addition to that, the lower area of S3 sample limits the dye adsorption on the sur-
Sample name
BET surface area (m2/ g)
Jsc (mA cm2)
Voc (V)
FF
Uncapped S1 S2 S3
0.04 2.33 7.78 3.17
0.927 5.195 8.165 6.274
0.526 0.674 0.761 0.676
0.42 0.63 0.51 0.66
g (%) 0.20 2.20 3.21 2.80
face which leads to the decrease in Jsc of the DSSC. The overall efficiency of the 0.05 M HMTA-capped ZnO DSSC exhibits an enhanced efficiency compared with the uncapped ZnO DSSC. This clearly demonstrates monodispersity in size and morphology of ZnO nanostructures are important for the enhanced functional characteristics of nanostructures. Moreover, ZnO nanosheets coated DSSC shows a higher efficiency than the reported values for various ZnO nanostructures such as 1.9% of nanoflowers [48], 1.3% of nanowires [49], 0.3% of nanowires array [50]. 4. Conclusions The effect of amine molecule concentration on morphology, structural, optical and DSSC characteristics of ZnO nanostructures obtained by the wet chemical route was investigated. ZnO nanostructure-coated photoanodes were prepared by the spray deposition technique and the resultant DSSC characteristics were determined. ZnO nanosheets with an average size of 20 nm were
Fig. 11. (a) Top and (b) cross sectional view of annealed photoanode of sample S2.
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synthesized using HMTA. Monodispersity and a distinguished ZnO nanosheet morphology were obtained by HMTA addition. Enhanced photoluminescence intensity and DSSC efficiency were obtained for 0.05 M HMTA. This indicates that an optimal capping molecule concentration is important to obtain excellent functional properties. Acknowledgements The authors would like to thank Prof. K. Murakami, Prof. K. Hara and Prof. H. Kominami for their support in the I–V and photoluminescence measurements. M. Navaneethan thanks MEXT-Japan for the award of a research fellowship. References [1] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, ZnO nanowire UV photodetectors with high internal gain, Nano Lett. 7 (2007) 1003. [2] S.J. Chang, T.J. Hsueh, I.C. Chen, B.R. Huang, Highly sensitive ZnO nanowire CO sensors with the adsorption of Au nanoparticles, Nanotechnology 19 (2008) 175502. [3] M.H. Asif, A. Fulati, O. Nur, M. Willander, C. Brannmark, P. Stralfors, S.I. Borjesson, F. Elinder, Functionalized zinc oxide nanorod with ionophoremembrane coating as an intracellular Ca2+ selective sensor, Appl. Phys. Lett. 95 (2009) 023703. [4] S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang, Z.L. Wang, Self-powered nanowire devices, Nat. Nano 5 (2010) 366. [5] C. Klingshirn, ZnO: from basics towards applications, Phys. Status Solidi B 244 (2007) 3027. [6] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001) 1897. [7] J.S. Huang, C.Y. Chou, C.F. Lin, Enhancing performance of organic–inorganic hybrid solar cells using a fullerene interlayer from all-solution processing, Solar Energy Mater. Solar Cells 94 (2010) 182. [8] I. Gonzalez-Valls, M. Lira-Cantu, Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review, Energy Environ. Sci. 2 (2009) 19. [9] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455. [10] E. Puyoo, G. Rey, E. Appert, V. Consonni, D. Bellet, Efficient dye-sensitized solar cells made from ZnO nanostructure composites, J. Phys. Chem. C 116 (2012) 18117. [11] M. McCune, W. Zhang, Y. Deng, High efficiency dye-sensitized solar cells based on three-dimensional multilayered ZnO nanowire arrays with ‘‘caterpillarlikelike’’ structure, Nano Lett. 12 (2012) 3656. [12] Z. Dong, X. Lai, J.E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang, L. Jiang, Accurate control of multishelled ZnO hollow microspheres for dye-sensitized solar cells with high efficiency, Adv. Mater. 24 (2012) 1046. [13] X. Lai, J.E. Halpert, D. Wang, Recent advances in micro-/nano-structured hollow spheres for energy applications: from simple to complex systems, Energy Environ. Sci. 5 (2012) 5604. [14] S. Ameena, M.S. Akhtar, H.-K. Seoc, Y.S. Kim, H.S. Shina, Influence of Sn doping on ZnO nanostructures from nanoparticles to spindle shape and their photoelectrochemical properties for dye sensitized solar cells, Chem. Eng. J. 187 (2012) 351. [15] N. Mir, M. Salavati-Niasari, F. Davar, Preparation of ZnO nanoflowers and Zn glycerolate nanoplates using inorganic precursors via a convenient rout and application in dye sensitized solar cells, Chem. Eng. J. 181–182 (2012) 779. [16] J.S. Jeong, J.Y. Lee, J.H. Cho, C.J. Lee, S.J. An, G.C. Yi, R. Gronsky, Growth behaviour of well-aligned ZnO nanowires on a Si substrate at low temperature and their optical properties, Nanotechnology 16 (2005) 2455. [17] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, J. Condens Phys. Matter 16 (2004) R829. [18] J. Yang, G. Liu, J. Lu, Y. Qiu, S. Yang, Electrochemical route to the synthesis of ultrathin ZnO nanorod/nanobelt arrays on zinc substrate, Appl. Phys. Lett. 90 (2007) 103109. [19] C.L. Kuo, T.J. Kuo, M.H. Huang, Hydrothermal synthesis of ZnO microspheres and hexagonal microrods with sheetlike and platelike nanostructures, J. Phys. Chem. B 109 (2005) 20115. [20] A. Irzh, N. Perkas, A. Gedanken, Microwave-assisted coating of PMMA beads by silver nanoparticles, Langmuir 23 (2007) 9891. [21] X. Tang, E.S. Guang Choo, L. Li, J. Ding, J. Xue, Synthesis of ZnO nanoparticles with tunable emission colors and their cell labeling applications, Chem. Mater. 22 (2010) 3383. [22] Y. Xia, T.D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S.C. Glotzer, N.A. Kotov, Self-assembly of self-limiting monodisperse supraparticles from polydisperse nanoparticles, Nat. Nanotechnol. 6 (2011) 580.
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