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Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells
Advanced Powder Technology 23 (2012) 655–660
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells Alisah Cagatay Cakir, Sule Erten-Ela ⇑ Ege University, Solar Energy Institute, 35100 Bornova-Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 6 August 2011 Accepted 13 August 2011 Available online 24 August 2011 Keywords: Dye sensitized solar cell Nanocrystalline materials Nanostructured zinc oxide X-ray diffraction (XRD) Scanning electron microscopy (SEM)
a b s t r a c t This paper reports additive-free, reproducible, low-temperature solution-based process for the preparation of crystalline ZnO nanorods by homogeneous precipitation from zinc acetate. Also, ZnO nanorod structured dye sensitized solar cells using ruthenium dye (Z907) have been fabricated and characterized. The formation and growth of zinc oxide nanorods are successfully achieved. We analyzed three different synthesis method using solution phase, autoclave and microwave. The calcination effects on the morphology of ZnO nanorods are also investigated. Analysis of ZnO nanorods shows that calcination at lower temperature is resulted in a nanorod growth. Additive-free, well-aligned ZnO nanorods are obtained with the length of 330–558 nm and diameters of 14–36 nm. The XRD, SEM, and PL spectra have been provided for the characterization of ZnO nanorods. Microwave-assisted ZnO nanostructured dye sensitized solar cell devices yielded a short-circuit photocurrent density of 6.60 mA/cm2, an open-circuit voltage of 600 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 2.35% under standard AM 1.5 sun light. Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Among environment-friendly and renewable energy sources, dye sensitized solar cells have always been high on the list of likely candidates [1,2]. As one of the most important oxide semiconductor materials, ZnO has attracted considerable attention due to its good optical, electrical, and piezoelectrical properties and its potential applications in the blue ultraviolet region owing to its direct wide bandgap (3.37 eV) and large excitation binding energy (60 meV at room temperature) [3–5]. The synthesis of one-dimensional (1-D) nano- or microstructured ZnO semiconductor materials has attracted considerable research activity because of their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for applications in optoelectronic devices and functional materials [6]. ZnO nanomaterials with 1-D structures, such as nanowires or nanorods, are especially attractive due to their tunable electronic and optoelectronic properties, and the potential applications in the nanoscale electronic and optoelectronic devices [7,8]. Numbers of methods have proven successful for the reproducible growth of ZnO crystals of specific size and shape, such as thermal evaporation, vapor-phase transport, laser ablation, gamma-ray irradiation, metal–organic chemical vapor deposition, hydrothermal growth, and template assisted solution processes. Among these processes, ⇑ Corresponding author. Tel.: +90 232 3111231; fax: +90 232 3886027. E-mail addresses: [email protected], [email protected] (S. Erten-Ela).
the hydrothermal methods in aqueous solutions are recognized as excellent procedures for the preparation of 1-D ZnO nanocrystallites, as the resulting particles have narrow size distribution, good crystallization, and high-quality growth orientation. Recently, ZnO nanocrystals with various shapes have been synthesized using different surfactants [9–12]. However, the reaction process is complicated and involves the use of additive which is environmentally unfriendly in large-scale industrial production. In this paper, an additive-free method has been developed to prepare zinc oxide nanorods from commercially available zinc acetate precursor using solution-phase reactions. One-dimensional (1-D) and additive-free ZnO nanorods are prepared by using simple solution phase method, autoclave and microwave oven. Also calcination effects on the morphology at low temperature are discussed. The as-prepared and calcined ZnO crystallites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) measurements. Dye sensitized solar cells have been fabricated and characterized using ZnO nanorods and ruthenium dye (Z907).
2. Experimental 2.1. Materials Zinc acetate dehydrate [Zn(CH3COO)22H2O] is purchased from Sigma–Aldrich company and is used as starting material without further purification.
0921-8831/$ - see front matter Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2011.08.003
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Fig. 1. Schematic drawing of dye sensitized solar cell.
2.2. Synthesis of ZnO nanorods in different methods In this paper, we focused on the synthesis of ZnO nanorods from zinc salts in water solution. We concentrated on the following three methods: hydrolysis of zinc acetate in solution phase reaction (Method A), in autoclave (Method B), and in microwave oven (Method C). In a typical experiment, zinc acetate is dissolved in 25 ml deionized water in a beaker. The concentration of Zinc acetate dehydrate is 0.55 M. The solution is stirred with magnetic bar at 100 °C for 1 h until a transparent mixture is obtained. Experiment is conducted with three methods. 2.2.1. (a) Synthesis of one-dimensional ZnO nanostructures by solution phase method (Method A) Subsequently, solution is refluxed, followed by hydrothermal reaction at 200 °C for 36 h. After that, solution is cooled to room temperature naturally. Finally the mixture is poured into a beaker and heated at 200 °C until water evaporated. After wet precipitate is dried in an oven at 90 °C for 12 h. 2.2.2. (b) Synthesis of one-dimensional ZnO nanostructures by autoclave (Method B) Subsequently, solution is loaded into a 500 ml Teflon-lined autoclave, followed by hydrothermal reaction at 200 °C for 36 h. After that, the autoclave is cooled to room temperature naturally. Finally the mixture is poured into a beaker and heated at 200 °C until water evaporated. And then, wet precipitate is dried in an oven at 90 °C for 12 h. 2.2.3. (c) Synthesis of one-dimensional ZnO nanostructures by microwave oven (Method C) Subsequently, solution is loaded into a 100 ml Teflon-lined container. Then solution is irradiated by microwave energy in the microwave oven at 200 °C for 60 min (CEM MARS-5, frequency 2.45 GHz, maximum power 700 W, multimode oven). Then the solution is poured into a beaker and heated at 200 °C until water evaporated. After wet precipitate is dried in an oven at 90 °C for 12 h. Finally, in order to investigate calcination effect, white powder obtained from three steps is calcined in a furnace at 200 °C for 36 h. In aqueous solution, zinc(II) is solvated by water, giving rise to aquo ions. In dilute solutions, zinc(II) can exist as several monomeric hydroxyl species [13,14]. These species include ZnOH + (aq), 2 Zn(OH)2(aq), Zn(OH)2(s), ZnðOHÞ 3 (aq), and ZnðOHÞ4 (aq). At a given zinc(II) concentration, the stability of these complexes is dependent on the pH and temperature of the solution. Solid ZnO nuclei is formed by the dehydration of these hydroxyl species. The ZnO crystal could continue to grow by the condensation of
the surface hydroxyl groups with the zinc-hydroxyl complexes [14,15]. The hydrolysis and condensation reactions of zinc salts result in one-dimensional ZnO crystals under a wide variety of conditions. In general, rod/wire growth is possible in slightly acidic to basic conditions (5 < pH < 12) at temperatures from 50 to 200 °C. Wires are formed at pH > 9 even in the absence of additives [14,16–18]. Basic conditions are crucial because divalent metal ions do not readily hydrolyze in acidic media [19]. The crystal morphology can be controlled by various species in the solution, which act as promoters or inhibitors for nucleation and growth. These species can include the zinc counterion, additives such as amines, and acids and bases [14]. There are lots of reports have been published on ZnO nanorod synthesis and application [20–26]. Literature data shows that additives and pH play an important role in solution that they have a strong effect on the formation of ZnO nanorods. The important point in our study, we did not use any additive or basic medium to form ZnO nanorods. ZnO nanorod formation and growth are controlled with the method and calcination effect. 2.2.4. (d) Fabrication of dye sensitized solar cells FTO (SnO2:F, Pilkington TEC-15; SnO2:F, TEC15, Rsheet:15 ohm/ square), electrically conductive oxide-coated glasses are used as transparent electrodes. The construction of the dye sensitized solar cell device requires first cleaning of the fluorine doped tin oxide (FTO) coated glass substrates in a detergent solution using an ultrasonic bath for 15 min, rinsed with water and ethanol. The calcined powder is used to make ZnO paste in ethanol. Doctor blading technique is used to prepare ZnO films. Then ZnO nanorod-coated electrodes are immersed into the commercially available ruthenium dye (Z907) solution (0.5 mM) and kept at room temperature overnight. The stained electrode and Pt-counter electrode are assembled into a sealed sandwich-type cell by heating with a hot-melt ionomer film (Surlyn 1702, Du-Pont) as a spacer between the electrodes. Platinized FTO glasses are used as counter electrode. Cells are prepared in a sandwich geometry. A drop of electrolyte solution (electrolyte of 1 M LiI, 0.044 M I2, 0.25 M TBP in Acetonitrile/ Valeronitrile (85:15 v/v)) is placed on the drilled hole in the counter electrode of the assembled cell and is driven into the cell. Finally, the hole is sealed using additional Bynel and a cover glass (0.1 mm thickness). Active areas of the cells are adjusted to 0.6 cm2. Schematic drawing and characterization of dye sensitized solar cells are shown in Figs. 1 and 2. 3. Materials characterization The obtained samples are characterized on a Rikagu X-ray diffractometer (XRD) with Cu Ka radiation (1.540 Å). The operation
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Fig. 2. J–V characteristics of dye sensitized solar cells with ruthenium dye based ZnO nanorod electrodes under simulated AM 1.5 (100 mWcm2) illumination.
Fig. 3. XRD patterns of samples before calcination (a), after calcination (b).
voltage and current are kept at 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) images are obtained using a Philips XL 30S FEG. The photoluminescence spectra are
recorded using a PTI-QM1 spectophotometer. UV–Vis absorption spectra are performed on an analytic JENA S 600 UV–Vis spectrophotometer.
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Fig. 4. SEM pictures of ZnO nanorods in simple solution phase for 36 h before calcination (a) and after 36 h calcination (b).
Fig. 5. SEM pictures of ZnO nanorods in autoclave for 36 h before calcination (a) and after 36 h calcination (b).
4. Results and discussion
69.2, 72.5, 76.9° can be indexed to the hexagonal wurtzite phase of zinc oxide (JCPDS card No. 36-1451).
4.1. UV–Vis absorption and emission spectra of ZnO nanorods 4.3. SEM pictures of ZnO nanorods using microwave and autoclave UV–Vis absorption spectrum and PL spectrum are recorded for ZnO nanorods. Absorption and emission maxima of ZnO nanorods are obtained at 374 nm and 390 nm, respectively.
4.2. X-ray diffraction The crystal structures of the ZnO nanorods are investigated using XRD. X-ray powder diffraction pattern of ZnO nanorods before and after calcinations using simple solution phase method, autoclave and microwave oven are supplied in Fig. 3. X-ray diffraction (XRD) is performed on the samples both before and after the formation of nanorods. Fig. 3(a) shows the XRD pattern of samples before calcination. There is no evidence of nanorod formation before calcination in Fig. 3(a). Fig. 3(b) shows the XRD pattern of ZnO nanorod after calcination. The XRD patterns of the sample synthesized at 200 °C for 36 h with different methods are quite identical. This XRD pattern indicated that the peaks corresponding to the ZnO phase are quite prominent compared to the Zn-related peaks. The XRD pattern reveals that, with the use of calcination method, ZnO nanorods are formed. The diffraction peaks positioned at 2h values of 31.8, 34.5, 36.3, 47.6, 56.6, 62.9, 66.3, 68.0,
In order to investigate the role of calcination time and method in the dimension and degree of orientation of the ZnO nanorods, we have carried out different experiments. SEM images show the results of these experiments. In our experiments, we kept constant the amount of the zinc acetate concentration, water content, reaction and calcination times and all other experimental parameters unchanged to investigate its effect on the ZnO nanorod. We have only changed the method from simple phase reaction to autoclave and microwave. In our study, each series of the experiments on the nanorod length and width dependence are conducted using one of the preparation methods. Fig. 4 shows the SEM images of ZnO nanorods synthesized using simple solution phase method (Method A). Fig. 4(a) shows the SEM image of this procedure before calcinations and there is no evidence of the formation of ZnO nanorod. Fig. 4(b) is the image of the ZnO nanorods after calcinations at 200 °C for 36 h, showing uniformity can achieve dense and highly orientated ZnO nanorod clusters compared with Method B and C (Figs. 5 and 6). The SEM picture shows that calcination leads to nanocluster formation and growth. The diameter and the length of the nanorods clusters are 21–24 nm and 330–402 nm, respectively.
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not detected in Fig. 6(b). There is a further increase in length when the method changed. The average length increased almost from 402 to 558 nm by changing the method from simple reaction phase to microwave oven. The average diameters of nanorod are decreased from 21–24 to 14–18 nm by changing the method from simple reaction phase to microwave oven dependent on calcination effect. It can be observed that the increases in the lengths of the nanorods are much higher compared to that of their diameters with changing method. It is observed that, in all the cases, the growths of the nanorods in the lateral dimensions are quite high. Thus, these results indicate that the presence of the microwave irradiation and calcination is essential to regulate the lateral growth of the ZnO nanorods. Nanorod cluster morphology is mostly detected in simple phase reactions. As a result, when all reaction parameters are kept constant for formation of nanorods, a series of experiments are developed using solution phase method, autoclave, microwave oven. It is found that the average length of ZnO nanorods in the microwave method increases after calcination which is due to the fact that this effect leads to formation of ZnO nanorods. Microwave irradiation and calcination resulted in well-shaped and longer nanorods. In this case, method plays an important role during hydrothermal synthesis. XRD and SEM results indicate that calcination at low temperature is crucial for the formation and growth of the ZnO nanorods. 4.4. The evaluation of the performance of nanorod ZnO structured dye sensitized solar cell
Fig. 6. SEM pictures of ZnO nanorods in microwave oven for 60 min before calcination (a) and after 36 h calcination (b).
Table 1 Photovoltaic characterization of dye sensitized nanorod ZnO structured dye sensitized solar cell using Z907 dye. 2
Microwave method (Method C) Autoclave method (Method B) Solution phase method (Method A)
Isc (mA/cm )
Voc (mV)
ff
g (%)
6.60 5.25 4.18
600 600 600
0.59 0.52 0.58
2.35 1.66 1.45
It is found that the length of ZnO nanorods could be well controlled by changing the method in the Zn2+ precursor solution. Fig. 5(a) shows the SEM image of Method B procedure using autoclave and there is no evidence to form ZnO nanorod. Fig. 5(b) shows the SEM picture after 36 h calcination at 200 °C using autoclave method with the diameter of 14–36 nm and length of 459– 500 nm (Method B). It shows that the lengths of nanorods after autoclave and calcination procedures are strengthed. The image in the inset of Fig. 4(b) reveals that ZnO nanorods are hexagonal. The image demonstrates that the orientation of the nanorods is quite good. In Fig. 5(b), the view of the nanorods reveals that the nanorods are well separated from each other not like in Fig. 4(b). Fig. 6 shows the SEM images of Method C using microwave oven. Fig. 6(a) shows the SEM image of Method C before using calcination technique. After calcination, uniform growths of aligned ZnO nanorods having diameters of 14–18 nm and lengths of 360–558 nm are observed (Fig. 6(b)). And, nano clusters are
Fig. 1 shows the schematic drawing of dye sensitized solar cell using ZnO nanorod electrode and Ru dye (Z907). JV data collection is made using Keithley 2400 Source-Meter. JV characteristics of dye sensitized solar cell in dark and under illumination are shown in Fig. 2. Dye sensitized solar cells are fabricated using three different types of ZnO nanorods. Under standard global AM 1.5 solar conditions, the microwave-assisted ZnO nanorod structured cell devices using Z-907 dye gave a short-circuit photocurrent density (Jsc) of 6.60 mA/cm2, an open-circuit voltage (Voc) of 600 mV, and a fill factor (ff) of 0.59, corresponding to an overall conversion efficiency g, derived from the equation g) Jsc Voc ff/light intensity, of 2.35% (see Table 1). 5. Conclusion We analyzed three different synthesis method using solution phase method, autoclave and microwave. Perfect single nanocrystals are obtained using microwave heating and calcination method. We report that microwave method is easily controllable, wellrepeatable, mild, and feasible for the preparation of ZnO nanorods. Dye sensitized solar cell consisting of Z907 dye and ZnO nanorod electrodes have been fabricated and characterized. We report the reasonably good efficiency under standard conditions obtained for Z907 using microwave-assisted ZnO electrodes that shows the open circuit voltage of 600 mV, short-circuit current density of 6.60 mA/cm2, filling factor of 0.59 and overall conversion efficiency of 2.35%. Acknowledgements We acknowledge financial support from Scientific and Technological Research Council of Turkey (TUBITAK). I thank Mechanical Engineer MSc. Cagatay Ela for proofreading and his fruitful advices. References [1] (a) B. Oregan, M. Gratzel, Nature 353 (1991) 737; (b) A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 269.
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[2] (a) S. Erten-Ela, M.D. Yilmaz, B. Icli, Y. Dede, S. Icli, E.U. Akkaya, Org. Lett. 10 (2008) 3299; (b) S. Kolemen, A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-Ela, M. Marszalek, J.H. Yum, S.M. Zakeeruddin, Md.K. Nazeeruddin, M. Graetzel, E.U. Akkaya, Chem. Sci. 2 (2011) 949; (c) S. Kolemen, Y. Cakmak, S. Erten-Ela, Y. Altay, J. Brendel, M. Thelakkat, E.U. Akkaya , Org. Lett. 12 (2010) 3812. [3] Z. Fan, J.G. Lu, J. Nanosci. Nanotechnol. 5 (2005) 1561. [4] Y. Li, M. Zheng, L. Ma, M. Zhong, W. Shen, Inorg. Chem. 47 (2008) 3140. [5] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Markoc, App. Phys. Rev. 98 (2005) 41301. [6] S. Erten-Ela, S. Cogal, S. Icli, Inorg. Chim. Acta 362 (2009) 1855. [7] (a) S. Erten-Ela, J. Brendel, M. Thelakkat, Chem. Phys. Lett. 510 (2011) 93; (b) S. Erten-Ela, G. Turkmen, Renewable Energy, 36 (2011) 1821. [8] S. Erten-Ela, S. Cogal, G. Turkmen, S. Icli, Curr. Appl. Phys. 10 (2010) 187. [9] L. Vayssieres, Adv. Mater. 15 (2003) 464. [10] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P.D. Yang, Angew. Chem. 42 (2003) 3031. [11] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [12] Y. Hu, T. Mei, J. Guo, T. White, Inorg. Chem. 46 (2007) 11031.
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Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells Alisah Cagatay Cakir, Sule Erten-Ela ⇑ Ege University, Solar Energy Institute, 35100 Bornova-Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 6 August 2011 Accepted 13 August 2011 Available online 24 August 2011 Keywords: Dye sensitized solar cell Nanocrystalline materials Nanostructured zinc oxide X-ray diffraction (XRD) Scanning electron microscopy (SEM)
a b s t r a c t This paper reports additive-free, reproducible, low-temperature solution-based process for the preparation of crystalline ZnO nanorods by homogeneous precipitation from zinc acetate. Also, ZnO nanorod structured dye sensitized solar cells using ruthenium dye (Z907) have been fabricated and characterized. The formation and growth of zinc oxide nanorods are successfully achieved. We analyzed three different synthesis method using solution phase, autoclave and microwave. The calcination effects on the morphology of ZnO nanorods are also investigated. Analysis of ZnO nanorods shows that calcination at lower temperature is resulted in a nanorod growth. Additive-free, well-aligned ZnO nanorods are obtained with the length of 330–558 nm and diameters of 14–36 nm. The XRD, SEM, and PL spectra have been provided for the characterization of ZnO nanorods. Microwave-assisted ZnO nanostructured dye sensitized solar cell devices yielded a short-circuit photocurrent density of 6.60 mA/cm2, an open-circuit voltage of 600 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 2.35% under standard AM 1.5 sun light. Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Among environment-friendly and renewable energy sources, dye sensitized solar cells have always been high on the list of likely candidates [1,2]. As one of the most important oxide semiconductor materials, ZnO has attracted considerable attention due to its good optical, electrical, and piezoelectrical properties and its potential applications in the blue ultraviolet region owing to its direct wide bandgap (3.37 eV) and large excitation binding energy (60 meV at room temperature) [3–5]. The synthesis of one-dimensional (1-D) nano- or microstructured ZnO semiconductor materials has attracted considerable research activity because of their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for applications in optoelectronic devices and functional materials [6]. ZnO nanomaterials with 1-D structures, such as nanowires or nanorods, are especially attractive due to their tunable electronic and optoelectronic properties, and the potential applications in the nanoscale electronic and optoelectronic devices [7,8]. Numbers of methods have proven successful for the reproducible growth of ZnO crystals of specific size and shape, such as thermal evaporation, vapor-phase transport, laser ablation, gamma-ray irradiation, metal–organic chemical vapor deposition, hydrothermal growth, and template assisted solution processes. Among these processes, ⇑ Corresponding author. Tel.: +90 232 3111231; fax: +90 232 3886027. E-mail addresses: [email protected], [email protected] (S. Erten-Ela).
the hydrothermal methods in aqueous solutions are recognized as excellent procedures for the preparation of 1-D ZnO nanocrystallites, as the resulting particles have narrow size distribution, good crystallization, and high-quality growth orientation. Recently, ZnO nanocrystals with various shapes have been synthesized using different surfactants [9–12]. However, the reaction process is complicated and involves the use of additive which is environmentally unfriendly in large-scale industrial production. In this paper, an additive-free method has been developed to prepare zinc oxide nanorods from commercially available zinc acetate precursor using solution-phase reactions. One-dimensional (1-D) and additive-free ZnO nanorods are prepared by using simple solution phase method, autoclave and microwave oven. Also calcination effects on the morphology at low temperature are discussed. The as-prepared and calcined ZnO crystallites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) measurements. Dye sensitized solar cells have been fabricated and characterized using ZnO nanorods and ruthenium dye (Z907).
2. Experimental 2.1. Materials Zinc acetate dehydrate [Zn(CH3COO)22H2O] is purchased from Sigma–Aldrich company and is used as starting material without further purification.
0921-8831/$ - see front matter Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2011.08.003
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Fig. 1. Schematic drawing of dye sensitized solar cell.
2.2. Synthesis of ZnO nanorods in different methods In this paper, we focused on the synthesis of ZnO nanorods from zinc salts in water solution. We concentrated on the following three methods: hydrolysis of zinc acetate in solution phase reaction (Method A), in autoclave (Method B), and in microwave oven (Method C). In a typical experiment, zinc acetate is dissolved in 25 ml deionized water in a beaker. The concentration of Zinc acetate dehydrate is 0.55 M. The solution is stirred with magnetic bar at 100 °C for 1 h until a transparent mixture is obtained. Experiment is conducted with three methods. 2.2.1. (a) Synthesis of one-dimensional ZnO nanostructures by solution phase method (Method A) Subsequently, solution is refluxed, followed by hydrothermal reaction at 200 °C for 36 h. After that, solution is cooled to room temperature naturally. Finally the mixture is poured into a beaker and heated at 200 °C until water evaporated. After wet precipitate is dried in an oven at 90 °C for 12 h. 2.2.2. (b) Synthesis of one-dimensional ZnO nanostructures by autoclave (Method B) Subsequently, solution is loaded into a 500 ml Teflon-lined autoclave, followed by hydrothermal reaction at 200 °C for 36 h. After that, the autoclave is cooled to room temperature naturally. Finally the mixture is poured into a beaker and heated at 200 °C until water evaporated. And then, wet precipitate is dried in an oven at 90 °C for 12 h. 2.2.3. (c) Synthesis of one-dimensional ZnO nanostructures by microwave oven (Method C) Subsequently, solution is loaded into a 100 ml Teflon-lined container. Then solution is irradiated by microwave energy in the microwave oven at 200 °C for 60 min (CEM MARS-5, frequency 2.45 GHz, maximum power 700 W, multimode oven). Then the solution is poured into a beaker and heated at 200 °C until water evaporated. After wet precipitate is dried in an oven at 90 °C for 12 h. Finally, in order to investigate calcination effect, white powder obtained from three steps is calcined in a furnace at 200 °C for 36 h. In aqueous solution, zinc(II) is solvated by water, giving rise to aquo ions. In dilute solutions, zinc(II) can exist as several monomeric hydroxyl species [13,14]. These species include ZnOH + (aq), 2 Zn(OH)2(aq), Zn(OH)2(s), ZnðOHÞ 3 (aq), and ZnðOHÞ4 (aq). At a given zinc(II) concentration, the stability of these complexes is dependent on the pH and temperature of the solution. Solid ZnO nuclei is formed by the dehydration of these hydroxyl species. The ZnO crystal could continue to grow by the condensation of
the surface hydroxyl groups with the zinc-hydroxyl complexes [14,15]. The hydrolysis and condensation reactions of zinc salts result in one-dimensional ZnO crystals under a wide variety of conditions. In general, rod/wire growth is possible in slightly acidic to basic conditions (5 < pH < 12) at temperatures from 50 to 200 °C. Wires are formed at pH > 9 even in the absence of additives [14,16–18]. Basic conditions are crucial because divalent metal ions do not readily hydrolyze in acidic media [19]. The crystal morphology can be controlled by various species in the solution, which act as promoters or inhibitors for nucleation and growth. These species can include the zinc counterion, additives such as amines, and acids and bases [14]. There are lots of reports have been published on ZnO nanorod synthesis and application [20–26]. Literature data shows that additives and pH play an important role in solution that they have a strong effect on the formation of ZnO nanorods. The important point in our study, we did not use any additive or basic medium to form ZnO nanorods. ZnO nanorod formation and growth are controlled with the method and calcination effect. 2.2.4. (d) Fabrication of dye sensitized solar cells FTO (SnO2:F, Pilkington TEC-15; SnO2:F, TEC15, Rsheet:15 ohm/ square), electrically conductive oxide-coated glasses are used as transparent electrodes. The construction of the dye sensitized solar cell device requires first cleaning of the fluorine doped tin oxide (FTO) coated glass substrates in a detergent solution using an ultrasonic bath for 15 min, rinsed with water and ethanol. The calcined powder is used to make ZnO paste in ethanol. Doctor blading technique is used to prepare ZnO films. Then ZnO nanorod-coated electrodes are immersed into the commercially available ruthenium dye (Z907) solution (0.5 mM) and kept at room temperature overnight. The stained electrode and Pt-counter electrode are assembled into a sealed sandwich-type cell by heating with a hot-melt ionomer film (Surlyn 1702, Du-Pont) as a spacer between the electrodes. Platinized FTO glasses are used as counter electrode. Cells are prepared in a sandwich geometry. A drop of electrolyte solution (electrolyte of 1 M LiI, 0.044 M I2, 0.25 M TBP in Acetonitrile/ Valeronitrile (85:15 v/v)) is placed on the drilled hole in the counter electrode of the assembled cell and is driven into the cell. Finally, the hole is sealed using additional Bynel and a cover glass (0.1 mm thickness). Active areas of the cells are adjusted to 0.6 cm2. Schematic drawing and characterization of dye sensitized solar cells are shown in Figs. 1 and 2. 3. Materials characterization The obtained samples are characterized on a Rikagu X-ray diffractometer (XRD) with Cu Ka radiation (1.540 Å). The operation
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Fig. 2. J–V characteristics of dye sensitized solar cells with ruthenium dye based ZnO nanorod electrodes under simulated AM 1.5 (100 mWcm2) illumination.
Fig. 3. XRD patterns of samples before calcination (a), after calcination (b).
voltage and current are kept at 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) images are obtained using a Philips XL 30S FEG. The photoluminescence spectra are
recorded using a PTI-QM1 spectophotometer. UV–Vis absorption spectra are performed on an analytic JENA S 600 UV–Vis spectrophotometer.
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Fig. 4. SEM pictures of ZnO nanorods in simple solution phase for 36 h before calcination (a) and after 36 h calcination (b).
Fig. 5. SEM pictures of ZnO nanorods in autoclave for 36 h before calcination (a) and after 36 h calcination (b).
4. Results and discussion
69.2, 72.5, 76.9° can be indexed to the hexagonal wurtzite phase of zinc oxide (JCPDS card No. 36-1451).
4.1. UV–Vis absorption and emission spectra of ZnO nanorods 4.3. SEM pictures of ZnO nanorods using microwave and autoclave UV–Vis absorption spectrum and PL spectrum are recorded for ZnO nanorods. Absorption and emission maxima of ZnO nanorods are obtained at 374 nm and 390 nm, respectively.
4.2. X-ray diffraction The crystal structures of the ZnO nanorods are investigated using XRD. X-ray powder diffraction pattern of ZnO nanorods before and after calcinations using simple solution phase method, autoclave and microwave oven are supplied in Fig. 3. X-ray diffraction (XRD) is performed on the samples both before and after the formation of nanorods. Fig. 3(a) shows the XRD pattern of samples before calcination. There is no evidence of nanorod formation before calcination in Fig. 3(a). Fig. 3(b) shows the XRD pattern of ZnO nanorod after calcination. The XRD patterns of the sample synthesized at 200 °C for 36 h with different methods are quite identical. This XRD pattern indicated that the peaks corresponding to the ZnO phase are quite prominent compared to the Zn-related peaks. The XRD pattern reveals that, with the use of calcination method, ZnO nanorods are formed. The diffraction peaks positioned at 2h values of 31.8, 34.5, 36.3, 47.6, 56.6, 62.9, 66.3, 68.0,
In order to investigate the role of calcination time and method in the dimension and degree of orientation of the ZnO nanorods, we have carried out different experiments. SEM images show the results of these experiments. In our experiments, we kept constant the amount of the zinc acetate concentration, water content, reaction and calcination times and all other experimental parameters unchanged to investigate its effect on the ZnO nanorod. We have only changed the method from simple phase reaction to autoclave and microwave. In our study, each series of the experiments on the nanorod length and width dependence are conducted using one of the preparation methods. Fig. 4 shows the SEM images of ZnO nanorods synthesized using simple solution phase method (Method A). Fig. 4(a) shows the SEM image of this procedure before calcinations and there is no evidence of the formation of ZnO nanorod. Fig. 4(b) is the image of the ZnO nanorods after calcinations at 200 °C for 36 h, showing uniformity can achieve dense and highly orientated ZnO nanorod clusters compared with Method B and C (Figs. 5 and 6). The SEM picture shows that calcination leads to nanocluster formation and growth. The diameter and the length of the nanorods clusters are 21–24 nm and 330–402 nm, respectively.
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not detected in Fig. 6(b). There is a further increase in length when the method changed. The average length increased almost from 402 to 558 nm by changing the method from simple reaction phase to microwave oven. The average diameters of nanorod are decreased from 21–24 to 14–18 nm by changing the method from simple reaction phase to microwave oven dependent on calcination effect. It can be observed that the increases in the lengths of the nanorods are much higher compared to that of their diameters with changing method. It is observed that, in all the cases, the growths of the nanorods in the lateral dimensions are quite high. Thus, these results indicate that the presence of the microwave irradiation and calcination is essential to regulate the lateral growth of the ZnO nanorods. Nanorod cluster morphology is mostly detected in simple phase reactions. As a result, when all reaction parameters are kept constant for formation of nanorods, a series of experiments are developed using solution phase method, autoclave, microwave oven. It is found that the average length of ZnO nanorods in the microwave method increases after calcination which is due to the fact that this effect leads to formation of ZnO nanorods. Microwave irradiation and calcination resulted in well-shaped and longer nanorods. In this case, method plays an important role during hydrothermal synthesis. XRD and SEM results indicate that calcination at low temperature is crucial for the formation and growth of the ZnO nanorods. 4.4. The evaluation of the performance of nanorod ZnO structured dye sensitized solar cell
Fig. 6. SEM pictures of ZnO nanorods in microwave oven for 60 min before calcination (a) and after 36 h calcination (b).
Table 1 Photovoltaic characterization of dye sensitized nanorod ZnO structured dye sensitized solar cell using Z907 dye. 2
Microwave method (Method C) Autoclave method (Method B) Solution phase method (Method A)
Isc (mA/cm )
Voc (mV)
ff
g (%)
6.60 5.25 4.18
600 600 600
0.59 0.52 0.58
2.35 1.66 1.45
It is found that the length of ZnO nanorods could be well controlled by changing the method in the Zn2+ precursor solution. Fig. 5(a) shows the SEM image of Method B procedure using autoclave and there is no evidence to form ZnO nanorod. Fig. 5(b) shows the SEM picture after 36 h calcination at 200 °C using autoclave method with the diameter of 14–36 nm and length of 459– 500 nm (Method B). It shows that the lengths of nanorods after autoclave and calcination procedures are strengthed. The image in the inset of Fig. 4(b) reveals that ZnO nanorods are hexagonal. The image demonstrates that the orientation of the nanorods is quite good. In Fig. 5(b), the view of the nanorods reveals that the nanorods are well separated from each other not like in Fig. 4(b). Fig. 6 shows the SEM images of Method C using microwave oven. Fig. 6(a) shows the SEM image of Method C before using calcination technique. After calcination, uniform growths of aligned ZnO nanorods having diameters of 14–18 nm and lengths of 360–558 nm are observed (Fig. 6(b)). And, nano clusters are
Fig. 1 shows the schematic drawing of dye sensitized solar cell using ZnO nanorod electrode and Ru dye (Z907). JV data collection is made using Keithley 2400 Source-Meter. JV characteristics of dye sensitized solar cell in dark and under illumination are shown in Fig. 2. Dye sensitized solar cells are fabricated using three different types of ZnO nanorods. Under standard global AM 1.5 solar conditions, the microwave-assisted ZnO nanorod structured cell devices using Z-907 dye gave a short-circuit photocurrent density (Jsc) of 6.60 mA/cm2, an open-circuit voltage (Voc) of 600 mV, and a fill factor (ff) of 0.59, corresponding to an overall conversion efficiency g, derived from the equation g) Jsc Voc ff/light intensity, of 2.35% (see Table 1). 5. Conclusion We analyzed three different synthesis method using solution phase method, autoclave and microwave. Perfect single nanocrystals are obtained using microwave heating and calcination method. We report that microwave method is easily controllable, wellrepeatable, mild, and feasible for the preparation of ZnO nanorods. Dye sensitized solar cell consisting of Z907 dye and ZnO nanorod electrodes have been fabricated and characterized. We report the reasonably good efficiency under standard conditions obtained for Z907 using microwave-assisted ZnO electrodes that shows the open circuit voltage of 600 mV, short-circuit current density of 6.60 mA/cm2, filling factor of 0.59 and overall conversion efficiency of 2.35%. Acknowledgements We acknowledge financial support from Scientific and Technological Research Council of Turkey (TUBITAK). I thank Mechanical Engineer MSc. Cagatay Ela for proofreading and his fruitful advices. References [1] (a) B. Oregan, M. Gratzel, Nature 353 (1991) 737; (b) A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 269.
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