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Synthesis of rod-cluster ZnO nanostructures and their application to dye-sensitized solar cells
Applied Surface Science 268 (2013) 561–565
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of rod-cluster ZnO nanostructures and their application to dye-sensitized solar cells Yongming Meng, Yu Lin ∗ , Jiyuan Yang College of Material Science and Engineering, Engineering Research Center of Environment-Friendly Functional Materials for Ministry of Education, Huaqiao University, Xiamen, Fujian 361021, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 October 2012 Received in revised form 29 December 2012 Accepted 31 December 2012 Available online 9 January 2013 Keywords: Hydrothermal route Rod-cluster ZnO nanostructures Dye-sensitized solar cell
a b s t r a c t This paper reports a simple hydrothermal route for the preparation of rod-cluster ZnO nanostructures and their application to dye-sensitized solar cells. The effects of reaction conditions on the morphologies of ZnO nanostructures and the performance of cells had been investigated including hydrothermal temperature, time and pH value of the reaction solution. The product synthesized with pH = 10 of the reacting solution at 120 ◦ C for 18 h comparatively shows the best photoelectric conversion efficiency of 2.42% on the basis of its structural superiority for light capture and dye absorption. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of economy and society, energy shortage and environmental pollution have become the outstanding problems which are severely hampering the economic and social progress. Consequently the exploitation and utilization of new energy, especially solar energy, has aroused more and more worldwide interest. Dye-sensitized solar cell (DSSC), a neoteric power conversion device, has attracted substantial attention since its first appearance in 1991 due to its low cost, simple fabrication and enormous application prospect [1–3]. To date, over 12% photoelectric conversion efficiency has been reported [4]. As the central component of DSSCs, photoanode has been a focus for researchers. Over the last few years, great efforts have been dedicated to acquiring the respectable photovoltaic performance of DSSCs assembled by the TiO2 electrode layers [5–7]. Simultaneously other superior semiconducting metal oxides are being looked for as the potential alternatives to TiO2 [8–11]. ZnO is a unique kind of metal oxide semiconductor with a 3.37 eV wide band gap and a high electron mobility of 115–155 cm2 V−1 s−1 , which provides a tremendous possibility for application in DSSCs [12]. One-dimensional (1D) ZnO nanostructures as the photoanodes in DSSCs have been thoroughly studied for their rapid charge transport ability to diminish the chance of
∗ Corresponding author. Tel.: +860 592 6162225; fax: +860 592 6162225. E-mail address: [email protected] (Y. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.171
electron recombination. However, the energy conversion efficiencies of the 1D-ZnO-DSSCs remain in relatively low levels as the insufficient surface area of 1D nanostructure for dye loading and light harvesting [13–15]. For the sake of overcoming the above drawbacks, hierarchical structures of ZnO are particularly anticipated to achieve the enhancement of surface area in recent years. Although the conversion efficiencies of ZnO-based DSSCs are lower than commonly used TiO2 for the slow electron-injection rate from an excited dye to ZnO and its instability in an acidic dye [16,17], ZnO is still considered as the most promising alternative material to TiO2 in the field of DSSCs since its easiness of crystallization and diversified growth. Numerous trials have been devoted to regulating the shape and size of ZnO nanomaterials in order to obtain high surface area that can strengthen the absorptive amount of dye molecules and improve the properties of DSSCs based on ZnO electrode [18–21]. ZnO nanostructures with different morphologies have been produced by various physical, chemical and electrochemical methods [22–25]. Undoubtedly the hydrothermal route is favorable with respect to other techniques owing to its low temperature, simplicity and large-scale products. In this paper, a simple hydrothermal method is used to fabricate ZnO rod-cluster hierarchical nanostructures on fluorine-doped tin oxide (FTO) glass substrates. The effects on the morphologies of ZnO nanostructures with different synthesis parameters including hydrothermal temperature, synthetic time and pH value of the reaction solution have been investigated. The photovoltaic performances of the DSSCs based on ZnO nanostructured films have been measured.
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Y. Meng et al. / Applied Surface Science 268 (2013) 561–565
2. Experimental 2.1. Synthesis of ZnO nanostructures For the synthesis of ZnO nanostructures, a closed cylindrical Teflon-lined stainless steel chamber with 50 ml capacity was used. All the reagents used in the experiments were of analytical grade, and no further purification was done. The reaction solution was prepared by dropping an appropriate amount of 25% NH3 ·H2 O into 40 ml 0.1 M aqueous solution of Zn(NO3 )2 under stirring to adjust the pH from 8 to 10. Then, a piece of FTO glass substrate (pretreated by sonication in absolute ethanol and distilled water successively and dried in air at 40 ◦ C) was placed flatly in the reaction solution. The reaction vessel was heated at a constant temperature from 95 ◦ C to 180 ◦ C. The reaction time was varied from 6 h to 18 h. After the reaction, the FTO substrate covered with ZnO nanostructures was washed with distilled water several times and annealed in a resistor furnace at 450 ◦ C for 1 h. The final product was cooled at room temperature for further characterization. 2.2. Characterization The surface morphologies of the as-deposited ZnO nanostructures were analyzed by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800). The crystal structures were characterized by using X-ray diffraction (XRD, AXS D8-Advance with Cu KR radiation). 2.3. Cell fabrication and photoelectrochemical measurements The FTO substrate covered with ZnO thin layer was immersed into 0.25 mM absolute ethanol solution of dye N719 for 1.5 h. When loaded with dye molecules, it was washed with ethanol, dried in air and used as the photo-electrode. An electrodeposited platinum conductive glass was served as the counter-electrode. Then the photo- and counter-electrodes were sandwiched together and several drops of liquid electrolyte were injected into the aperture between the two electrodes for wholly permeating the ZnO thin film. Finally, a piece of cyano acrylate adhesive was used as a sealant. The liquid electrolyte contained 0.4 M sodium iodide, 0.1 M tetrabutyl ammonium iodide, 0.5 M 4-tert-butylpyridine and 0.05 M iodine in an acetonitrile solution. Thus, a DSSC with ZnO photoanode was assembled. Photoelectrochemical tests were carried out by measuring the J–V characteristic curves under simulated AM 1.5 solar illumination at 100 mW cm−2 from a xenon arc lamp (CHF-XM500, Trusttech Co., Ltd., China) in an ambient atmosphere and recorded by using a CHI 660 C electrochemical workstation (CH Instrument Inc., China). 3. Results and discussion The XRD measurements were carried out to confirm the formation of ZnO products on FTO substrate. Typical XRD spectrum of the bare FTO substrate and the sample hydrothermally synthesized at 120 ◦ C with pH = 10 for 12 h is shown in Fig. 1. All the diffraction peaks can be indexed as the hexagonal wurtzite ZnO phase in contrast with JCPDS card no. 36-1451, which manifests that the ZnO nanostructures of good crystal form without other impure phases have been deposited on FTO substrate successfully. Eight peaks can be observed in the as-prepared samples which may be caused by the different crystal growth orientations under the conditions of free liquid phase via a hydrothermal method without any extra electric and magnetic field. Among them, the (0 0 2) peak presents the highest intensity which turns out to be that more ZnO nanorods along c-axis have been formed in this reaction system.
Fig. 1. XRD patterns of (a) bare FTO substrate and (b) ZnO nanostructures synthesized at 120 ◦ C with pH = 10 for 12 h. Table 1 Photovoltaic performance data of DSSCs based on ZnO synthesized with pH = 10 for 12 h at different temperatures. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
95 ◦ C 120 ◦ C 180 ◦ C
0.571 0.506 0.642
5.14 6.32 3.58
0.441 0.606 0.555
1.29 1.94 1.27
Fig. 2a–c shows the surface morphologies of the ZnO nanostructures produced with pH = 10 for 12 h at different temperatures of 95 ◦ C, 120 ◦ C and 180 ◦ C. In Fig. 2a, several slender ZnO nanorods attach to each other at the bottom and form the pyramid-like structure. It implies that this shape of ZnO is attributed to self-assembly of the nanorods because the reaction system can not furnish sufficient energy for atomic mobility at lower temperature and the interaction between ZnO nanorods is not strong enough to enhance their self-assembly [26]. Fig. 2b indicates the tight ZnO nanorodcluster morphology synthesized at 120 ◦ C with pH = 10 for 12 h. This result originates from the fact that nucleation atoms acquire enough thermal energy at a higher solution temperature which quickly elevates their surface mobility and makes them become more activated, leading to an integration of ZnO nanorods. It is obvious that the ZnO rod-cluster in Fig. 2c is much looser than that of Fig. 2b. This can be explained by that the increasing of reaction temperature is helpful for aggregation of the ZnO nanorods but also accelerates the collision between nucleation atoms during the ZnO nanorods growing procedure. This kind of collision may cause the dispersion and desorption of nucleation atoms, which reduces the probability for tight aggregation of the ZnO nanorods [26,27]. Fig. 2d shows the J–V curves for DSSCs assembled by the ZnO nanostructures synthesized at different reaction temperatures. The detail parameters can be seen from the Table 1. The performance of DSSCs reaches the top photoelectric conversion efficiency at 1.94% with the highest short-circuit current density (Jsc ) of 6.32 mA/cm2 and a Table 2 Photovoltaic performance data of DSSCs based on ZnO synthesized at 120 ◦ C for 12 h with different pH values. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
pH = 8 pH = 9 pH = 10
0.855 0.565 0.506
3.53 3.87 6.32
0.558 0.599 0.606
1.68 1.31 1.94
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Fig. 2. SEM images of ZnO synthesized at (a) 95 ◦ C, (b) 120 ◦ C and (c) 180 ◦ C with pH = 10 for 12 h and (d) their current–voltage curves of DSSCs.
filling factor (FF) of 0.606 when the reaction temperature is 120 ◦ C. Meanwhile the open-circuit voltage (Voc ) is lower than the one for the other two samples, which can be attributed to its more surface area available for the photoelectron recombination in spite of the higher electron injection from a dye to ZnO since the larger amount of dye loaded. Fig. 3a–c presents the surface morphologies of ZnO prepared at 120 ◦ C for 12 h with various pH values by changing the dosage of ammonia solution. From Fig. 3a, flower-like ZnO made of numerous uneven nanoparticles can be observed. As shown in Fig. 3b and c, rod-cluster nanostructures constituted with many ZnO nanorods are obtained. It proves that the ammonia concentration in reacting solution plays a key role to the formation of ZnO nanostructures. A high ammonia concentration may result in the large nucleation of Zn2+ and rapidly form the amine complex of [Zn(NH3 )4 ]2+ as the nucleation precursor [28]. The more ammonia solution is added, the more [Zn(NH3 )4 ]2+ will be performed and turned to Zn(OH)2 . During the reaction period, ZnO will crystallize from Zn(OH)2 along the c-axis based on the minimum surface energy in this direction. When the nucleation of ZnO is performed, more nucleation atoms will gather and grow individually along the c-axis into the rod-like crystal. The attachment of ZnO nanorods in Fig. 3b and c can be illustrated by the dynamic equilibration toward the upright direction near the nucleation center in alkaline solution environment [29]. The appearance of some ZnO nanorods with sharp points is possibly caused by the corrosion under high pH value of the reacting solution. Fig. 3d and Table 2 show the performance of cells assembled by the ZnO nanostructures synthesized at different pH values. It is notable that the Jsc and FF rise gradually in company with the increasing of pH value. Impressively the Voc of sample pH = 8 reaches to 0.855 V. To some extent, this higher photovoltage is related to the lower amount of its surface states. For this reason, the prominent photovoltage is exhibited in the sample of pH = 8. SEM photographs of ZnO produced at different reaction time are revealed in Fig. 4a–c. According to these pictures, we can find that the ZnO nanorods are clustering with a dandelion-like morphology
to the extension of the reaction time. In Fig. 4a, a stellate cluster is formed by gathering at the bottom of some ZnO nanorods and plenty of single rods also come up. The shape becomes more and more stereoscopic as the reaction time elevates, but the dandelionlike ZnO nanostructure in Fig. 4c shows a larger size than that of Fig. 4b. The reason can be demonstrated as follows. In the initial stage of the reaction, the supersaturation of the nucleation precursor may be in charge of the majority of ZnO nanorods. As the proceeding of the reaction, the nucleation precursor will be consumed and its concentration in solution descends which leads to the decreasing of growth rate. In the meanwhile, the nanorods aggregate one by one owing to the surface effect and the interaction among different rods under the high temperature and pressure conditions. The extension of the reaction time offers a big possibility to the aggregation of ZnO nanorods until the dynamic balance in the end. The performances of DSSCs fabricated with the ZnO prepared under different reaction time are exhibited in Fig. 4d and Table 3. It is apparent that the Jsc and Á heighten as the reaction time increases. And the sample of 18 h shows a superior photoelectric performance receiving the Jsc of 7.29 mA/cm2 and Á of 2.42%. By comparison, it is obvious that the final sample synthesized at 120 ◦ C with pH = 10 for 18 h presents the maximum conversion efficiency. It may be associated to its ordered and less defective structure consisting of many well crystallized 1DZnO nanorods shown in Fig. 4c. On the one hand, the structure of ZnO nanorods can effectively reduce the potential barrier of the grain boundary for electronic transmission between the
Table 3 Photovoltaic performance data of DSSCs based on ZnO synthesized at 120 ◦ C with pH = 10 for different reaction time. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
6h 12 h 18 h
0.575 0.506 0.572
3.59 6.32 7.29
0.622 0.606 0.580
1.29 1.94 2.42
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Fig. 3. SEM images of ZnO synthesized with pH = (a) 8, (b) 9 and (c) 10 at 120 ◦ C for 12 h and (d) their current–voltage curves of DSSCs.
nanoparticles, thereby enhancing the charge transfer capacity so as to improve the efficiency of photoelectric conversion [30]. On the other hand, the three-dimensional hierarchical structure of ZnO not only provides a high surface area for dye absorption but also strengthens the capture of the light and
vastly prolongs the distance of light traveling within the ZnO layer, which supplies more opportunities for the photons utilized by the N719 dye molecules [31,32]. For the two reasons, the excellent photoelectric conversion efficiency appears in the last sample.
Fig. 4. SEM images of ZnO synthesized for (a) 6 h, (b) 12 h and (c) 18 h with pH = 10 at 120 ◦ C and (d) their current–voltage curves of DSSCs.
Y. Meng et al. / Applied Surface Science 268 (2013) 561–565
4. Conclusions In summary, we have developed a simple hydrothermal method for the fabrication of ZnO hierarchical structure constituted with a cluster of 1D nanorods on FTO substrate. The influence of reaction conditions on the morphologies of ZnO and the performances of cells have been studied in this paper. The product synthesized with pH = 10 of the reacting solution at 120 ◦ C for 18 h comparatively shows the best photoelectric conversion efficiency of 2.42% on the basis of its structural superiority for light capture and dye absorption. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. JB-ZR1137), and the Natural Science Foundation of Fujian Province (Grant no. 2009J01251). References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737–740. [2] M. Grätzel, Journal of Photochemistry and Photobiology C 4 (2003) 145–153. [3] A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chemical Reviews 110 (2010) 6595–6663. [4] A. Yella, H.W. Lee, H.N. Tsao, C.Y. Yi, A.K. Chandiran, Md.K. Nazeeruddin, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629–634. [5] H.Y. Byun, R. Vittal, D.Y. Kim, K.J. Kim, Langmuir 20 (2004) 6853–6857. [6] S. Meng, J. Ren, E. Kaxiras, Nano Letters 8 (2008) 3266–3272. [7] G.T. Dai, L. Zhao, S.M. Wang, J.H. Hu, B.H. Dong, H.B. Lu, J. Li, Journal of Alloys and Compounds 539 (2012) 264–270. [8] A. Le Viet, R. Jose, M.V. Reddy, B.V.R. Chowdari, S. Ramakrishna, The Journal of Physical Chemistry C 114 (2010) 21795–21800. [9] J.F. Qian, P. Liu, Y. Xiao, Y. Jiang, Y.L. Cao, X.P. Ai, H.X. Yang, Advanced Materials 21 (2009) 3663–3667.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of rod-cluster ZnO nanostructures and their application to dye-sensitized solar cells Yongming Meng, Yu Lin ∗ , Jiyuan Yang College of Material Science and Engineering, Engineering Research Center of Environment-Friendly Functional Materials for Ministry of Education, Huaqiao University, Xiamen, Fujian 361021, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 October 2012 Received in revised form 29 December 2012 Accepted 31 December 2012 Available online 9 January 2013 Keywords: Hydrothermal route Rod-cluster ZnO nanostructures Dye-sensitized solar cell
a b s t r a c t This paper reports a simple hydrothermal route for the preparation of rod-cluster ZnO nanostructures and their application to dye-sensitized solar cells. The effects of reaction conditions on the morphologies of ZnO nanostructures and the performance of cells had been investigated including hydrothermal temperature, time and pH value of the reaction solution. The product synthesized with pH = 10 of the reacting solution at 120 ◦ C for 18 h comparatively shows the best photoelectric conversion efficiency of 2.42% on the basis of its structural superiority for light capture and dye absorption. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of economy and society, energy shortage and environmental pollution have become the outstanding problems which are severely hampering the economic and social progress. Consequently the exploitation and utilization of new energy, especially solar energy, has aroused more and more worldwide interest. Dye-sensitized solar cell (DSSC), a neoteric power conversion device, has attracted substantial attention since its first appearance in 1991 due to its low cost, simple fabrication and enormous application prospect [1–3]. To date, over 12% photoelectric conversion efficiency has been reported [4]. As the central component of DSSCs, photoanode has been a focus for researchers. Over the last few years, great efforts have been dedicated to acquiring the respectable photovoltaic performance of DSSCs assembled by the TiO2 electrode layers [5–7]. Simultaneously other superior semiconducting metal oxides are being looked for as the potential alternatives to TiO2 [8–11]. ZnO is a unique kind of metal oxide semiconductor with a 3.37 eV wide band gap and a high electron mobility of 115–155 cm2 V−1 s−1 , which provides a tremendous possibility for application in DSSCs [12]. One-dimensional (1D) ZnO nanostructures as the photoanodes in DSSCs have been thoroughly studied for their rapid charge transport ability to diminish the chance of
∗ Corresponding author. Tel.: +860 592 6162225; fax: +860 592 6162225. E-mail address: [email protected] (Y. Lin). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.171
electron recombination. However, the energy conversion efficiencies of the 1D-ZnO-DSSCs remain in relatively low levels as the insufficient surface area of 1D nanostructure for dye loading and light harvesting [13–15]. For the sake of overcoming the above drawbacks, hierarchical structures of ZnO are particularly anticipated to achieve the enhancement of surface area in recent years. Although the conversion efficiencies of ZnO-based DSSCs are lower than commonly used TiO2 for the slow electron-injection rate from an excited dye to ZnO and its instability in an acidic dye [16,17], ZnO is still considered as the most promising alternative material to TiO2 in the field of DSSCs since its easiness of crystallization and diversified growth. Numerous trials have been devoted to regulating the shape and size of ZnO nanomaterials in order to obtain high surface area that can strengthen the absorptive amount of dye molecules and improve the properties of DSSCs based on ZnO electrode [18–21]. ZnO nanostructures with different morphologies have been produced by various physical, chemical and electrochemical methods [22–25]. Undoubtedly the hydrothermal route is favorable with respect to other techniques owing to its low temperature, simplicity and large-scale products. In this paper, a simple hydrothermal method is used to fabricate ZnO rod-cluster hierarchical nanostructures on fluorine-doped tin oxide (FTO) glass substrates. The effects on the morphologies of ZnO nanostructures with different synthesis parameters including hydrothermal temperature, synthetic time and pH value of the reaction solution have been investigated. The photovoltaic performances of the DSSCs based on ZnO nanostructured films have been measured.
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2. Experimental 2.1. Synthesis of ZnO nanostructures For the synthesis of ZnO nanostructures, a closed cylindrical Teflon-lined stainless steel chamber with 50 ml capacity was used. All the reagents used in the experiments were of analytical grade, and no further purification was done. The reaction solution was prepared by dropping an appropriate amount of 25% NH3 ·H2 O into 40 ml 0.1 M aqueous solution of Zn(NO3 )2 under stirring to adjust the pH from 8 to 10. Then, a piece of FTO glass substrate (pretreated by sonication in absolute ethanol and distilled water successively and dried in air at 40 ◦ C) was placed flatly in the reaction solution. The reaction vessel was heated at a constant temperature from 95 ◦ C to 180 ◦ C. The reaction time was varied from 6 h to 18 h. After the reaction, the FTO substrate covered with ZnO nanostructures was washed with distilled water several times and annealed in a resistor furnace at 450 ◦ C for 1 h. The final product was cooled at room temperature for further characterization. 2.2. Characterization The surface morphologies of the as-deposited ZnO nanostructures were analyzed by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800). The crystal structures were characterized by using X-ray diffraction (XRD, AXS D8-Advance with Cu KR radiation). 2.3. Cell fabrication and photoelectrochemical measurements The FTO substrate covered with ZnO thin layer was immersed into 0.25 mM absolute ethanol solution of dye N719 for 1.5 h. When loaded with dye molecules, it was washed with ethanol, dried in air and used as the photo-electrode. An electrodeposited platinum conductive glass was served as the counter-electrode. Then the photo- and counter-electrodes were sandwiched together and several drops of liquid electrolyte were injected into the aperture between the two electrodes for wholly permeating the ZnO thin film. Finally, a piece of cyano acrylate adhesive was used as a sealant. The liquid electrolyte contained 0.4 M sodium iodide, 0.1 M tetrabutyl ammonium iodide, 0.5 M 4-tert-butylpyridine and 0.05 M iodine in an acetonitrile solution. Thus, a DSSC with ZnO photoanode was assembled. Photoelectrochemical tests were carried out by measuring the J–V characteristic curves under simulated AM 1.5 solar illumination at 100 mW cm−2 from a xenon arc lamp (CHF-XM500, Trusttech Co., Ltd., China) in an ambient atmosphere and recorded by using a CHI 660 C electrochemical workstation (CH Instrument Inc., China). 3. Results and discussion The XRD measurements were carried out to confirm the formation of ZnO products on FTO substrate. Typical XRD spectrum of the bare FTO substrate and the sample hydrothermally synthesized at 120 ◦ C with pH = 10 for 12 h is shown in Fig. 1. All the diffraction peaks can be indexed as the hexagonal wurtzite ZnO phase in contrast with JCPDS card no. 36-1451, which manifests that the ZnO nanostructures of good crystal form without other impure phases have been deposited on FTO substrate successfully. Eight peaks can be observed in the as-prepared samples which may be caused by the different crystal growth orientations under the conditions of free liquid phase via a hydrothermal method without any extra electric and magnetic field. Among them, the (0 0 2) peak presents the highest intensity which turns out to be that more ZnO nanorods along c-axis have been formed in this reaction system.
Fig. 1. XRD patterns of (a) bare FTO substrate and (b) ZnO nanostructures synthesized at 120 ◦ C with pH = 10 for 12 h. Table 1 Photovoltaic performance data of DSSCs based on ZnO synthesized with pH = 10 for 12 h at different temperatures. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
95 ◦ C 120 ◦ C 180 ◦ C
0.571 0.506 0.642
5.14 6.32 3.58
0.441 0.606 0.555
1.29 1.94 1.27
Fig. 2a–c shows the surface morphologies of the ZnO nanostructures produced with pH = 10 for 12 h at different temperatures of 95 ◦ C, 120 ◦ C and 180 ◦ C. In Fig. 2a, several slender ZnO nanorods attach to each other at the bottom and form the pyramid-like structure. It implies that this shape of ZnO is attributed to self-assembly of the nanorods because the reaction system can not furnish sufficient energy for atomic mobility at lower temperature and the interaction between ZnO nanorods is not strong enough to enhance their self-assembly [26]. Fig. 2b indicates the tight ZnO nanorodcluster morphology synthesized at 120 ◦ C with pH = 10 for 12 h. This result originates from the fact that nucleation atoms acquire enough thermal energy at a higher solution temperature which quickly elevates their surface mobility and makes them become more activated, leading to an integration of ZnO nanorods. It is obvious that the ZnO rod-cluster in Fig. 2c is much looser than that of Fig. 2b. This can be explained by that the increasing of reaction temperature is helpful for aggregation of the ZnO nanorods but also accelerates the collision between nucleation atoms during the ZnO nanorods growing procedure. This kind of collision may cause the dispersion and desorption of nucleation atoms, which reduces the probability for tight aggregation of the ZnO nanorods [26,27]. Fig. 2d shows the J–V curves for DSSCs assembled by the ZnO nanostructures synthesized at different reaction temperatures. The detail parameters can be seen from the Table 1. The performance of DSSCs reaches the top photoelectric conversion efficiency at 1.94% with the highest short-circuit current density (Jsc ) of 6.32 mA/cm2 and a Table 2 Photovoltaic performance data of DSSCs based on ZnO synthesized at 120 ◦ C for 12 h with different pH values. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
pH = 8 pH = 9 pH = 10
0.855 0.565 0.506
3.53 3.87 6.32
0.558 0.599 0.606
1.68 1.31 1.94
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Fig. 2. SEM images of ZnO synthesized at (a) 95 ◦ C, (b) 120 ◦ C and (c) 180 ◦ C with pH = 10 for 12 h and (d) their current–voltage curves of DSSCs.
filling factor (FF) of 0.606 when the reaction temperature is 120 ◦ C. Meanwhile the open-circuit voltage (Voc ) is lower than the one for the other two samples, which can be attributed to its more surface area available for the photoelectron recombination in spite of the higher electron injection from a dye to ZnO since the larger amount of dye loaded. Fig. 3a–c presents the surface morphologies of ZnO prepared at 120 ◦ C for 12 h with various pH values by changing the dosage of ammonia solution. From Fig. 3a, flower-like ZnO made of numerous uneven nanoparticles can be observed. As shown in Fig. 3b and c, rod-cluster nanostructures constituted with many ZnO nanorods are obtained. It proves that the ammonia concentration in reacting solution plays a key role to the formation of ZnO nanostructures. A high ammonia concentration may result in the large nucleation of Zn2+ and rapidly form the amine complex of [Zn(NH3 )4 ]2+ as the nucleation precursor [28]. The more ammonia solution is added, the more [Zn(NH3 )4 ]2+ will be performed and turned to Zn(OH)2 . During the reaction period, ZnO will crystallize from Zn(OH)2 along the c-axis based on the minimum surface energy in this direction. When the nucleation of ZnO is performed, more nucleation atoms will gather and grow individually along the c-axis into the rod-like crystal. The attachment of ZnO nanorods in Fig. 3b and c can be illustrated by the dynamic equilibration toward the upright direction near the nucleation center in alkaline solution environment [29]. The appearance of some ZnO nanorods with sharp points is possibly caused by the corrosion under high pH value of the reacting solution. Fig. 3d and Table 2 show the performance of cells assembled by the ZnO nanostructures synthesized at different pH values. It is notable that the Jsc and FF rise gradually in company with the increasing of pH value. Impressively the Voc of sample pH = 8 reaches to 0.855 V. To some extent, this higher photovoltage is related to the lower amount of its surface states. For this reason, the prominent photovoltage is exhibited in the sample of pH = 8. SEM photographs of ZnO produced at different reaction time are revealed in Fig. 4a–c. According to these pictures, we can find that the ZnO nanorods are clustering with a dandelion-like morphology
to the extension of the reaction time. In Fig. 4a, a stellate cluster is formed by gathering at the bottom of some ZnO nanorods and plenty of single rods also come up. The shape becomes more and more stereoscopic as the reaction time elevates, but the dandelionlike ZnO nanostructure in Fig. 4c shows a larger size than that of Fig. 4b. The reason can be demonstrated as follows. In the initial stage of the reaction, the supersaturation of the nucleation precursor may be in charge of the majority of ZnO nanorods. As the proceeding of the reaction, the nucleation precursor will be consumed and its concentration in solution descends which leads to the decreasing of growth rate. In the meanwhile, the nanorods aggregate one by one owing to the surface effect and the interaction among different rods under the high temperature and pressure conditions. The extension of the reaction time offers a big possibility to the aggregation of ZnO nanorods until the dynamic balance in the end. The performances of DSSCs fabricated with the ZnO prepared under different reaction time are exhibited in Fig. 4d and Table 3. It is apparent that the Jsc and Á heighten as the reaction time increases. And the sample of 18 h shows a superior photoelectric performance receiving the Jsc of 7.29 mA/cm2 and Á of 2.42%. By comparison, it is obvious that the final sample synthesized at 120 ◦ C with pH = 10 for 18 h presents the maximum conversion efficiency. It may be associated to its ordered and less defective structure consisting of many well crystallized 1DZnO nanorods shown in Fig. 4c. On the one hand, the structure of ZnO nanorods can effectively reduce the potential barrier of the grain boundary for electronic transmission between the
Table 3 Photovoltaic performance data of DSSCs based on ZnO synthesized at 120 ◦ C with pH = 10 for different reaction time. Sample
Voc (V)
Jsc (mA/cm2 )
FF
Á (%)
6h 12 h 18 h
0.575 0.506 0.572
3.59 6.32 7.29
0.622 0.606 0.580
1.29 1.94 2.42
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Fig. 3. SEM images of ZnO synthesized with pH = (a) 8, (b) 9 and (c) 10 at 120 ◦ C for 12 h and (d) their current–voltage curves of DSSCs.
nanoparticles, thereby enhancing the charge transfer capacity so as to improve the efficiency of photoelectric conversion [30]. On the other hand, the three-dimensional hierarchical structure of ZnO not only provides a high surface area for dye absorption but also strengthens the capture of the light and
vastly prolongs the distance of light traveling within the ZnO layer, which supplies more opportunities for the photons utilized by the N719 dye molecules [31,32]. For the two reasons, the excellent photoelectric conversion efficiency appears in the last sample.
Fig. 4. SEM images of ZnO synthesized for (a) 6 h, (b) 12 h and (c) 18 h with pH = 10 at 120 ◦ C and (d) their current–voltage curves of DSSCs.
Y. Meng et al. / Applied Surface Science 268 (2013) 561–565
4. Conclusions In summary, we have developed a simple hydrothermal method for the fabrication of ZnO hierarchical structure constituted with a cluster of 1D nanorods on FTO substrate. The influence of reaction conditions on the morphologies of ZnO and the performances of cells have been studied in this paper. The product synthesized with pH = 10 of the reacting solution at 120 ◦ C for 18 h comparatively shows the best photoelectric conversion efficiency of 2.42% on the basis of its structural superiority for light capture and dye absorption. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. JB-ZR1137), and the Natural Science Foundation of Fujian Province (Grant no. 2009J01251). References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737–740. [2] M. Grätzel, Journal of Photochemistry and Photobiology C 4 (2003) 145–153. [3] A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chemical Reviews 110 (2010) 6595–6663. [4] A. Yella, H.W. Lee, H.N. Tsao, C.Y. Yi, A.K. Chandiran, Md.K. Nazeeruddin, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629–634. [5] H.Y. Byun, R. Vittal, D.Y. Kim, K.J. Kim, Langmuir 20 (2004) 6853–6857. [6] S. Meng, J. Ren, E. Kaxiras, Nano Letters 8 (2008) 3266–3272. [7] G.T. Dai, L. Zhao, S.M. Wang, J.H. Hu, B.H. Dong, H.B. Lu, J. Li, Journal of Alloys and Compounds 539 (2012) 264–270. [8] A. Le Viet, R. Jose, M.V. Reddy, B.V.R. Chowdari, S. Ramakrishna, The Journal of Physical Chemistry C 114 (2010) 21795–21800. [9] J.F. Qian, P. Liu, Y. Xiao, Y. Jiang, Y.L. Cao, X.P. Ai, H.X. Yang, Advanced Materials 21 (2009) 3663–3667.
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