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Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells
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RAPID COMMUNICATION
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells Yu-Fen Wang, Ke-Nan Li, Yang-Fan Xu, Cheng-Yong Su, Dai-Bin Kuangn MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China Received 17 April 2013; received in revised form 14 June 2013; accepted 14 June 2013
KEYWORDS
Abstract
Dye-sensitized solar cells; Zn2SnO4; Nanosheets; Light scattering; Photovoltaic performance
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles are synthesized for the first time through the facile hydrothermal process in the presence of F−. Dye-sensitized solar cell based on hierarchical Zn2SnO4 nanosheets photoelectrode shows a remarkable enhancement in power conversion efficiency (4.82%) compared to that of nanoparticles (4.01%) because of its superior light scattering ability, faster electron transport rate and slower charge recombination rate, which were confirmed by the UV−vis diffuse reflectance spectroscopy, intensity-modulated photocurrent/ photovoltage spectroscopy and electrochemical impedance spectroscopy. Further TiCl4 treatment of Zn2SnO4 nanosheets photoelectrode results in an impressive photovoltaic performance of 5.44%. & 2013 Elsevier Ltd. All rights reserved.
Introduction Zinc stannate (Zn2SnO4) has been attracting immense interesting because of its promising applications in transparent conducting electrode, photocatalyst, sensor, Li-ion batteries and solar cells. Since the initial report of Zn2SnO4 as photoanode of dye-sensitized solar cell (DSSC) [1,2], it has been one of research hotspots as optoelectronic materials. Zn2SnO4 has a wide band gap of 3.6 eV and electron mobility of 10–15 cm2 V−1 s−1 [3], which is larger than those of typical binary oxide (e.g. TiO2, band gap: 3.2 eV, electron n
Corresponding author. Fax: +86 20 8411 3015. E-mail address: [email protected] (D.-B. Kuang).
mobility: 10−5 cm2 V−1 s−1) [4] implying superior property for decreasing the dye photobleaching by the UV region of solar spectrum. Compared to ZnO, Zn2SnO4 also shows excellent stability in acid media which would in that case decrease the photocurrent and photovoltaic performance by forming the Zn-dye+ complex [5]. Furthermore, the ternary oxide possesses tunable work function, band gap energy, and electric resistivity by simply varying the relative Zn/Sn ratio. Although there are many intrinsic advantages of the Zn2SnO4, it was still rarely investigated as photoelectrode in DSSC compared to other counterparts like TiO2, ZnO and SnO2. Most researches of Zn2SnO4 photoelectrodes were based on nanoparticles with 8–60 nm or octahedral, but the power conversion efficiency was still less than 4.7% [2,6–11]. Recent report on the Zn2SnO4 nanowire has shown poor
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.06.008 Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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photovoltaic performance (o2.8%) [12,13] because of its low surface area. Over the past years, hierarchical structured TiO2 [14–23], ZnO [24–28] and SnO2 [29–31] spheres, octahedra or nanoarrays [32,33], which consist of nanoparticles have shown enhanced photovoltaic performance owing to their superior light scattering ability, comparably faster electron transport rate or slower electron recombination rate. Hence the development of hierarchical Zn2SnO4 photoelectrode would be more desirable, which may improve the power conversion efficiency significantly. In this communication, the hierarchical Zn2SnO4 nanosheets (NSs) consisting of nanoparticles (NPs) are prepared through a facile hydrothermal process in the diethylene glycol (DEG) and H2O mixture solvent containing Zn(CH3COO)2∙2H2O, SnCl4∙ 5H2O, diethanolamine and NH4F. After the hydrothermal reaction of 200 1C for 24 h, the resulted precipitates were washed 3 times using water and ethanol and then dried at 60 1C. As comparison, the hydrothermal fabrication in the absence of NH4F was also performed. The DSSC based on the Zn2SnO4 NSs shows a 20% enhancement of power conversion efficiency compared with the Zn2SnO4 NPs based cells. Finally, the photovoltaic performance of 5.44% is obtained for the Zn2SnO4 NSs photoelectrode after subsequent TiCl4 treatment.
Experimental Synthesis In a typical synthesis of hierarchical Zn2SnO4 NSs, 1.756 g Zn (CH3COO)2 2H2O, 1.4024 g SnCl4 5H2O and diethanolamine was dissolved in a mixed solvent of deionized water and DEG in a 100 mL Teflon-lined stainless steel autoclave, then 0.8888 g NH4F was added to this solution under vigorous stirring, continuously stirred for another 30 min. Subsequently, the autoclave was sealed and put into electronic oven, maintained at 200 1C for 24 h. When the reaction was finished, the autoclave was cooled down to room temperature naturally. The resulted precipitates were collected by centrifuging at 5000 rpm for 5 min and washed 3 times of absolute ethanol and distilled water in an ultrasonic cleaning bath for 5 min, respectively, and finally dried at 60 1C for further characterizations. For comparison, the synthesis of Zn2SnO4 NPs was also performed in the absence of NH4F, while keeping other experimental conditions constant.
Fabrication of Zn2SnO4 paste 1.0 g hierarchical Zn2SnO4 NSs or NPs samples were ground for 40 min in the mixtures of 8.0 mL ethanol, 0.2 mL acetic acid, 3.0 g terpineol and 0.5 g ethyl cellulose to form slurry, and then the mixtures were sonicated for 5 min in an ultrasonic bath, finally to form a viscous white Zn2SnO4 paste.
Preparation of the Zn2SnO4 photoelectrode The hierarchical Zn2SnO4 paste was screen-printed onto the FTO glass with the film thickness of 15 μm. And the Zn2SnO4 films were gradually heated under an air flow at 325 1C for 5 min, at 375 1C for 5 min, at 450 1C for 15 min, and then at 500 1C for 15 min. One batch of hierarchical
Zn2SnO4 films were immersed into a 40 mM TiCl4 aqueous solution at 70 1C for 30 min and washed with water and ethanol, then sintered at 520 1C for 30 min, after cooling down to 70–80 1C, the fabricated Zn2SnO4 electrodes was immersed into acetonitrile/tert-butanol (volume ratio 1:1) containing N719 dye (Ru[LL′-(NCS)2], L = 2,2′-bipyridyl-4,4′dicarboxylic acid, L = 2, 2′-bipyridyl-4, 4′-ditetrabutylammonium carboxylate, 5.0 10−4 M, Solaronix Co.), and was kept for 24 h at room temperature.
Fabrication of Zn2SnO4 based dye-sensitized solar cells The Pt-coated FTO (fluorine-doped tin oxide) glass as a counter electrode, was prepared by dropping H2PtCl6 (5.0 10−4 M) solution on the FTO glass followed by heating at 400 1C for 15 min in air. The electrolyte composition is 1-propyl-3-methylimidazolium iodide (PMII, 0.6 M), I2 (0.03 M), LiI (0.05 M), Guanidine thiocyanate (GuNCS) (GSCN, 0.1 M, Aldrich), and 4tert-butylpyridine (t-BP, 0.5 M, Aldrich) in acetonitrile and valeronitrile (85:15 v/v). The active area of the dye-coated Zn2SnO4 film was 0.16 cm2.
Characterizations The phase purity of the products was characterize by powder X-ray diffraction (XRD) on a Bruker D8 Advance Xray diffractometer using Cu Kα radiation (λ = 1.5418 Å). The Field emission scanning electron microscopy (FE-SEM, JSM6330 F) were applied to investigate the size and morphology as well as element mapping, and transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM) were performed on a JEOL-2010 h transmission electron microscope to further determine the intrinsic structure. The N2 adsorption–desorption isotherms of the hierarchical Zn2SnO4 NSs and NPs were measured by using an ASAP 2010 Surface Area Analyzer (Micromeritics Instrument Corporation). The thickness of Zn2SnO4 film was measured by using a profilometer (Ambios, XP-1). Shimadzu UV-3150 UV–vis-NIR Spectrophotometer was used to measure UV–vis diffuse reflectance spectrum. X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCALAB MK II X-ray photoelectron spectrometer. The current–voltage characteristics were performed using a Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mW cm−2) provided by solar simulator (91192, 1 kW Xe lamp, Oriel). The incident light intensity was calibrated with a NREL-calibrated Si solar cell. Incident photon-tocurrent conversion efficiency (IPCE) signal was recorded on a Keithley 2000 multimeter under the illumination of a 150 W tungsten lamp with a Spectral Product DK240 monochrometer. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) characterized on the electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under a modulated blue light emitting diodes (457 nm) driven by a Zahner (PP211) source supply. The electrochemical impedance spectroscopy (EIS) measurements were performed with a Zennium electrochemical workstation (ZAHNER) at bias potential of −0.75 V in dark with the frequency ranging from 10 mHz to 1 MHz.
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells
Results and discussions Powder X-ray diffraction (XRD) patterns (Fig. 1a) of the samples prepared without/with NH4F show a cubic structure of Zn2SnO4 (JCPDS Card no. 24-1470) with cell constants of a= b= c= 8.657 Å, implying a highly crystalline structure. The average particles size calculated from the XRD for the Zn2SnO4 NSs and Zn2SnO4 NPs were 7.2 nm and 5.6 nm, respectively, indicating the F− influences not only the morphology but also the size of the sample. Field emission scanning electron microscopic (FE-SEM) images show the sample prepared in the presence of NH4F turns to be nanosheets (Fig. 1b). The rough surface of the Zn2SnO4 NSs shows the hierarchical structure with sizes of 200–300 nm composed of nanoparticles (∼7.4 nm) confirmed by the TEM characterization (Fig. 1c and d), which is larger than that of the Zn2SnO4 NPs obtained in the absence of NH4F (Fig. S1,∼5.6 nm). The high electronegativity of F− compared with Cl− may reduce the chemical reactivity of the precursor, which results in fewer critical nucleis. Hence, sufficient nutrients are left to promote the larger Zn2SnO4 crystallite growth during the hydrothermal process [34–37]. The HRTEM and the corresponding Fast Fourier Transform (FFT) images of the Zn2SnO4 NSs (inset in Fig. 1d) and Zn2SnO4 NPs (Fig. S1b) reveal they are single crystalline nature. Fig. 1d shows that the Zn2SnO4 nanoparticle
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(building units of NSs) exhibits clear lattice fringes with the lattice spacing of 0.500 nm, which can be indexed as (111) planes. The formation of the hierarchical Zn2SnO4 NSs is ascribed to the influence of the F− in the hydrothermal process. The F− may interact strongly with (111) facets, which reduces the surface energy and kinetically inhibites the crystal growth along (111) facets, thus resulting in the exposure of (111) facets and inducing the oriental attachment which finally leads to the hierarchical Zn2SnO4 NSs consisting of NPs. Other fluoride salt (e.g. NaF, KF) also results in the similar hierarchical Zn2SnO4 nanosheets structure (Fig. S2). X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface composition of the Zn2SnO4 NPs and Zn2SnO4 NSs. The full-scale XPS spectra (Fig. S3a) show the two samples are mainly composed of Zn, Sn, C, and O. The peaks centered at 1021.41 eV and 1044.48 eV are assigned to the Zn 2p3/2 and Zn 2p1/2 of Zn2SnO4 NPs and Zn2SnO4 NSs (Fig. S3b). The Sn 3d5/2 and Sn 3d3/2 peaks of the Zn2SnO4 NSs are at 486.27 eV and 494.70 eV (Fig. 2a), respectively, which positively shift 0.14 eV compared to that of Zn2SnO4 NPs, suggesting that they have different bonding environments. The positive shift of the Sn 3d peaks of the Zn2SnO4 NSs implies there is an interaction between Sn4+ and F− since the F− might be doped into Zn2SnO4 NSs due to the similar atom radius of F− (0.136 nm) and O2− (0.140 nm),
Figure 1 (a) Powder XRD patterns of the as-prepared hierarchical Zn2SnO4 NSs and NPs. (b) The FE-SEM images of as-prepared Zn2SnO4 NSs. (c) TEM image of Zn2SnO4 NSs, inset in upper is the corresponding diffraction pattern images of Zn2SnO4 NSs. (d) The high-magnification TEM images of Zn2SnO4 nanoparticles in hierarchical Zn2SnO4 NSs. Inset is FFT image of single nanoparticle near to the Zn2SnO4 NSs. Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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Figure 2 XPS spectra: (a) the higher resolution curves of Sn 3d of Zn2SnO4 NPs and Zn2SnO4 NSs. (b) The higher resolution curves of F 1S of Zn2SnO4 NSs.
Figure 3 (a) Photocurrent density–voltage (J–V) and (b) Incident photon-to-current (IPCE) curves of the DSSCs based on different Zn2SnO4 photoelectrodes with the film thickness of ∼15 μm, ■: Zn2SnO4 NPs, : Zn2SnO4 NSs, : Zn2SnO4 NSs–TiO2.
hence the F− may replace the O2− and connect directly to Sn4+ in Zn2SnO4 NSs lattice. Furthermore, the F 1S region (Fig. 2b) is observed at 684.06 eV for the Zn2SnO4 NSs which confirms that F− reacts with Sn4+ to form F–Sn–O bonds during the hydrothermal process [38]. The total content of F in the Zn2SnO4 NSs is determined to be 1.56% according to the XPS survey spectra (Fig. S3a). The influence and detailed mechanism of F− on the morphology of the Zn2SnO4 NSs need further study. The typical photocurrent density (J)–voltage (V) curves of the DSSCs based on N719 sensitized Zn2SnO4 NPs and NSs photoelectrodes are shown in Fig. 3a and the photovoltaic parameters derived from the J–V curves are summarized in Table 1. It is observed that the Jsc, Voc and η for the Zn2SnO4 NSs photoelectrode are 8.29 mA cm−2, 743 mV and 4.82%, respectively, which are much higher than those of 7.05 mA cm−2, 733 mV, 4.01% for the DSSC based on Zn2SnO4 NPs photoelectrode. The 20% enhancement of the photovoltaic performance for the Zn2SnO4 NSs is mainly ascribed to the improvement of Jsc and Voc. The specific Brunauer–Emmett– Teller (BET) surface area of the hierarchical Zn2SnO4 NSs and Zn2SnO4 NPs are 108.1 m2/g and 129.1 m2/g, respectively, obtained from the N2 adsorption–desorption measurements (Fig. S4), which leads to the dye amounts on hierarchical Zn2SnO4 NSs (20.18 10−8 mol cm−2) are less than those on the Zn2SnO4 NPs (27.57 10−8 mol cm−2). However, the enhanced Jsc for the former is probably attributed to the superior light scattering ability and charge collection efficiency of the hierarchical Zn2SnO4 NSs. Fig. S5 (UV–vis diffuse spectra) reveals the Zn2SnO4 NSs has better diffuse
reflectance compared to the Zn2SnO4 NPs in the whole visible range (400–800 nm), which implies the former could maximize the use of solar light and hence enhances the light harvesting efficiency and Jsc. The incident photon-to-current efficiency (IPCE) spectra (Fig. 3b) shows the value of the Zn2SnO4 NSs based DSSC is higher than that of Zn2SnO4 NPs based DSSC which is consistent with the higher Jsc. The intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) of Zn2SnO4 NPs and Zn2SnO4 NSs based DSSCs were further measured to characterize the electron transport and recombination. The electron transport time (recombination time) is calculated from the expression τd =1/2πfd (τr =1/2πfr), where fd (fr) is the characteristic frequency minimum of the IMPS (IMVS) imaginary component. The τd (Fig. 4a) and τr (Fig. 4b) decreases with increasing light intensity, which is attributed to the fact that the deep traps are filled by the more photoelectrons generated at higher light intensity, resulting in electron trapping/detrapping involves shallower levels [39]. Obviously, the τd of the DSSC based on Zn2SnO4 NSs is smaller than that of Zn2SnO4 NPs under various light intensity implying the two-dimensional (2D) Zn2SnO4 NSs composed of NPs is favorable for the electron transportation (Scheme 1a). The longer lifetime of the DSSC based on Zn2SnO4 NSs compared to the Zn2SnO4 NPs is agreement with the larger Voc for the former. The electron diffusion coefficient (Dn =d2/(4 τd), d is film thickness, see Fig. S6) and the effective electron diffusion length (Ln =(Dn τr)1/2) of the DSSCs based on the Zn2SnO4 NSs or NPs photoelectrodes further reveals that the Zn2SnO4 NSs is much better than those Zn2SnO4 NPs. The electron collection
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells
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Table 1 Detailed photovoltaic parameters (Jsc, Voc, FF, and η) of dye-sensitized solar cells with different Zn2SnO4 photoelectrodes with ∼15.0 μm. DSSCs
Jsc (mA cm−2)
Voc (mV)
η (%)
FF
Adsorbed dye ( 10−8 mol cm−2)
Zn2SnO4 NPs Zn2SnO4 NSs Zn2SnO4NSs–TiO2
7.05 8.29 9.86
733 743 751
4.01 4.82 5.44
0.776 0.782 0.735
27.57 20.18 22.96
Figure 4 (a) Light intensity dependent electron transport time constant, (b) electron recombination time constant, (c) electron collection efficiency and (d) effective electron diffusion length of the DSSCs based on different Zn2SnO4 photoelectrodes with the film thickness of ∼15 μm, ■: Zn2SnO4 NPs, : Zn2SnO4 NSs, : Zn2SnO4 NSs–TiO2.
Scheme 1 (a) Schematic electron transport form Zn2SnO4 NSs–TiO2 hybrid films to FTO glass. (b) Schematic diagram of the band structure and charge separation in Zn2SnO4 NSs–TiO2 hybrid.
efficiency (ηcc =1−τd/τr) is estimated from the IMPS and IMVS spectra [40,41], which shows the ηcc of Zn2SnO4 NSs is much superior than the Zn2SnO4 NPs cell under various light intensity (Fig. 4c). Hence, the larger ηcc combines with the superior light scattering ability for the Zn2SnO4 NSs compared to the Zn2SnO4 NPs are responsible for the higher Jsc for the former. The detailed IMPS and IMVS parameters (τd, τr, Dn, Ln, and ηcc) of DSSCs measured under light intensity of 150 W/m2 are summarized in Table S1. Based on the discussions above, the
faster electron transport rate, slower electron recombination rate and superior light scattering ability for the Zn2SnO4 NSs compared to the Zn2SnO4 NPs are responsible for the enhancement of power conversion efficiency. The electrochemical impedance spectra (EIS) of the DSSCs based on Zn2SnO4 NSs and NPs photoelectrode are shown in Fig. 5, which provide additional information and deeper understanding on the interfacial reactions of photoexcited electrons in DSSCs. The second semicircle (intermediate
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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Y.-F. Wang et al. was demonstrated, which shows significant photovoltaic performance in dye-sensitized solar cell. NH4F is found to play inclusive role in the formation of the hierarchical Zn2SnO4 NSs. IMPS, IMVS, EIS and UV–vis reflectance spectra reveal that the faster electron transport rate, slower recombination rate and superior light scattering ability for the hierarchical Zn2SnO4 NSs compared to the nanoparticles are responsible for the 20% enhancement of the power conversion efficiency. We anticipate that this opens up a new way for other novel morphologies of Zn2SnO4, which can be potentially applied in high efficiency DSSCs or quantum dot sensitized solar cells.
Figure 5 Impedance spectra (Nyquist polts) of DSSCs based on different Zn2SnO4 films electrode measured at −0.75 V bias in the dark.
frequencies) in the Nyquist plot corresponds to the electron transfer at the Zn2SnO4/dye/electrolyte interface, larger electron recombination resistance is observed for Zn2SnO4 NSs compared to the Zn2SnO4 NPs result in a longer electron lifetime for the former, which is in agreement with the IMVS results described above. It leads to the higher Voc for the Zn2SnO4 NSs, which is confirmed by the J–V data. The influence of incorporating shell of TiO2 on the photovoltaic parameters is further investigated. After TiCl4 treatment of the Zn2SnO4 NSs photoelectrode (Zn2SnO4 NSs– TiO2), the photocurrent density and photovoltage increase from 8.29 mA cm−2 to 9.86 mA cm−2, and 743 mV to 751 mV, respectively, resulting in a significant improvement of power conversion efficiency from 4.82% to 5.44%, this can be ascribed to the significant reduction of the electron back transfer from Zn2SnO4 NSs to the redox electrolyte (I− 3 ). Element mapping image clearly shows the TiO2 homogeneously cover on the surface of Zn2SnO4 NSs (Fig. S7). The conduction band (CB) of the Zn2SnO4 NSs can be obtained through the measurements of the UV–vis-NIR absorption spectrum (Fig. S8) and XPS spectrum (Fig. S9). The schematic band alignment diagram of the Zn2SnO4 NSs–TiO2 is shown in Scheme 1b, the CB potential of the TiO2 coating is ca. 0.09 eV more negatively than the Zn2SnO4 NSs, such a band-structure-matched Zn2SnO4 NSs–TiO2 hybrid photoelectrode can be imaged as the “bridge” for the electron transport from Zn2SnO4 to FTO electrode, which can inhibit the recombination reaction between the electron in the CB of Zn2SnO4 NSs–TiO2 and I− 3 in the electrolyte resulting in longer electron lifetime (see IMVS, EIS) hence increase the Voc and Jsc. One possible explanation for this observation is that the TiO2 coating on the Zn2SnO4 NSs surface will minimize defects at this internal surface, enabling ease of electron transfer from the TiO2 to the Zn2SnO4 NSs and also avoid any extra internal trap sites which would otherwise be present in a poorly constructed junction [42]. Furthermore, the additional TiO2 shell layer also increases the dye loading (Table 1) which can contribute to the enhancement of the Jsc as well.
Conclusions In summary, the facile hydrothermal fabrication of hierarchical Zn2SnO4 NSs consisting of nanoparticles as the photoelectrode
Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (20873183, U0934003), the Program for New Century Excellent Talents in University (NCET-11-0533), the Fundamental Research Funds for the Central Universities, the Research Fund for the Doctoral Program of Higher Education (20100171110014), and Sun Yat-Sen Innovative Talents Cultivation Program for Doctoral Graduate Student.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.06.008.
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Ke-Nan Li received his Bachelor's degree in Sun Yat-sen University in 2011. Now he is a Ph.D. student at School of chemistry and chemical engineering of Sun Yat-sen University. His research interest is focused on the Tin Oxide materials and their applications in Dye-sensitized Solar Cells.
Yang-Fan Xu is currently an undergraduate of Sun Yat-Sen University. His current research interest focuses on the synthesis of hierarchically nanomaterials and their applications in solar cells.
Cheng-Yong Su is Cheung Kong Professor of Chemistry at Sun Yat-sen University. He obtained his Ph.D in 1996 from Lanzhou University, then worked at Stuttgart University as an Alexander von Humboldt Research Fellow in 2001, and moved to the University of South Carolina in 2002 as a Postdoctoral Fellow. His current research interest is in the field of supramolecular coordination chemistry and materials, focusing on clean environment and energy related metal-organic materials, catalysis and nanoscience. Dai-Bin Kuang received his Ph.D. in physical chemistry from Sun Yat-sen University (China) in 2003. He worked at the Max Planck Institute of Colloids and Interfaces (Germany) from 2003–2004 and also at Ecole Polytechnique Fédérale de Lausanne (Switzerland) from 2004–2008 as postdoctoral researcher. In 2008, he became a professor in the school of chemistry and chemical engineering at Sun Yat-sen University. His current research interest is in the field of new energy materials, focusing on functional nanostructured materials and their applications in photocatalysis and solar cells (such as dye-sensitized solar cells and quantum dot-sensitized solar cells).
Yu-Fen Wang received her Bachelor's degree in Liaocheng University in 2008. Now she is a Ph.D. student at School of chemistry and chemical engineering of Sun Yat-sen University. Her research interests include fabrications of hierarchically nanostructured materials and their applications in Dye-sensitized Solar Cells.
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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RAPID COMMUNICATION
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells Yu-Fen Wang, Ke-Nan Li, Yang-Fan Xu, Cheng-Yong Su, Dai-Bin Kuangn MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China Received 17 April 2013; received in revised form 14 June 2013; accepted 14 June 2013
KEYWORDS
Abstract
Dye-sensitized solar cells; Zn2SnO4; Nanosheets; Light scattering; Photovoltaic performance
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles are synthesized for the first time through the facile hydrothermal process in the presence of F−. Dye-sensitized solar cell based on hierarchical Zn2SnO4 nanosheets photoelectrode shows a remarkable enhancement in power conversion efficiency (4.82%) compared to that of nanoparticles (4.01%) because of its superior light scattering ability, faster electron transport rate and slower charge recombination rate, which were confirmed by the UV−vis diffuse reflectance spectroscopy, intensity-modulated photocurrent/ photovoltage spectroscopy and electrochemical impedance spectroscopy. Further TiCl4 treatment of Zn2SnO4 nanosheets photoelectrode results in an impressive photovoltaic performance of 5.44%. & 2013 Elsevier Ltd. All rights reserved.
Introduction Zinc stannate (Zn2SnO4) has been attracting immense interesting because of its promising applications in transparent conducting electrode, photocatalyst, sensor, Li-ion batteries and solar cells. Since the initial report of Zn2SnO4 as photoanode of dye-sensitized solar cell (DSSC) [1,2], it has been one of research hotspots as optoelectronic materials. Zn2SnO4 has a wide band gap of 3.6 eV and electron mobility of 10–15 cm2 V−1 s−1 [3], which is larger than those of typical binary oxide (e.g. TiO2, band gap: 3.2 eV, electron n
Corresponding author. Fax: +86 20 8411 3015. E-mail address: [email protected] (D.-B. Kuang).
mobility: 10−5 cm2 V−1 s−1) [4] implying superior property for decreasing the dye photobleaching by the UV region of solar spectrum. Compared to ZnO, Zn2SnO4 also shows excellent stability in acid media which would in that case decrease the photocurrent and photovoltaic performance by forming the Zn-dye+ complex [5]. Furthermore, the ternary oxide possesses tunable work function, band gap energy, and electric resistivity by simply varying the relative Zn/Sn ratio. Although there are many intrinsic advantages of the Zn2SnO4, it was still rarely investigated as photoelectrode in DSSC compared to other counterparts like TiO2, ZnO and SnO2. Most researches of Zn2SnO4 photoelectrodes were based on nanoparticles with 8–60 nm or octahedral, but the power conversion efficiency was still less than 4.7% [2,6–11]. Recent report on the Zn2SnO4 nanowire has shown poor
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.06.008 Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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photovoltaic performance (o2.8%) [12,13] because of its low surface area. Over the past years, hierarchical structured TiO2 [14–23], ZnO [24–28] and SnO2 [29–31] spheres, octahedra or nanoarrays [32,33], which consist of nanoparticles have shown enhanced photovoltaic performance owing to their superior light scattering ability, comparably faster electron transport rate or slower electron recombination rate. Hence the development of hierarchical Zn2SnO4 photoelectrode would be more desirable, which may improve the power conversion efficiency significantly. In this communication, the hierarchical Zn2SnO4 nanosheets (NSs) consisting of nanoparticles (NPs) are prepared through a facile hydrothermal process in the diethylene glycol (DEG) and H2O mixture solvent containing Zn(CH3COO)2∙2H2O, SnCl4∙ 5H2O, diethanolamine and NH4F. After the hydrothermal reaction of 200 1C for 24 h, the resulted precipitates were washed 3 times using water and ethanol and then dried at 60 1C. As comparison, the hydrothermal fabrication in the absence of NH4F was also performed. The DSSC based on the Zn2SnO4 NSs shows a 20% enhancement of power conversion efficiency compared with the Zn2SnO4 NPs based cells. Finally, the photovoltaic performance of 5.44% is obtained for the Zn2SnO4 NSs photoelectrode after subsequent TiCl4 treatment.
Experimental Synthesis In a typical synthesis of hierarchical Zn2SnO4 NSs, 1.756 g Zn (CH3COO)2 2H2O, 1.4024 g SnCl4 5H2O and diethanolamine was dissolved in a mixed solvent of deionized water and DEG in a 100 mL Teflon-lined stainless steel autoclave, then 0.8888 g NH4F was added to this solution under vigorous stirring, continuously stirred for another 30 min. Subsequently, the autoclave was sealed and put into electronic oven, maintained at 200 1C for 24 h. When the reaction was finished, the autoclave was cooled down to room temperature naturally. The resulted precipitates were collected by centrifuging at 5000 rpm for 5 min and washed 3 times of absolute ethanol and distilled water in an ultrasonic cleaning bath for 5 min, respectively, and finally dried at 60 1C for further characterizations. For comparison, the synthesis of Zn2SnO4 NPs was also performed in the absence of NH4F, while keeping other experimental conditions constant.
Fabrication of Zn2SnO4 paste 1.0 g hierarchical Zn2SnO4 NSs or NPs samples were ground for 40 min in the mixtures of 8.0 mL ethanol, 0.2 mL acetic acid, 3.0 g terpineol and 0.5 g ethyl cellulose to form slurry, and then the mixtures were sonicated for 5 min in an ultrasonic bath, finally to form a viscous white Zn2SnO4 paste.
Preparation of the Zn2SnO4 photoelectrode The hierarchical Zn2SnO4 paste was screen-printed onto the FTO glass with the film thickness of 15 μm. And the Zn2SnO4 films were gradually heated under an air flow at 325 1C for 5 min, at 375 1C for 5 min, at 450 1C for 15 min, and then at 500 1C for 15 min. One batch of hierarchical
Zn2SnO4 films were immersed into a 40 mM TiCl4 aqueous solution at 70 1C for 30 min and washed with water and ethanol, then sintered at 520 1C for 30 min, after cooling down to 70–80 1C, the fabricated Zn2SnO4 electrodes was immersed into acetonitrile/tert-butanol (volume ratio 1:1) containing N719 dye (Ru[LL′-(NCS)2], L = 2,2′-bipyridyl-4,4′dicarboxylic acid, L = 2, 2′-bipyridyl-4, 4′-ditetrabutylammonium carboxylate, 5.0 10−4 M, Solaronix Co.), and was kept for 24 h at room temperature.
Fabrication of Zn2SnO4 based dye-sensitized solar cells The Pt-coated FTO (fluorine-doped tin oxide) glass as a counter electrode, was prepared by dropping H2PtCl6 (5.0 10−4 M) solution on the FTO glass followed by heating at 400 1C for 15 min in air. The electrolyte composition is 1-propyl-3-methylimidazolium iodide (PMII, 0.6 M), I2 (0.03 M), LiI (0.05 M), Guanidine thiocyanate (GuNCS) (GSCN, 0.1 M, Aldrich), and 4tert-butylpyridine (t-BP, 0.5 M, Aldrich) in acetonitrile and valeronitrile (85:15 v/v). The active area of the dye-coated Zn2SnO4 film was 0.16 cm2.
Characterizations The phase purity of the products was characterize by powder X-ray diffraction (XRD) on a Bruker D8 Advance Xray diffractometer using Cu Kα radiation (λ = 1.5418 Å). The Field emission scanning electron microscopy (FE-SEM, JSM6330 F) were applied to investigate the size and morphology as well as element mapping, and transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM) were performed on a JEOL-2010 h transmission electron microscope to further determine the intrinsic structure. The N2 adsorption–desorption isotherms of the hierarchical Zn2SnO4 NSs and NPs were measured by using an ASAP 2010 Surface Area Analyzer (Micromeritics Instrument Corporation). The thickness of Zn2SnO4 film was measured by using a profilometer (Ambios, XP-1). Shimadzu UV-3150 UV–vis-NIR Spectrophotometer was used to measure UV–vis diffuse reflectance spectrum. X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCALAB MK II X-ray photoelectron spectrometer. The current–voltage characteristics were performed using a Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mW cm−2) provided by solar simulator (91192, 1 kW Xe lamp, Oriel). The incident light intensity was calibrated with a NREL-calibrated Si solar cell. Incident photon-tocurrent conversion efficiency (IPCE) signal was recorded on a Keithley 2000 multimeter under the illumination of a 150 W tungsten lamp with a Spectral Product DK240 monochrometer. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) characterized on the electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under a modulated blue light emitting diodes (457 nm) driven by a Zahner (PP211) source supply. The electrochemical impedance spectroscopy (EIS) measurements were performed with a Zennium electrochemical workstation (ZAHNER) at bias potential of −0.75 V in dark with the frequency ranging from 10 mHz to 1 MHz.
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells
Results and discussions Powder X-ray diffraction (XRD) patterns (Fig. 1a) of the samples prepared without/with NH4F show a cubic structure of Zn2SnO4 (JCPDS Card no. 24-1470) with cell constants of a= b= c= 8.657 Å, implying a highly crystalline structure. The average particles size calculated from the XRD for the Zn2SnO4 NSs and Zn2SnO4 NPs were 7.2 nm and 5.6 nm, respectively, indicating the F− influences not only the morphology but also the size of the sample. Field emission scanning electron microscopic (FE-SEM) images show the sample prepared in the presence of NH4F turns to be nanosheets (Fig. 1b). The rough surface of the Zn2SnO4 NSs shows the hierarchical structure with sizes of 200–300 nm composed of nanoparticles (∼7.4 nm) confirmed by the TEM characterization (Fig. 1c and d), which is larger than that of the Zn2SnO4 NPs obtained in the absence of NH4F (Fig. S1,∼5.6 nm). The high electronegativity of F− compared with Cl− may reduce the chemical reactivity of the precursor, which results in fewer critical nucleis. Hence, sufficient nutrients are left to promote the larger Zn2SnO4 crystallite growth during the hydrothermal process [34–37]. The HRTEM and the corresponding Fast Fourier Transform (FFT) images of the Zn2SnO4 NSs (inset in Fig. 1d) and Zn2SnO4 NPs (Fig. S1b) reveal they are single crystalline nature. Fig. 1d shows that the Zn2SnO4 nanoparticle
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(building units of NSs) exhibits clear lattice fringes with the lattice spacing of 0.500 nm, which can be indexed as (111) planes. The formation of the hierarchical Zn2SnO4 NSs is ascribed to the influence of the F− in the hydrothermal process. The F− may interact strongly with (111) facets, which reduces the surface energy and kinetically inhibites the crystal growth along (111) facets, thus resulting in the exposure of (111) facets and inducing the oriental attachment which finally leads to the hierarchical Zn2SnO4 NSs consisting of NPs. Other fluoride salt (e.g. NaF, KF) also results in the similar hierarchical Zn2SnO4 nanosheets structure (Fig. S2). X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface composition of the Zn2SnO4 NPs and Zn2SnO4 NSs. The full-scale XPS spectra (Fig. S3a) show the two samples are mainly composed of Zn, Sn, C, and O. The peaks centered at 1021.41 eV and 1044.48 eV are assigned to the Zn 2p3/2 and Zn 2p1/2 of Zn2SnO4 NPs and Zn2SnO4 NSs (Fig. S3b). The Sn 3d5/2 and Sn 3d3/2 peaks of the Zn2SnO4 NSs are at 486.27 eV and 494.70 eV (Fig. 2a), respectively, which positively shift 0.14 eV compared to that of Zn2SnO4 NPs, suggesting that they have different bonding environments. The positive shift of the Sn 3d peaks of the Zn2SnO4 NSs implies there is an interaction between Sn4+ and F− since the F− might be doped into Zn2SnO4 NSs due to the similar atom radius of F− (0.136 nm) and O2− (0.140 nm),
Figure 1 (a) Powder XRD patterns of the as-prepared hierarchical Zn2SnO4 NSs and NPs. (b) The FE-SEM images of as-prepared Zn2SnO4 NSs. (c) TEM image of Zn2SnO4 NSs, inset in upper is the corresponding diffraction pattern images of Zn2SnO4 NSs. (d) The high-magnification TEM images of Zn2SnO4 nanoparticles in hierarchical Zn2SnO4 NSs. Inset is FFT image of single nanoparticle near to the Zn2SnO4 NSs. Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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Figure 2 XPS spectra: (a) the higher resolution curves of Sn 3d of Zn2SnO4 NPs and Zn2SnO4 NSs. (b) The higher resolution curves of F 1S of Zn2SnO4 NSs.
Figure 3 (a) Photocurrent density–voltage (J–V) and (b) Incident photon-to-current (IPCE) curves of the DSSCs based on different Zn2SnO4 photoelectrodes with the film thickness of ∼15 μm, ■: Zn2SnO4 NPs, : Zn2SnO4 NSs, : Zn2SnO4 NSs–TiO2.
hence the F− may replace the O2− and connect directly to Sn4+ in Zn2SnO4 NSs lattice. Furthermore, the F 1S region (Fig. 2b) is observed at 684.06 eV for the Zn2SnO4 NSs which confirms that F− reacts with Sn4+ to form F–Sn–O bonds during the hydrothermal process [38]. The total content of F in the Zn2SnO4 NSs is determined to be 1.56% according to the XPS survey spectra (Fig. S3a). The influence and detailed mechanism of F− on the morphology of the Zn2SnO4 NSs need further study. The typical photocurrent density (J)–voltage (V) curves of the DSSCs based on N719 sensitized Zn2SnO4 NPs and NSs photoelectrodes are shown in Fig. 3a and the photovoltaic parameters derived from the J–V curves are summarized in Table 1. It is observed that the Jsc, Voc and η for the Zn2SnO4 NSs photoelectrode are 8.29 mA cm−2, 743 mV and 4.82%, respectively, which are much higher than those of 7.05 mA cm−2, 733 mV, 4.01% for the DSSC based on Zn2SnO4 NPs photoelectrode. The 20% enhancement of the photovoltaic performance for the Zn2SnO4 NSs is mainly ascribed to the improvement of Jsc and Voc. The specific Brunauer–Emmett– Teller (BET) surface area of the hierarchical Zn2SnO4 NSs and Zn2SnO4 NPs are 108.1 m2/g and 129.1 m2/g, respectively, obtained from the N2 adsorption–desorption measurements (Fig. S4), which leads to the dye amounts on hierarchical Zn2SnO4 NSs (20.18 10−8 mol cm−2) are less than those on the Zn2SnO4 NPs (27.57 10−8 mol cm−2). However, the enhanced Jsc for the former is probably attributed to the superior light scattering ability and charge collection efficiency of the hierarchical Zn2SnO4 NSs. Fig. S5 (UV–vis diffuse spectra) reveals the Zn2SnO4 NSs has better diffuse
reflectance compared to the Zn2SnO4 NPs in the whole visible range (400–800 nm), which implies the former could maximize the use of solar light and hence enhances the light harvesting efficiency and Jsc. The incident photon-to-current efficiency (IPCE) spectra (Fig. 3b) shows the value of the Zn2SnO4 NSs based DSSC is higher than that of Zn2SnO4 NPs based DSSC which is consistent with the higher Jsc. The intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) of Zn2SnO4 NPs and Zn2SnO4 NSs based DSSCs were further measured to characterize the electron transport and recombination. The electron transport time (recombination time) is calculated from the expression τd =1/2πfd (τr =1/2πfr), where fd (fr) is the characteristic frequency minimum of the IMPS (IMVS) imaginary component. The τd (Fig. 4a) and τr (Fig. 4b) decreases with increasing light intensity, which is attributed to the fact that the deep traps are filled by the more photoelectrons generated at higher light intensity, resulting in electron trapping/detrapping involves shallower levels [39]. Obviously, the τd of the DSSC based on Zn2SnO4 NSs is smaller than that of Zn2SnO4 NPs under various light intensity implying the two-dimensional (2D) Zn2SnO4 NSs composed of NPs is favorable for the electron transportation (Scheme 1a). The longer lifetime of the DSSC based on Zn2SnO4 NSs compared to the Zn2SnO4 NPs is agreement with the larger Voc for the former. The electron diffusion coefficient (Dn =d2/(4 τd), d is film thickness, see Fig. S6) and the effective electron diffusion length (Ln =(Dn τr)1/2) of the DSSCs based on the Zn2SnO4 NSs or NPs photoelectrodes further reveals that the Zn2SnO4 NSs is much better than those Zn2SnO4 NPs. The electron collection
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells
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Table 1 Detailed photovoltaic parameters (Jsc, Voc, FF, and η) of dye-sensitized solar cells with different Zn2SnO4 photoelectrodes with ∼15.0 μm. DSSCs
Jsc (mA cm−2)
Voc (mV)
η (%)
FF
Adsorbed dye ( 10−8 mol cm−2)
Zn2SnO4 NPs Zn2SnO4 NSs Zn2SnO4NSs–TiO2
7.05 8.29 9.86
733 743 751
4.01 4.82 5.44
0.776 0.782 0.735
27.57 20.18 22.96
Figure 4 (a) Light intensity dependent electron transport time constant, (b) electron recombination time constant, (c) electron collection efficiency and (d) effective electron diffusion length of the DSSCs based on different Zn2SnO4 photoelectrodes with the film thickness of ∼15 μm, ■: Zn2SnO4 NPs, : Zn2SnO4 NSs, : Zn2SnO4 NSs–TiO2.
Scheme 1 (a) Schematic electron transport form Zn2SnO4 NSs–TiO2 hybrid films to FTO glass. (b) Schematic diagram of the band structure and charge separation in Zn2SnO4 NSs–TiO2 hybrid.
efficiency (ηcc =1−τd/τr) is estimated from the IMPS and IMVS spectra [40,41], which shows the ηcc of Zn2SnO4 NSs is much superior than the Zn2SnO4 NPs cell under various light intensity (Fig. 4c). Hence, the larger ηcc combines with the superior light scattering ability for the Zn2SnO4 NSs compared to the Zn2SnO4 NPs are responsible for the higher Jsc for the former. The detailed IMPS and IMVS parameters (τd, τr, Dn, Ln, and ηcc) of DSSCs measured under light intensity of 150 W/m2 are summarized in Table S1. Based on the discussions above, the
faster electron transport rate, slower electron recombination rate and superior light scattering ability for the Zn2SnO4 NSs compared to the Zn2SnO4 NPs are responsible for the enhancement of power conversion efficiency. The electrochemical impedance spectra (EIS) of the DSSCs based on Zn2SnO4 NSs and NPs photoelectrode are shown in Fig. 5, which provide additional information and deeper understanding on the interfacial reactions of photoexcited electrons in DSSCs. The second semicircle (intermediate
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008
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Y.-F. Wang et al. was demonstrated, which shows significant photovoltaic performance in dye-sensitized solar cell. NH4F is found to play inclusive role in the formation of the hierarchical Zn2SnO4 NSs. IMPS, IMVS, EIS and UV–vis reflectance spectra reveal that the faster electron transport rate, slower recombination rate and superior light scattering ability for the hierarchical Zn2SnO4 NSs compared to the nanoparticles are responsible for the 20% enhancement of the power conversion efficiency. We anticipate that this opens up a new way for other novel morphologies of Zn2SnO4, which can be potentially applied in high efficiency DSSCs or quantum dot sensitized solar cells.
Figure 5 Impedance spectra (Nyquist polts) of DSSCs based on different Zn2SnO4 films electrode measured at −0.75 V bias in the dark.
frequencies) in the Nyquist plot corresponds to the electron transfer at the Zn2SnO4/dye/electrolyte interface, larger electron recombination resistance is observed for Zn2SnO4 NSs compared to the Zn2SnO4 NPs result in a longer electron lifetime for the former, which is in agreement with the IMVS results described above. It leads to the higher Voc for the Zn2SnO4 NSs, which is confirmed by the J–V data. The influence of incorporating shell of TiO2 on the photovoltaic parameters is further investigated. After TiCl4 treatment of the Zn2SnO4 NSs photoelectrode (Zn2SnO4 NSs– TiO2), the photocurrent density and photovoltage increase from 8.29 mA cm−2 to 9.86 mA cm−2, and 743 mV to 751 mV, respectively, resulting in a significant improvement of power conversion efficiency from 4.82% to 5.44%, this can be ascribed to the significant reduction of the electron back transfer from Zn2SnO4 NSs to the redox electrolyte (I− 3 ). Element mapping image clearly shows the TiO2 homogeneously cover on the surface of Zn2SnO4 NSs (Fig. S7). The conduction band (CB) of the Zn2SnO4 NSs can be obtained through the measurements of the UV–vis-NIR absorption spectrum (Fig. S8) and XPS spectrum (Fig. S9). The schematic band alignment diagram of the Zn2SnO4 NSs–TiO2 is shown in Scheme 1b, the CB potential of the TiO2 coating is ca. 0.09 eV more negatively than the Zn2SnO4 NSs, such a band-structure-matched Zn2SnO4 NSs–TiO2 hybrid photoelectrode can be imaged as the “bridge” for the electron transport from Zn2SnO4 to FTO electrode, which can inhibit the recombination reaction between the electron in the CB of Zn2SnO4 NSs–TiO2 and I− 3 in the electrolyte resulting in longer electron lifetime (see IMVS, EIS) hence increase the Voc and Jsc. One possible explanation for this observation is that the TiO2 coating on the Zn2SnO4 NSs surface will minimize defects at this internal surface, enabling ease of electron transfer from the TiO2 to the Zn2SnO4 NSs and also avoid any extra internal trap sites which would otherwise be present in a poorly constructed junction [42]. Furthermore, the additional TiO2 shell layer also increases the dye loading (Table 1) which can contribute to the enhancement of the Jsc as well.
Conclusions In summary, the facile hydrothermal fabrication of hierarchical Zn2SnO4 NSs consisting of nanoparticles as the photoelectrode
Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (20873183, U0934003), the Program for New Century Excellent Talents in University (NCET-11-0533), the Fundamental Research Funds for the Central Universities, the Research Fund for the Doctoral Program of Higher Education (20100171110014), and Sun Yat-Sen Innovative Talents Cultivation Program for Doctoral Graduate Student.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.06.008.
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Ke-Nan Li received his Bachelor's degree in Sun Yat-sen University in 2011. Now he is a Ph.D. student at School of chemistry and chemical engineering of Sun Yat-sen University. His research interest is focused on the Tin Oxide materials and their applications in Dye-sensitized Solar Cells.
Yang-Fan Xu is currently an undergraduate of Sun Yat-Sen University. His current research interest focuses on the synthesis of hierarchically nanomaterials and their applications in solar cells.
Cheng-Yong Su is Cheung Kong Professor of Chemistry at Sun Yat-sen University. He obtained his Ph.D in 1996 from Lanzhou University, then worked at Stuttgart University as an Alexander von Humboldt Research Fellow in 2001, and moved to the University of South Carolina in 2002 as a Postdoctoral Fellow. His current research interest is in the field of supramolecular coordination chemistry and materials, focusing on clean environment and energy related metal-organic materials, catalysis and nanoscience. Dai-Bin Kuang received his Ph.D. in physical chemistry from Sun Yat-sen University (China) in 2003. He worked at the Max Planck Institute of Colloids and Interfaces (Germany) from 2003–2004 and also at Ecole Polytechnique Fédérale de Lausanne (Switzerland) from 2004–2008 as postdoctoral researcher. In 2008, he became a professor in the school of chemistry and chemical engineering at Sun Yat-sen University. His current research interest is in the field of new energy materials, focusing on functional nanostructured materials and their applications in photocatalysis and solar cells (such as dye-sensitized solar cells and quantum dot-sensitized solar cells).
Yu-Fen Wang received her Bachelor's degree in Liaocheng University in 2008. Now she is a Ph.D. student at School of chemistry and chemical engineering of Sun Yat-sen University. Her research interests include fabrications of hierarchically nanostructured materials and their applications in Dye-sensitized Solar Cells.
Please cite this article as: Y.-F. Wang, et al., Hierarchical Zn2SnO4 nanosheets consisting of nanoparticles for efficient dye-sensitized solar cells, Nano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2013.06.008