Synthesis of a ZnS Shell on the ZnO Nanowire and ... - ACS Publications

Nov 17, 2010 - Mudan Wang , Chenxu Chen , Haiying Qin , Lei Zhang , Youtong Fang .... Xiang Chen , Zhiming Bai , Xiaoqin Yan , Haoge Yuan , Guangjie ...
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J. Phys. Chem. C 2010, 114, 21360–21365

Synthesis of a ZnS Shell on the ZnO Nanowire and Its Effect on the Nanowire-Based Dye-Sensitized Solar Cells Jooyoung Chung, Jihyun Myoung, Jisook Oh, and Sangwoo Lim* Department of Chemical and Biomolecular Engineering, Yonsei UniVersity, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Korea ReceiVed: September 29, 2010; ReVised Manuscript ReceiVed: NoVember 5, 2010

A thin ZnS shell was formed on ZnO nanowires using ZnSO4 and thiourea in an NH4OH (ZTA) precursor solution, and its effect on the performance of a dye-sensitized solar cell (DSSC) was investigated. As compared to hydrothermal and successive ionic layer adsorption and reaction methods, it was observed that the ZnS shell was effectively synthesized on the ZnO nanowires using the ZTA solution. ZnO nanowires with a ZnS shell had lower absorption intensities over the entire wavelength range compared with those of nanowires without the ZnS shell, which can cause dye to absorb more light in the DSSC operation. Because the intensity of visible emission decreased after the formation of a ZnS shell on the ZnO nanowires, it is proposed that surface defects that provide recombination sites will be reduced at the anode/electrolyte interface. The cell performance of the ZnO nanowire-based DSSC was greatly improved by the deposition of a thin ZnS shell onto the ZnO nanowires in the ZTA solution, which resulted from the reduced visible absorption of the semiconductor and defect sites on the semiconductor surface. 1. Introduction Dye-sensitized solar cells (DSSCs) have great potential as renewable energy devices due to their relatively low manufacturing costs1,2 and high durability of the dye. Over the past years, TiO2, ZnO, and SnO2 nanoparticles with large surface areas were predominantly used as anode materials for DSSCs. However, the improvement in cell efficiency has reached its limitation because injected electrons from the dye molecules experience about 106 trapping/detrapping events at the nanoparticle grain boundary and the nanoparticles/electrolyte interface.3 As a result, the electrons require 10 ms to pass through the nanoparticle materials.4,5 In this regard, some of the research efforts have been made to nanowire-based DSSCs.6 The most attractive aspect of using 1-D nanowires is that they transport electrons, minimizing trapping/detrapping events. In addition, nanowires have excellent electron transport capabilities resulting from internal electrical fields in the direction of the c axis of wurtzite crystals and the suppression of recombination for injected electrons from surrounding electrolytes.7 Various 1-D nanostructure materials have been studied as potential anode materials for DSSCs.8-10 Among theses materials, TiO2 nanowires do not provide the advantage of electron transport because they agglomerate in a polycrystalline network.9 Another candidate anode material is ZnO nanowire because its well-aligned nanostructure can be easily synthesized, and the morphology can be modified.11 This type of nanostructure enables rapid electron transport with a large electron diffusion coefficient of 1.8 × 10-3 cm2/s.10 Although the cell efficiency of the ZnO nanowire-based DSSCs is still lower than that of TiO2 nanoparticle-based ones, extensive effort has been made to improve the performance of ZnO nanowire-based DSSCs.7,12-15 In particular, a study on DSSCs fabricated using ZnO nanowires mainly focused on increasing the surface area in order to adsorb * To whom correspondence should be addressed. Phone: +82 2 21235754. Fax: +82 2 3126401. E-mail: [email protected].

more dye molecules.12-15 In other investigations, the effect of hydrothermal synthesis time on the performance of a DSSC was investigated.12,13 A linear relationship between nanowire length and cell performance has been observed.12,13 Modification of the nanowire structure, such as the branched nanowire14 or the nanoparticle/nanowire composite structure,15 was found to increase cell performance due to the larger resulting surface area. On the other hand, because of the importance of the surface condition of the ZnO nanowire, an effort has been made to understand the effects of various surface treatments on DSSC cell performance. Inferior interactions between ZnO nanowires and the dye were shown to decrease the light-harvesting efficiency.16 In our recent report on the annealing effect of ZnO nanowires on the performances of DSSCs, it was concluded that dye loading was facilitated by the increased number of hydroxide functional groups on the surface of ZnO, contributing to an improvement in DSSC efficiency.17 One of the most promising surface treatments found to improve DSSC cell performance is applying a thin layer on the ZnO nanowire surface to passivate the recombination sites, decreasing the dark current and increasing the VOC (open-circuit voltage).18 Type II band alignment of heterostructures can be formed through nanowire coatings19 through the induction of a charge separation at the interface of the two different materials,19 localizing the electrons and holes in different regions of the nanostructures.20 The corresponding characteristic of band alignment is beneficial for photovoltaic devices because electrons are confined within the core material. The type II band alignment also forms appropriate conduction band edge lines to facilitate electron transfer.21 However, depending on the type of shell material, cell efficiency can be degraded. For example, the coating of Al2O3 over the ZnO nanowires decreased the overall cell efficiency and JSC (short-circuit current density), because the Al2O3 shell layer acts as an insulating barrier and decreases injection efficiency.18 To avoid decreasing the injection efficiency, a Zn-based material may be a useful option as a shell layer on ZnO nanowires because it can help prevent the

10.1021/jp109355h  2010 American Chemical Society Published on Web 11/17/2010

Synthesis of a ZnS Shell on the ZnO Nanowire for DSSCs

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TABLE 1: Conditions of the ZnS Shell Deposition onto the ZnO Nanowires sample

solution and time

temperature

as-grown H-1 H-2 S-1 S-2 Z-1 Z-2 Z-3 Z-4 Z-5

Na2S 0.1 M (30 min) f Zn(NO3)2 0.1 M (30 min) [4 cycles] Na2S 0.01 M (30 min) f Zn(NO3)2 0.01 M (30 min) [1 cycle] Na2S 0.1 M (20 s) f Zn(NO3)2 0.1 M (20 s) [90 cycles] Na2S 0.01 M (20 s) f Zn(NO3)2 0.01 M (20 s) [30 cycles] ZnSO4/thiourea/NH4OH ) 0.05:5:50 mM (60 min) [1 cycle] ZnSO4/thiourea/NH4OH ) 0.05:5:50 mM (10 min) [1 cycle] ZnSO4/thiourea/NH4OH ) 0.05:5:500 mM (10 min) [1 cycle] ZnSO4/thiourea/NH4OH ) 0.05:5:5 mM (10 min) [1 cycle] ZnSO4/thiourea/NH4OH ) 0.05:50:50 mM (10 min) [1 cycle]

60 °C 60 °C RT RT 60 °C 60 °C 60 °C 60 °C 60 °C

shell layer from acting as an insulating layer and causing a decrease in JSC. The ZnS layer is a promising option as a shell layer on the ZnO nanowire for the application of solar cells. S atoms in the ZnS layer are able to fill up the oxygen vacancies on the ZnO nanowire,22 which can reduce the recombination of electrons in DSSCs. Because ZnS prevents tunneling of the electrons from the ZnO core material to the ZnS shell layer due to a higher band gap of ZnS, excited electrons are confined inside the ZnO.22 In addition, the ZnS layer on the ZnO nanowire forms a type II band alignment.23 Dark exciton formation in the ZnO/ZnS nanowire reduces the recombination rate of excitons so that the ZnS layer may improve the carrier collection in the photovoltaic application.23 Nevertheless, the effects of the ZnO/ZnS core/ shell nanowire on the DSSC performance have not been investigated until now. Therefore, in this study, we propose a new method in which a thin ZnS shell is formed on the ZnO nanowires and investigate its effect on DSSC performance. 2. Experimental Section 2.1. Synthesis of ZnO Nanowires. A ZnO seed layer was deposited onto fluorine-doped tin oxide (FTO) glass (Hartford Glass Co., Rs ) 15 ohm/sq) using a sputtering process. Radio frequency (rf) sputtering was performed using a ZnO (99.99% purity) target in an Ar atmosphere at 1 mTorr. The ZnO seed layer-coated glass was annealed in a quartz tube furnace at 300 °C and 1 Torr in an O2 ambient atmosphere for 1 h. The annealing of the seed layer improves adhesion between the FTO and the seed layer, facilitating vertical growth of well-aligned ZnO nanowires.17 For the synthesis of ZnO nanowires, the precursor was prepared by dissolving 30 mM of zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%, Aldrich) in deionized water. Ammonium hydroxide (NH4OH, 28% NH3 in water, J.T. Baker) was added to obtain a precursor solution pH of 10.3. ZnO nanowire arrays were grown on the substrates in the precursor solution at 60 °C for 24 h. The precursor solution was replaced with a fresh solution every 6 h in order to achieve sufficiently long nanowires. The nanowire arrays were then rinsed with deionized water to remove residual surface salts. 2.2. Coating of the ZnS Shell onto the ZnO Nanowires. The ZnS layer was synthesized on the ZnO nanowires using three different methods. First, a hydrothermal method was applied using two precursors based on the previous report.22 As-grown ZnO nanowires were immersed in a solution of 0.01 or 0.1 M sodium sulfide nonahydrate (Na2S · 9H2O, 98.0%, Aldrich) at 60 °C for 30 min and then immersed in a solution of 0.01 or 0.1 M zinc nitrate hexahydrate at 60 °C for 30 min. Those steps were performed once or repeated four times. The second method was the successive ionic layer adsorption and reaction (SILAR)24 using the same precursors as with the

hydrothermal method. As-grown ZnO nanowires were immersed in 0.01 or 0.1 M zinc nitrate hexahydrate at room temperature for 20 s, and then they were immersed in 0.01 or 0.1 M sodium sulfide nonahydrate at room temperature for 20 s. Those steps were repeated 30 or 90 times. To remove loosely adsorbed ions, the substrate was rinsed in deionized water between each step. The third method was a new method first used in this study to deposit a ZnS layer onto ZnO nanowires. As-grown ZnO nanowires were immersed in the precursor solution prepared by dissolving zinc sulfate heptahydrate (ZnSO4 · 7H2O, 99%, Aldrich) and thiourea (NH2CSNH2, 99%, Aldrich) in an ammonium hydroxide solution (NH4OH, 5.0 N in H2O, Fluka) (ZTA solution) at 60 °C. The process conditions used to synthesize a ZnS layer on the ZnO nanowires are summarized in Table 1. Samples H-1 and H-2 were synthesized via a hydrothermal method. The samples prepared using the SILAR method are labeled S-1 and S-2. The synthesis conditions of the ZnS shells using the ZTA solution (Z-1 to Z-5) are also summarized in the table. 2.3. Cell Fabrication. ZnO nanowires with and without a ZnS shell were immersed in 0.5 mM N719 dye (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)bistetra-butylammonium) in anhydrous ethanol at room temperature for 2 h. For the preparation of the cathode, a Pt layer was deposited on a counter FTO glass using a dc sputtering process using a Pt (99.99% purity) target in an Ar (99.999% purity) ambient atmosphere at room temperature and 5 mTorr. Glass covered with ZnO nanowire arrays and Pt-coated glass were separated by 50 µm thick hot-melt spacers (Type 1702, Himilan of Mitsui-DuPont Polychemical) and were combined to form a 1 cm2 active area of the DSSC, and the inner space of the cell was filled with a liquid electrolyte (0.5 M LiI, 50 mM I2, 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile). 2.4. Measurement and Characterization. The morphologies of the nanowires were investigated using a field emission scanning electron microscope (FE-SEM, Hitachi S-4200). The formation of the ZnS shell structures on the ZnO nanowires was examined using a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-3010) and X-ray photoelectron spectroscopy (XPS, ThermoVG, Sigma Probe). The optical properties of ZnO nanowires with and without a ZnS shell were investigated using photoluminescence spectroscopy (PL, SPEX 1403) and a UV-vis-NIR spectrophotometer (Mecasys Co., Optizen 2120). The crystallinities of the ZnO and ZnO/ZnS core/shell structures were investigated using X-ray diffraction (XRD, Bruker D8 Discover). Current density-voltage measurements were performed using a potentiostat (CompactStat) under AM 1.5G of simulated sunlight (model 11000, Sun 2000 Series Solar Simulators, Abet-Technologies) with a 0.008 cm2 opened hard mask in order to compare the performances

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Figure 1. FE-SEM images of ZnO nanowires: (a) as-grown, (b) Z-3, (c) as-grown, (d) Z-1, and (e) Z-4. Conditions of the ZnS shell preparation are listed in Table 1.

Figure 2. XPS spectra of the ZnO nanowires and ZnO/ZnS core/shell nanostructures. Conditions of the ZnS shell preparation are listed in Table 1.

of the DSSCs fabricated using ZnO nanowires or ZnO/ZnS core/ shell nanostructures. 3. Results and Discussion Figure 1 shows the top-down and cross-sectional FE-SEM images of the ZnO nanowire and ZnO/ZnS core/shell nanostructures prepared in a ZTA solution. In all cases, the length of the ZnO nanowires was 5.3 µm and a significant change in the diameter was not observed before and after the ZnS shell formation regardless of the variety of the ZnS shell formation process However, for the cases of samples H-1 and S-1, a filmtype morphology was obtained rather than a coating of ZnO nanowires. Because the film type anode material provides only an outer surface layer to adsorb dye molecules and a fairly low JSC is expected due to the small surface area,25 no DSSC was fabricated and no SEM images are shown for samples H-1 and S-1. On the other hand, for samples H-2 and S-2, the morphologies of the nanowire structures remained after coating with a ZnS layer and their diameter and length were identical to those of other samples (data not shown). Therefore, it is noted from Figure 1 that no significant changes in morphology were observed after coating the ZnS shell onto the ZnO nanowires in various methods. Thus, surface area is not a critical factor that affects the cell efficiency of various samples in this study. To confirm the formation of a ZnS shell on the ZnO nanowires, XPS measurements were performed. The XPS results in Figure 2 show that an S 2p peak appeared after the ZnS layer synthesis process, which indicates that a new layer, composed of sulfur, was formed on the ZnO nanowires. The S 2p peaks of samples H-2 and S-2 were located at 167.4 and 169.4 eV, respectively. However, those peaks can be attributed to oxygenbonded sulfur, based on the fact that the S 2p peaks of the SO3

and the SO42- were located at 16726 and 169.1 eV,27 respectively. Because an S 2p peak related to Zn-S bonding, reported to appear at 162.8 eV,28 was not observed, it was suggested that ZnS was not successfully formed through the hydrothermal or SILAR methods used in this study. However, for the ZnO/ZnS core/shell structure prepared in the ZTA solution (Z-4), an S 2p peak appeared at 162.3 eV, which is much lower than those of samples H-2 and S-2 and close to that of ZnS (162.8 eV).28 The XPS results indicate that a ZnS shell was successfully synthesized on the ZnO nanowires as ZnS structures using the ZTA method. The crystallinities of the nanowires were investigated using XRD measurements, and results are shown in Figure 3. Because it was observed from XPS measurements that a stable ZnS layer was prepared only with the ZTA method, only the samples prepared in ZTA solutions were analyzed using XRD. As shown in Figure 3, all samples had hexagonal wurtzite structures with preferential orientations along the c axes. Although the formation of a ZnS layer was observed using the XPS analysis, no additional peaks related to ZnS were observed in XRD measurements. This is consistent with the previous report that there was no significant change in the XRD pattern when the 12 nm thick ZnS shell was formed on the 50 nm thick ZnO nanowire.29 However, considering that the thickness of ZnS is not negligible with respect to a ZnO nanowire thickness, observation of no additional XRD peak in the presence of the ZnS shell on the ZnO nanowire may be due to the amorphous structure of the ZnS layer.30 Because the full width at half-maximum (fwhm) in all (002) peaks is obtained as 0.18°, it is suggested that the crystal size of the ZnO nanostructure calculated using the Scherrer formula31 is not changed after the formation of the ZnS layer. Figure 4 shows HR-TEM images of ZnO nanowires with and without a ZnS layer. Figure 4a shows the as-grown ZnO nanowire that has an atomically smooth surface. Its enlarged image in Figure 4b clearly shows the lattice spacing of 0.52 nm along the [0002] direction of the wurtzite ZnO structure. As seen in Figure 4c-h, thin ZnS layers were deposited on all the ZnO nanowires treated in ZTA solution. The thickness of the ZnS shell was observed to be 1.7-7.7 nm. The thickness of the ZnS layer increased from 1.7 to 7.7 nm when the reaction time increased from 10 to 60 min (Z-2 to Z-1). However, the thickness of the ZnS layer was not significantly changed with a molar concentration ratio of precursors, ZnSO4, thiourea, and NH4OH (Z-3 to Z-5). There may be two mechanisms to synthesize the ZnS layer: ZnO dissolution-based synthesis or surface reaction of Zn2+ and S2-. In the former mechanism, the surface of ZnO nanowires can be converted to ZnS via an ionexchange reaction and through the diffusion of zinc and sulfide ions on the surface of the ZnO.32,33 In that case, the diameter of the ZnO nanowire is decreased. However, this possibility is excluded in this study because no meaningful change in the fwhm of the (002) XRD peak, which is related to the ZnO

Synthesis of a ZnS Shell on the ZnO Nanowire for DSSCs

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Zn2+ + S2- T ZnS(s)

Figure 3. XRD patterns of the ZnO nanowires with and without a ZnS shell. Conditions of the ZnS shell preparation are listed in Table 1.

crystal size, was observed. Therefore, in this study, the latter one where thiourea reacts with OH- to produce S2-34 and this S2- reacts with Zn2+ dissolved from zinc sulfate to form ZnS is suggested as follows: NH3 + H2O T NH+ 4 + OH

(1)

SC(NH2)2 + OH- T SH- + H2O + H2CN2

(2)

SH- + OH- T S2- + H2O

(3)

ZnSO4 T Zn2+ + SO24

(4)

Figure 5 shows the UV-vis absorption spectra of the ZnO

(5)

nanowires with and without a ZnS shell. Here, the spectra were referenced to the ZnO seed layer on the FTO glass. Here, absorption spectra of ZnO nanowires were measured without dye loading. The UV-vis measurement of ZnO nanowires in the presence of the dye was not conducted because the UV-vis light that went through the anode materials will be absorbed in the N719 dye and this will hinder the analysis of the optical characteristics of ZnO nanostructures, which receives more attention in this study. The ZnO/ZnS core/shell nanostructures had lower absorption intensities over the entire wavelength range than did the ZnO nanowires without a ZnS shell. This is attributed to the wider band gap of ZnS, 3.64 eV,35 compared with that of ZnO (3.3 eV). Dye produces excited electrons by absorbing UV light, absorbing more light when the anode is composed of ZnO/ZnS core/shell nanostructures. This absorption characteristic of ZnO/ZnS nanostructures will be beneficial for improving the efficiency of the DSSC. The room-temperature PL spectra of the ZnO and ZnS core/ shell nanostructures are shown in Figure 6. Here, the spectra were normalized by the UV emission peak at 378 nm. The red or blue shift of the UV emission is not observed with a ZnS shell structure, which is consistent with the result in the previous report.36 The visible emission is commonly attributed to the defects in the ZnO surface. For instance, free-carrier depletion at the surface and its effect on the ionization state of the oxygen vacancy affects the green emission.37,38 It was noted that the peak intensity of the visible emission of the ZnO/ZnS core/ shell nanostructure was much less than that of the ZnO nanowires without a shell structure, as shown in Figure 6. Therefore, it was suggested that the formation of a ZnS layer on the ZnO nanowires reduced surface defects. This is another advantage of depositing a thin ZnS layer on the ZnO nanowires, in addition to the reduction of recombination sites on the surface at the anode/electrolyte interface. The effects of ZnO/ZnS core/shell nanostructures on the DSSC performance were investigated, and the results are summarized in Table 2. The efficiency of the DSSC prepared using ZnO nanowires without a ZnS shell was 0.11% in this study. When the ZnS layer was present on the ZnO nanowires and was formed via hydrothermal or SILAR methods (samples H-2 and S-2), the cell efficiency of the DSSC was decreased, and both the JSC and the VOC were decreased. However, the performances and efficiencies of the DSSCs fabricated using ZnO/ZnS core/shell nanostructures prepared in a ZTA solution were significantly improved. The efficiency of the DSSC increased 24-fold for the samples Z-1 and Z-3 as compared with the “as-grown” sample. The photocurrent density versus voltage curves of each cell prepared using ZnO nanowires with ZnS layers prepared via the ZTA method are shown in Figure 7. The JSC significantly increased after the formation of a ZnS layer on the ZnO nanowires. The increased JSC may result from the reduced visible absorption of the anode semiconductor, which has a reduced filter effect. The results also suggested that the ZnS layer formed in the ZTA solution effectively passivated the defect sites on the ZnO nanowires so as to inhibit the recombination of electrons at the anode/dye/electrolyte interfaces. However, a linear relationship between the peak intensity of the visible emission in the PL spectra and the cell efficiency is not observed. On the other hand, the thickness of the ZnS layer may affect the JSC and the cell efficiency. The DSSC prepared using ZnO nanowires covered with the thinnest ZnS layer (Z-2) exhibited the lowest efficiency among the DSSCs

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Figure 4. HR-TEM images of ZnO nanowires: (a, b) as-grown, (c, d) Z-1, (e) Z-2, (f) Z-3, (g) Z-4, and (h) Z-5. Conditions of the ZnS shell preparation are listed in Table 1.

TABLE 2: Performances of the DSSCs Prepared Using ZnO Nanowires with and without a ZnS Shell and N719 Dye

Figure 5. UV-vis absorption spectra of ZnO nanowires with and without a ZnS shell. Conditions of the ZnS shell preparation are listed in Table 1.

Figure 6. PL spectra of ZnO nanowires with and without a ZnS shell. The spectra were normalized by the UV emission peak at 378 nm. Conditions of the ZnS shell preparation are listed in Table 1.

sample

JSC [A/m2]

VOC [V]

FF

efficiency [%]

as-grown H-2 S-2 Z-1 Z-2 Z-3 Z-4 Z-5

11.4 6.1 1.7 127.7 55.9 119.8 99.8 63.8

0.50 0.28 0.25 0.51 0.50 0.53 0.52 0.51

0.26 0.25 0.27 0.42 0.34 0.44 0.34 0.34

0.11 0.04 0.01 2.70 1.02 2.72 1.75 1.20

results showed that the S 2p peak of the ZnS layer appeared at 162.3 eV for samples prepared in the ZTA solution, indicating that the ZnS shell was successfully synthesized on the ZnO nanowires. This shell was not properly formed through either the hydrothermal or the SILAR method. In the UV-vis absorption spectra, the ZnO/ZnS core/shell nanostructures had lower absorption intensities throughout the entire wavelength range than did the ZnO nanowires without a ZnS shell, allowing the dye to absorb more light into the DSSC. The PL measurements showed that the intensities of visible emission-related defects were much lower in the ZnO/ZnS core/shell nanostructure than were those of the ZnO nanowires without a shell structure. It was suggested that the formation of a ZnS layer on the ZnO nanowires reduced the surface defects, which induced recombination at the anode/electrolyte interface. The DSSC with the ZnO/ZnS core/shell nanowires prepared in the ZTA solution exhibited a significant increase in cell performance compared

fabricated with the ZnS shell. However, although the ZnS layer thicknesses of the samples Z-3 to Z-5 were similar, their JSC and efficiency were distinguished. It might result from the different ZnO/ZnS interface formation depending on the concentration ratio in the ZTA solution. Finally, it is concluded that the reduced light filter effect, optimized ZnS shell thickness, and the improved ZnO/ZnS interface to suppress the recombination of injected electrons improve the efficiency of ZnO nanowire-based DSSC. 4. Conclusions In summary, a 14 nm thick ZnS layer was synthesized on ZnO nanowires using a noble ZTA precursor solution, and its effect on the performance of a DSSC was studied. The XPS

Figure 7. Photocurrent density vs voltage curves of the DSSCs fabricated using as-grown nanowires and ZnO/ZnS core/shell nanostructures with N719 dye.

Synthesis of a ZnS Shell on the ZnO Nanowire for DSSCs with that with nanowires without a shell structure. It was concluded that the remarkable increase in the DSSC efficiency by the deposition of a ZnS shell on the ZnO nanowire resulted from the reduced visible absorption of the anode semiconductor, which diminishes the filter effect, and the reduced defect sites on the surface, which suppresses recombination of injected electrons. Acknowledgment. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0010573). This work was also supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090093823). References and Notes (1) Greg, S.; Carlo, B.; Roberto, A. Sol. Energy Mater. Sol. Cells 1994, 32, 259. (2) Roger, G. L.; Michael, J. N. Prog. PhotoVoltaics 1997, 5, 309. (3) van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2001, 105, 11194. (4) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2005, 109, 12093. (5) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (6) Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. B 2006, 110, 16159. (7) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (8) Sagawa, T.; Yoshikawa, S.; Imahori, H. J. Phys. Chem. Lett. 2010, 1, 1020. (9) Jiu, J.; Isoda, S.; Wang, F.; Adachi, M. J. Phys. Chem. B 2006, 110, 2087. (10) Ku, C.-H.; Wu, J.-J. Appl. Phys. Lett. 2007, 91, 093117. (11) Song, J.; Lim, S. J. Phys. Chem. C 2007, 111, 596. (12) Gao, H.; Fang, G.; Wang, M.; Liu, N.; Yuan, L.; Li, C.; Ai, L.; Zhang, J.; Zhou, C.; Wu, S.; Zhao, X. Mater. Res. Bull. 2008, 43, 3345. (13) Hua, G.; Zhang, Y.; Zhang, J.; Cao, X.; Xu, W.; Zhang, L. Mater. Lett. 2008, 62, 4109.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21365 (14) Suh, D.-I.; Lee, S.-Y.; Kim, T.-H.; Chun, J.-M.; Suh, E.-K.; Yang, O.-B.; Lee, S.-K. Chem. Phys. Lett. 2007, 442, 348. (15) Ku, C.-H.; Wu, J.-J. Nanotechnology 2007, 18, 505706. (16) Bedja, I.; Kamat, P. V.; Hau, X.; Lappin, A. G.; Hotchandani, S. Langmuir 1997, 13, 2398. (17) Chung, J.; Lee, J.; Lim, S. Physica B 2010, 405, 2593. (18) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. J. Phys. Chem. B 2006, 110, 22652. (19) Peng, P.; Milliron, D. J.; Hughes, S. M.; Johnson, J. C.; Alivisatos, A. P.; Saykally, R. J. Nano Lett. 2005, 5, 1809. (20) Zhong, H.; Zhou, Y.; Yang, Y.; Yang, C.; Li, Y. J. Phys. Chem. C 2007, 111, 6538. (21) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. AdV. Funct. Mater. 2008, 18, 2411. (22) Li, J.; Zhao, D.; Meng, X.; Zhang, Z.; Zhang, J.; Shen, D.; Lu, Y.; Fan, X. J. Phys. Chem. B 2006, 110, 14685. (23) Schrier, J.; Demchenko, D. O.; Wang, L.-W.; Alivisatos, A. P. Nano Lett. 2007, 7, 2377. (24) Valkonen, M. P.; Lindroos, S.; Kanniainen, T.; Leskela¨, M.; Resch, R.; Friedbacher, G.; Grasserbauer, M. J. Mater. Res. 1998, 13, 1688. (25) Pasquier, A. D.; Chen, H.; Lu, Y. Appl. Phys. Lett. 2006, 89, 253513. (26) Rodriguez, J. A.; Jirsak, T.; Freitag, A.; Hanson, J. C.; Larese, J. Z.; Chaturvedi, S. Catal. Lett. 1999, 62, 113. (27) Lu, S. W.; Schmidt, H. K. Mater. Res. Bull. 2008, 43, 583. (28) Vdovenkova, T.; Vdovenkov, A.; Tornqvist, R. Thin Solid Films 1999, 343, 332. (29) Wang, K.; Chen, J. J.; Zeng, Z. M.; Tarr, J.; Zhou, W. L.; Zhang, Y.; Yan, Y. F.; Jiang, C. S.; Pern, J.; Mascarenhas, A. Appl. Phys. Lett. 2010, 96, 123105. (30) Du, N.; Zhang, H.; Chen, B.; Wu, J.; Yang, D. Nanotechnology 2007, 18, 115619. (31) Martini, S.; Herrera, M. L. J. Am. Oil Chem. Soc. 2002, 79, 315. (32) Wang, Z.; Qian, X.-F.; Li, Y.; Yin, J.; Zhu, Z.-K. J. Solid State Chem. 2005, 178, 1589. (33) Dloczik, L.; Engelhardt, R.; Ernst, K.; Fiechter, S.; Sieber, I.; Ko¨nenkamp, R. Appl. Phys. Lett. 2001, 78, 3687. (34) Oladeji, I. O.; Chow, L. J. Electrochem. Soc. 1997, 144, 2342. (35) Makhova, L. V.; Konovalov, I.; Szargan, R.; Aschkenov, N.; Schubert, M.; Chasse´, T. Phys. Status Solidi C 2005, 2, 1206. (36) Yi, R.; Qiu, G.; Liu, X. J. Solid State Chem. 2009, 182, 2791. (37) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68, 403. (38) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983.

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