Highly Efficient and Durable Quantum Dot Sensitized ZnO Nanowire

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Highly Efficient and Durable Quantum Dot Sensitized ZnO Nanowire Solar Cell Using Noble-Metal-Free Counter Electrode Minsu Seol,† Easwaramoorthi Ramasamy,‡ Jinwoo Lee,*,‡ and Kijung Yong*,† †

Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ‡ Advanced Functional Nanomaterials Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ABSTRACT:

A highly efficient quantum dot sensitized solar cell has been fabricated using a CdSe/CdS cosensitized ZnO nanowire array as a photoelectrode (PE), ordered mesocellular carbon foam (MSU-F-C) as a counter electrode (CE), and a polysulfide electrolyte as a hole transporter. The nanowire structure provides efficient photoelectron collection and light harvesting, and CdSe/CdS cosensitization allows utilization of the whole visible wavelength region of the incident solar spectrum. The MSU-F-C used here provides an extremely high surface area and the ordered large size mesopores with an interconnected pore structure, which facilitate diffusion of redox relay in the electrolyte. As a result, it exhibits low charge transfer resistance (Rct) between the CE/electrolyte interface and thus presents highly efficient photovoltaic performance, compared to conventional noble-metal-based CEs. The cell with MSU-F-C CE yields the highest power conversion efficiency of 3.60%, with Voc, Jsc, and FF of 685 mV, 12.6 mA/cm2, and 0.42, respectively. Furthermore, it exhibits high durability in the polysulfide electrolyte with remarkable stability irrespective of the solvent used in the electrolyte solution.

1. INTRODUCTION In last two decades, significant progress has been made on the research of dye-sensitized solar cells (DSSCs) as a cost-effective alternative to conventional solid-state photovoltaics,1,2 and recently there is growing interest in quantum dot sensitized solar cells (QDSSCs), which use inorganic quantum dots (QDs) as sensitizer materials instead of organic dyes.3,4 Use of semiconductor QDs as sensitizers has many fascinating advantages; their high extinction coefficients5 and large intrinsic dipole moment6,7 originated from the bulk properties of the semiconductor. The tunable band gap by size controlling5,8 and multiple exciton generation by impact ionization9,10 are derived from the nanoscale dimension as a consequence of the quantum confinement effect. QDSSCs are typically composed of visible-light-absorbing QDs sensitized wide band-gap metal oxide as a photoelectrode (PE), catalysts deposited F-doped tin oxide (FTO) as a counter electrode (CE), and the redox relay included electrolyte. Various QDs including CdS,11,12 CdSe,13 and PbS14 have been utilized, and r 2011 American Chemical Society

recently, CdSe/CdS cosensitization has been investigated.15
20 The CdSe/CdS cosensitizer provides enhanced light absorption and charge-carrier transfer efficiency, and as a result, it shows dramatically enhanced power conversion efficiencies with a value of 3
4%, compared to CdS or CdSe alone.16 Regarding the wide band-gap metal oxide nanostructure as a sensitizer scaffold, conventional TiO2 nanoporous films have been widely used in both DSSCs and QDSSCs. While the nanoporous films have an extremely high surface area, nanometersized QDs (typically 3
5 nm) can clog the pores and hinder penetration of the redox electrolyte. Use of open structures including nanowires (NWs) and nanotubes facilitates diffusion of redox mediators and allows more QD loading. In spite of their low surface areas they can attain an efficient light-harvesting Received: June 22, 2011 Revised: September 28, 2011 Published: September 28, 2011 22018

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The Journal of Physical Chemistry C efficiency due to the high extinction coefficient of inorganic QDs.21 Some researchers applied a ZnO NW array as a sensitizer scaffold in QDSSCs,20,22,23 because the vertically aligned onedimensional electron pathways of the NW structure and the high electron mobility of ZnO (205
1000 cm2 V s
1 for ZnO, 0.1
4 cm2 V s
1 for TiO2)24 promote te collection of photogenerated electrons injected from QD sensitizers. In addition to the potential of improving electron transport, they enhance light harvesting by scattering the incident light.25 In QDSSCs, the polysulfide redox relay-based electrolyte26,27 and the standard platinized CE are most commonly used. However, the Pt-based CE exhibits poor activity in the polysulfide electrolyte despite its high electrocatalytic activity for triiodide reduction in DSSCs. It is mainly due to chemical adsorption of sulfur compounds on the surface of Pt, which may impede electron transfer at the CE/electrolyte interface.15 Therefore, instead of Pt, Au-based CE has also been used, which shows a maximum power conversion efficiency of more than 4%.15,16 Our previous research showed efficient QDSSCs using a CdSe/CdS/ZnO NW array as a PE and Au-based CE. However, since both Pt and Au are naturally low-abundant and high-cost materials, research developing noblemetal-free CE is needed. Several researchers suggest alternative noble-metal-free materials capable of replacing Au or Pt as electrocatalysts, including CoS,28 Cu2S,29 and carbon-based materials. Among them, carbon is the most widely investigated material as a Pt-free counter electrode in DSSCs30,31 using polyiodide electrolyte because it is the low-cost earth-abundant material with good corrosion resistance. Various carbon-based materials were studied, including activated carbon, carbon nanotubes, and graphene.32,33 To achieve a comparable efficiency to the noble-metal-based CE, carbon-based CE should be equipped with two important characteristics: large surface area compensating for the slow intrinsic reaction kinetics of carbon in the electrolyte, and active electrolyte diffusion into the carbon layer.34 In the present study, as an alternative noble-metal-free CE, ordered mesocellular carbon foam (MSU-F-C) has been applied to fabricate highly efficient CdSe/CdS/ZnO NW-based QDSSC. The well-interconnected carbon framework provides an extremely high surface area and the ordered large-sized mesopores with an interconnected pore structure facilitating diffusion of redox relay in the electrolyte.35,36 For comparison, noble-metalbased QDSSCs were also prepared using Au and Pt. Basic J
V characteristics and incident photon to current conversion efficiency (IPCE) were measured to compare the performance of cells with different CEs, and electrochemical impedance spectroscopy (EIS) was employed to scrutinize and compare characteristics of each CE. Durability with room-temperature aging in the dark was also tested. This work suggests that the MSU-F-C CE with an excellent electrocatalytic activity in the polysulfide electrolyte can successfully replace the current noble-metal-based CEs.

2. EXPERIMENTAL SECTION Preparation of CdSe/CdS/ZnO NW Photoelectrode. CdSe/ CdS/ZnO NW arrays were prepared by a three-step solutionbased method. Arrays of ZnO NWs were grown on FTO substrates (TEC, 8 Ω/sq.) by an ammonia solution method.37 A 50 nm thick ZnO buffer film was sputtered on the FTO glass at room temperature. The sputtered films were immersed in the aqueous solution containing 0.01 M Zn(NO3)2 3 6H2O and 0.5 M NH4OH for 20 h at 95 °C. After 10 h, substrates were introduced into another fresh solution.

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ZnO NW electrodes were in situ sensitized with CdS and CdSe using successive ion layer adsorption and reaction (SILAR) and chemical bath deposition (CBD), respectively.16 The electrodes were dipped in 100 mM aqueous CdSO4 for 30 s, rinsed with deionized water for 30 s, dipped for another 30 s in 100 mM aqueous Na2S, and rinsed with water for 30 s. All these processes constitute one SILAR cycle and are repeated for 50 cycles in order to get a suitable CdS loading on the ZnO NWs. CdSe was in situ deposited on the CdS/ZnO NWs by CBD. The electrodes were immersed in an aqueous solution containing Cd(CH3COO)2: Na2SeSO3:NH4OH = 2.5 mM:2.5 mM:45 mM for 3 h at 95 °C. This procedure was repeated three times to get suitable CdSe loading on the CdS/ZnO NWs. Preparation of CEs. MSU-F-C was synthesized using furfuryl alcohol, with mesocellular foam silica as the template.38 A 1 g amount of silica template was impregnated with 1.5 mL of furfuryl alcohol. Then it was polymerized at 85 °C for 4 h in vacuum and carbonized at 850 °C for 2 h under N2 flow. Finally, the MSU-F-C was obtained by etching the silica template in 10 wt % HF solution. A 6 mg amount of the as-prepared MSU-F-C powder was ultrasonically dispersed in 6 mL of ethanol and then spray coated on an FTO glass maintained at 120 °C. Au counter electrode prepared by thermal evaporation and thermally platinized CE were also prepared in order to compare with MSU-F-C. Fabrication of Solar Cells. The CE and CdSe/CdS/ZnO NW PE (active area 0.25 cm2) were sandwiched between a 60 μm thick hot-melt ionomer film (Surlyn) under heating (130 °C, 1 min). Polysulfide electrolyte was injected via the predrilled holes of the CE, and each hole was sealed using a small piece of Surlyn and a microscope cover glass. Polysulfide used here was composed of 0.5 M Na2S, 2 M S, and 0.2 M KCl in methanol/water (7:3 by volume ratio). We also prepared methanol-free electrolyte composed of 0.5 M Na2S, 2 M S, and 0.2 M KCl in water. Electrode and Cell Characterization. The morphologies of the samples were observed by a field emission scanning electron microscope (SEM, XL30S, Philips), and the detailed microscopic structure was observed by a high-resolution transmission electron microscope (HRTEM, JEM2010F, JEOL). Photocurrent density
voltage characteristics of the QDSSCs were measured under simulated air mass 1.5 G solar spectrum. The intensity was adjusted to 100 mW/cm2 using a NREL-certified silicon reference cell equipped with a KG-5 filter. An active area of 0.25 cm2 was accurately defined using a mask placed in front of the cell. IPCE spectra were recorded using a PEC-S2015 instrument (Peccell Technologies, Japan). EIS analysis was carried out at open-circuit condition over the frequency range from 0.1 Hz to 100 kHz. A thin layer symmetric cell was fabricated by stacking two similar electrodes on each other with Surlyn spacer and sealed by heating on a hot plate. The polysulfide electrolyte composed of 0.5 M Na2S, 2 M S, and 0.2 M KCl in methanol/ water (7:3 by volume ratio) was introduced into the cell gap through the vacuum backfilling method. EIS analysis was carried out at 0 V over the frequency range from 0.1 Hz to 100 kHz.

3. RESULTS AND DISCUSSION Illustrations of a QDSSC device using CdSe/CdS/ZnO NWs as the PE and the MSU-F-C as the CE and the basic charge transport mechanisms at each side are shown in Figure 1a, 1b, and 1e, respectively. As expected from the band-gap diagram, use of CdS and CdSe as a cosensitizer enables absorption in the almost entirely visible spectrum region, and the band edges of 22019

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Figure 1. Schematic illustrations of (a) a sandwich-type QDSSC and the charge transport mechanism at the (b) CdSe/CdS cosensitized ZnO NW PE and (e) MSU-F-C CE. SEM images of the (c) PE and (f) CE, and TEM images of the (d) PE and (g) CE.

Table 1. Photovoltaic Properties of QDSSCs with Different CEs under 100 mW/cm2, AM 1.5G Illumination

Figure 2. (a) Current density
voltage characteristics and (b) IPCE spectra of MSU-F-C, Au, and Pt CE-based QDSSCs. (a) Dotted lines are measured in the dark and continuous lines are measured under 1 sun (100 mW/cm2, AM 1.5G) illumination.

CdSe/CdS/ZnO form a stepwise cascade structure of type-II band alignment, which facilitates separation of electron
hole pairs generated at the sensitizer layer (Figure 1b).39,40 Incident light generates electron
hole pairs in the CdSe/CdS sensitizer layer, and the electrons are injected into the ZnO NW, which allows fast charge transport to the transparent conductive oxide (TCO) by the well-defined one-dimensional electron pathway (Figure 1a). ZnO NWs used here were grown by an ammonia solution method,37 with an average diameter and length of 100 nm and 10 μm, respectively (Figure 1c). The HRTEM image (Figure 1d) of the CdSe/CdS/

Voc (mV)

Jsc (mA/cm2)

FF

η (%)

MS U-F-C

685

12.6

0.42

3.60

Au

681

11.8

0.39

3.16

Pt

682

9.9

0.25

1.71

ZnO NW shows that polycrystalline CdS and CdSe QDs layers with average grain sizes of ∼5 nm were deposited on the single-crystalline ZnO NW. The remaining holes at the sensitizer layer are captured by the reducing species in the polysulfide electrolyte, and the oxidized counter parts are reduced at the CE. The MSU-F-C CE used here has a bimodal pore system composed of ordered ∼22 nm wide mesocellular pores and 4 nm pores generated from silicate walls (Figure 1f and 1g). The ordered large size mesopores with an interconnected pore structure facilitate diffusion of redox species in the electrolyte, and the extremely large surface area (∼900 m2/g) of it provides more active sites to reduce the redox species (Figure 1e). Figure 2a displays the current density
voltage (J
V) characteristics of CdSe/CdS-cosensitized ZnO NW solar cells using MSU-F-C, Au, and Pt as the CE under dark and simulated air mass 1.5 global (AM 1.5G) full sunlight intensity. A polysulfide electrolyte composed of 0.5 M Na2S, 2 M S, and 0.2 M KCl in methanol:water (7:3 by volume ratio) was used. Detailed photovoltaic parameters, namely, the open-circuit voltage (Voc), shortcircuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) are gathered in Table 1. For all cases Voc was ∼680 mV, which is slightly higher than that of the TiO2- or SnO2-based QDSSCs. It is known that the conduction band minimum of ZnO is higher than that of SnO2 and TiO2,18,41 and thus, with a consistent redox level of the polysulfide electrolyte, it may cause the high Voc in this study. The cell with MSU-F-C CE yielded the highest η of 3.60%, with Voc, Jsc, and FF of 685 mV, 12.6 mA/cm2, and 0.42, respectively. The enhancement of η is mainly attributed to the increase of Jsc and FF. The IPCE spectra for the corresponding cells are shown in Figure 2b. It clearly reveals that the use of CdSe/CdS cosensitizer enables utilization of the whole visible spectrum region up to 750 nm, and the maximum IPCE of 77% at 570 nm was obtained for the cell with MSU-F-C CE. The high Jsc, FF, and dark current of the cell with MSU-F-C CE reveal that reduction of the polysulfide redox species is fast at the CE, compared to Au and Pt CEs. 22020

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Figure 3. Nyquist plots of (a) QDSSCs and (b) thin layer symmetric cells with various CEs. (Inset) Expanded range of the low-value region.

On the other hand, it is interesting to observe that the cell with Pt CE showed comparable IPCE with other CEs at the whole spectrum region, even though it exhibited a far lower Jsc than them in J
V results. We believe that it is due to the difference of the light intensity power used in the IPCE measurement system and the 1 sun J
V measurement. The IPCE measurement is carried out under low-intensity monochromatic illumination, where relatively small amounts of electron
hole pairs are generated at the sensitizer, and under this condition regeneration of holes at PE is not much affected by the catalytic activities of CEs. Therefore, with the same PE and electrolyte, all cells of different CEs exhibit similar IPCE spectra. However, under highintensity illumination like 1 sun, the electron
hole pair generation rate is significantly enhanced and the catalytic role of the CE becomes important. The low Jsc of the cell with Pt CE implies that hole regeneration is not sufficient, which may be due to the poor electrocatalytic activity of the Pt for reducing the oxidized species of the redox system. The lowest FF and dark current also suggest the poor electrocatalytic activity of it. To reveal the electrochemical characteristics of MSU-F-C, Au, and Pt electrodes, EIS measurements were carried out with QDSSCs under open-circuit conditions. Figure 3a displays the Nyquist plots of the corresponding cells. The existence of a single arc suggests that the high-frequency arc related to the charge transfer process at the CE/electrolyte interface merges with the low-frequency arc related to electron transport in the ZnO NW PE. While it is difficult to separate the contribution from each part in the serial charge transport process, we can still compare the catalytic activity of CEs by the size of arcs of corresponding

Figure 4. Current density
voltage characteristics of QDSSCs with methanol-free and methanol-containing electrolyte, measured under 1 sun (100 mW/cm2, AM 1.5G) illumination. Cells with (a) MSU-F-C, (b) Au, and (c) Pt CEs.

cells because all the cells utilize the same PE and electrolyte. The magnified view in Figure 3a indicates that the cell with MSU-F-C CE has the smallest arc, implying that it has the best catalytic activity among them; thus, it exhibits the highest FF and η. Due to the higher conductivity of Au, the cell with Au CE showed a lower starting point at the high-frequency region compared with MSU-F-C CE. In the current study, thermally platinized CE was used and thus showed lower conductivity compared to evaporation-prepared Au CE. On the other hand, the cell with Pt CE had the largest arc due to the worst catalytic activity in the polysulfide electrolyte. 22021

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Figure 5. (a, c, e) Current density
voltage characteristics of QDSSCs, and (b, d, f) Nyquist plots of thin layer symmetrical cells employing various electrodes with room-temperature aging in dark.

To analyze the resistance existing at the CE/electrolyte interface, EIS spectra were obtained from the thin layer symmetrical cells fabricated with two identical electrodes, incorporating the polysulfide electrolyte, as shown in Figure 3b. Charge transfer resistance (Rct), which indicates the electron transfer resistance at the electrode/ electrolyte interface and thus varies inversely with electrocatalytic activity for the reduction of Sx2
, could be obtained by fitting the arc observed at higher frequencies in Nyquist plots (Figure 3b) to the Randles circuit. Pt showed the highest Rct, as expected from the poor photovoltaic performance using it. The reason for the high Rct of Pt is known that a sulfur-containing compound in the polysulfide electrolyte, such as S2
or thiol, chemisorbs preferentially and strongly on a Pt surface, thereby decreasing the surface activity and conductivity of it.15 The magnified figure (inset in Figure 3b) showed that MSU-FC and Au had similar Rct which is smaller by a factor of 5
6 than

that of Pt, clearly indicating that they had better catalytic activity in the polysulfide electrolyte. Rct is related with the series resistance (Rs) according to the equation42 Rs ¼ RFTO þ Rct þ Rdif f

ð1Þ

RFTO and Rdiff are the resistance of FTO substrate and the diffusion impedance of electrolyte, respectively. Furthermore, Rs is related to the shape of the J
V curve according to the diode equation43   qðV þ JRs Þ V þ JRs
1
J ¼ Jph
J0 exp ð2Þ mkB T Rsh Jph, J0, m, and Rsh are the photocurrent density modeled as a current source, reverse saturation current density, ideality factor, 22022

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The Journal of Physical Chemistry C and shunt resistance, respectively. Simulating J
V curves from the diode equation shows that the increasing Rs decreases FF.34 At the short-circuit condition (V = 0), Rs also influences the deviation of Jsc. Thus, the two equations imply that the low Rct decreases Rs and thus increases FF and Jsc. It is well matched with the results of high FF and Jsc obtained from the cells with MSU-F-C and Au, as shown in Figure 2 and Table 1. While carbon has a lower catalytic activity than Au due to the slow intrinsic kinetics,42 the extremely high surface area with ordered large-size mesopores of MSU-F-C leads to an increased reduction rate of Sx2
as well as facile penetration of the polysulfide electrolyte. Mora
Sero et al. pointed out that use of methanol in the electrolyte may provide an erroneously high η, because the methanol oxidation at the PE is nonregenerative and acts to provide a sacrificial donor.21 To test these ideas, we prepared cells with methanol-free electrolyte (0.5 M Na2S, 2 M S, 0.2 M KCl in water) and measured their J
V characteristics under dark and simulated AM 1.5G full sunlight intensity, as shown in Figure 4. For all cases, Voc slightly increased when the methanolfree electrolyte was used, indicating that it has more positive redox potential compared to the methanol-containing one. However, the cells with Au and Pt CE exhibited 30
40% decreased η, due to the decrease of FF and Jsc. Lee et al. reported that methanol used as a cosolvent with water improves penetrating and wetting of the electrolyte in the TiO2 matrix by reducing the surface tension.26 In our experiments, we found that the use of methanol increased the solubility of sulfur which is poorly soluble in water. Therefore, in methanol-free electrolyte solution, poor solubility induces a low dissolved amount of polysulfide redox relay, and with noble-metalbased CE, such as Pt and Au, depletion of the redox relay can occur due to adsorption of sulfur compounds on the CE surface. As a result, for the case with the noble-metal-based CEs, use of methanolfree electrolyte decreases Jsc and FF and thus η. On the other hand, for the case of MSU-F-C, although the use of methanol-free electrolyte slightly decreases Jsc due to a lower dissolved amount of polysulfide redox relay and the high surface tension of the electrolyte, Voc increases slightly and FF remains similar in value because depletion of the redox relay is not serious in MSU-F-C CE due to the relatively weak adsorption on the surface. Thus, η of the cell with MSU-F-C CE exhibits a similar value irrespective of the use of methanol in electrolyte solution, which clearly shows that there is no overestimating problem in evaluation of η. The stability in aging test is an important issue in liquid
electrolyte-based sensitized solar cells including QDSSC and DSSC. We checked the durability of cells with various CEs. Figure 5 shows the J
V characteristics of QDSSCs and Nyquist plots of thin layer symmetrical cells with room-temperature aging in the dark. The methanol-containing electrolyte was used in all cells. For all cases, Rct slightly decreased after 1 day (Figure 5b, 5d, and 5f) due to stabilization of the CE/electrolyte interface. As a result, Jsc and FF increase slightly for MSU-F-C and Pt or remain in similar values for Au. However, after 7 days aging in the dark, Au and Pt electrodes showed larger Rct and thus exhibited lower Jsc and FF than the as-prepared cells. Adsorption of sulfur compounds on the noble-metal surface continues during aging in the dark, which continuously increases Rct. In addition, owing to the slower adsorption of sulfur compounds on Au than Pt, the photovoltaic performance of the as-prepared cell was better but the performance degradation after 7 days aging in the dark was more severe for Au than Pt. On the other hand, for the case of the MSU-F-C electrode, Rct values after 7 days and even 11 days aging in the dark were still

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smaller than the as-prepared one, which clearly shows that MSUF-C is more durable in the polysulfide electrolyte than the others. Nevertheless, J
V characteristics (Figure 5a) after 7 days and 11 days showed lower performance than the as-made one, which may be due to possible degradation in the interfacial properties at the PE/electrolyte. We believe that it can be improved by introducing the passivation layer between the PE and the electrolyte. By modifying the surface of the PE with TiO2 or ZnS, which are inert in the polysulfide electrolyte, we can avoid the direct contact between the QDs and the electrolyte and also suppress back electron transfer.44
46

4. CONCLUSION We have shown that MSU-F-C can be a good candidate to replace the conventional noble-metal-based CE in QDSSCs. The MSU-F-C electrode exhibits promising electrocatalytic activity toward the polysulfide redox couple, which is superior to that of Pt and Au CE, due to the low Rct originated from the extremely high surface area and facile diffusion of the redox couple. Thus, with CdSe/CdS cosensitized ZnO NW PE which has advantages for light harvesting and charge transport, MSU-F-C CE shows the highest photovoltaic performance, with a maximum η, Voc, Jsc, and FF of 3.60%, 685 mV, 12.6 mA/cm2, and 0.42, respectively. We also confirmed that there are no significant differences between the use of electrolyte with and without methanol when MSU-F-C CE is used. Furthermore, the MSU-F-C electrode is highly durable in the polysulfide electrolyte, compared with Au and Pt electrodes. Our results demonstrate that the ordered mesocellular carbon form based CE is a potential alternative to the noble-metal-based CEs for low-cost and highly durable QDSSCs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J.L.); [email protected] (K.Y.).

’ ACKNOWLEDGMENT This work was supported by grants from the National Research Foundation (NRF2010-0009545, NRF2010-0015975) and by the Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008-005-J00501). ’ REFERENCES (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–740. (2) Gratzel, M. Nature 2001, 414, 338–344. (3) R€uhle, S.; Shalom, M.; Zaban, A. ChemPhysChem 2010, 11, 2290–2304. (4) Mora-Ser o, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Acc. Chem. Res. 2009, 42, 1848–1857. (5) Yu, W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854–2860. (6) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241–245. (7) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183–3188. (8) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. J. Am. Chem. Soc. 2008, 130, 4007–4015. (9) Schaller, R.; Sykra, M.; Pietryga, J.; Klimov, V. Nano Lett. 2006, 6, 424–429. (10) Trinh, M.; Houtepen, A.; Schins, J.; Hanrath, T.; Piris, J.; Knulst, W.; Goossens, A.; Siebbeles, L. Nano Lett. 2008, 8, 1713–1718. (11) Shalom, M.; R€uhle, S.; Hod, I.; Yahav, S.; Zaban, A. J. Am. Chem. Soc. 2009, 131, 9876–9877. 22023

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dx.doi.org/10.1021/jp205844r |J. Phys. Chem. C 2011, 115, 22018–22024