LETTER pubs.acs.org/JPCL
Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2/Sn2 at the Counter Electrode James G. Radich,† Ryan Dwyer,‡ and Prashant V. Kamat*,†,‡ †
Radiation Laboratory, Department of Chemical & Biomolecular Engineering and ‡Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States
bS Supporting Information ABSTRACT: Polysulfide electrolyte that is employed as a redox electrolyte in quantum dot sensitized solar cells provides stability to the cadmium chalcogenide photoanode but introduces significant redox limitations at the counter electrode through undesirable surface reactions. By designing reduced graphene oxide (RGO)-Cu2S composite, we have now succeeded in shuttling electrons through the RGO sheets and polysulfideactive Cu2S more efficiently than Pt electrode, improving the fill factor by ∼75%. The composite material characterized and optimized at different compositions indicates a Cu/RGO mass ratio of 4 provides the best electrochemical performance. A sandwich CdSe quantum dot sensitized solar cell constructed using the optimized RGO-Cu2S composite counter electrode exhibited an unsurpassed power conversion efficiency of 4.4%. SECTION: Energy Conversion and Storage
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emiconductor quantum dots offer new opportunities to develop next generation solar cells.15 Advantages of using quantum dots as photon harvesters include tunable band gaps,6 high molar extinction coefficients,6,7 hot electron injection,8 and large intrinsic dipole moments.9,10 All of these are desirable properties of a semiconductor material used to convert solar energy to electrical power. The quantum dot solar cell (QDSC) in principle adopts the design of dye-sensitized solar cell (DSSC),11 in which photoexcited electrons are injected into TiO2 nanocrystallites from excited semiconductor quantum dots. Chemical bath deposition1217 (CBD) and successive ionic layer adsorption and reaction1830 (SILAR) techniques are simplest and convenient to deposit CdSe quantum dots on mesoscopic TiO2 films. These methods provide close physical proximity between the electron transport layer and the sensitizer. Such strategies are important to maximize the charge injection efficiency given the distance-dependent electron transfer rates shown in recent studies with colloidal quantum dots.31,32 However, challenges to overcome limitations such as charge recombination at semiconductor electrolyte interface and poor redox activity of counter electrode toward sulfide/polysulfide couple need to be tackled before quantum dots become a viable candidate in QDSC.33 We now focus on improving the redox activity of the counter electrode in the polysulfide electrolyte and maximize power conversion efficiency. The use of platinum as a counter electrode in QDSC has been problematic with CdS- and CdSe-based photoanodes because it inhibits the charge transfer to polysulfide species (Sn2). Reaction 1 depicts the simplest polysulfide reduction reaction with r 2011 American Chemical Society
S2 as the sole product. 2 S2 n þ e f nS
ð1Þ
Sulfur compounds are known to chemisorb on platinum surfaces and induce poisoning effects toward electrode performance.34 Whereas polysulfide electrolyte (e.g., 1 M Na2S + 1 M S) is beneficial to the stability of the photoanode in liquid junction QDSC designs, power conversion efficiencies have remained low. Charge transfer to the oxidized redox polysulfide species at the counter electrode is considered to be a major hurdle in attaining high fill factor and high power conversion efficiency. Poor charge transfer rate at the counter electrode results in high overpotential for the reduction reaction that creates a bottleneck for the electron flow, thereby promoting back electron transfer at the photoanode. These effects are realized from the low current density and fill factor of the QDSC.33 A ZnS capping layer is commonly used as a means to mitigate this back electron transfer reaction at photoanode,35 but faster discharge of electrons at the counter electrode should eliminate electron buildup at the counter electrode. Prior research shows that metal-chalcogenides such as CoS, PbS, and Cu2S exhibit high electrocatalytic activity for polysulfide reduction with Cu2S exhibiting superior activity and stability, although each presents its own issues over long periods of time when used in conjunction with photoanodes.36 One can directly Received: August 5, 2011 Accepted: September 13, 2011 Published: September 13, 2011 2453
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expose the metal foils of Cu, Co, or Pb to sulfide solution to obtain an interfacial layer of metal sulfide. The problem is that such a preparative method suffers from continual corrosion and ultimately mechanical instability. For example, an anodic corrosion process that oxidizes the Cu to Cu+ rapidly forms Cu2S film on Cu foil, but no protective layer is formed to prevent further reaction and ultimate disintegration of the film. To demonstrate this effect, we exposed a small foil of 90/10 Cu/Zn brass to polysulfide solution, and complete disintegration of the material was evident after 1 week (Figure S1 of the Supporting Information). To overcome the challenges at the counter electrode polysulfide interface, we have developed a composite material consisting of reduced graphene oxide (RGO) and Cu2S. The high surface area of the 2-D structure promotes high numbers of the Cu2S reactive sites spread across the surface of RGO sheets. RGO serves to shuttle electrons across the 2-D mat to the sulfideactive Cu2S catalyst sites where the electrons are used to reduce the oxidized polysulfide. Scheme 1 depicts the response of RGOCu2S and platinum electrodes toward polysulfide reduction during the operation of QDSC. The ability of GO (or RGO) to stabilize metal nanoparticles has been discussed in previous studies.37 For example, Ag, Au, and Pt nanoparticles anchored on the RGO sheets exhibit good dispersion property.3739 In the present study, we made use of disproportionation reaction of Cu+ (via copper(I) acetate, i.e., CuAc) as a means to prepare Cu0 nanoparticles on RGO sheets (reaction 2). The complexation of the Cu+ cation with the graphene oxide is driven by the electrostatic interactions between the positive copper cationic species and the highly electronegative oxygen moieties found dispersed on the graphene oxide (GO) sheet. As the Cu+ ion undergoes disproportionation in ethanol, the suspended GO sheets act as nucleation sites for the Cu0, whereas the Cu2+ dissolves into the ethanol and is subsequently removed via washing. Ethanol
2Cuþ sf Cu0 þ Cu2þ
ð2Þ
Sonicating the GO-CuAc mixture for 30 min provided the necessary exfoliation of the GO sheets and allowed Cu+ and Cu0 to complex with GO. In addition, sonication process may also play a role in reducing Cu+ to Cu0. The mechanism of sonolytic reduction of metal ions in RGO suspension has been reported in a previous study.39 A small fraction of unreduced Cu+ remains complexed to the GO sheets in the form of CuAc, possibly by hydrogen bonding and other associations between the acetate species and the GO. This was evidenced through FTIR spectra showing the GO (CO: 1050 cm1 and OH: 3400 cm1) and acetate (1440 and 1608 cm1) absorption bands. However, once the complexation occurred, the stretching in the OH and CO regions was suppressed significantly, indicative of associations between the surface GO oxygens and the acetate or Cu0 (Figure S2 of the Supporting Information). Absorption spectra were recorded to monitor the evolution of the Cu2+ ion during the disproportionation reaction. The concentration of GO-bound copper (Cu+/Cu0), as determined from the absorbance change at 700 nm (Cu2+) in the supernatant, allowed us to establish the Cu/GO ratio after washing until the supernatant was clear. The suspensions were then spin-coated onto fluorine-doped tin oxide (FTO) and reduced under UV irradiation for 1 h. The composite electrode was then immersed in polysulfide solution as the Cu0/ Cu+ bound to RGO was converted to Cu2S. The synthetic steps involved in the preparation of RGO/Cu2S composite are illustrated in Scheme 2.
Scheme 1. Depiction of a QDSC Design with Emphasis on the Comparison of the Kinetics of Redox Reaction at the Two Different Counter Electrodesa
a
Electron-hole pairs are generated at the photoanode where charge separation leads electrons through the circuit and holes to the polysulfide electrolyte. Significantly impaired charge transfer occurs when platinum is used in conjunction with the polysulfide redox couple, increasing polarization resistance and thus decreasing the overall cell performance.
The morphology of the composite material was investigated using SEM and TEM, as shown in Figure 1. The SEM image shown in Figure 1a is of pristine RGO film prepared by excluding CuAc. The layered structure of the RGO sheets is apparent from the micrograph. Figure 1b shows the copper-RGO composite film prior to exposure to sulfide solution, indicating high coverage of the GO sheet by Cu0/CuAc at the Cu/GO ratio of 4. Figure 1c,d shows the complex morphology of the composite after reduction of the GO and immersion into polysulfide. The resulting material contains Cu2S clusters of disk-like morphology. The TEM images in Figures 1e,f (EDX inset) confirm the presence Cu2S nanoparticles on the individual RGO sheets. Additionally, the (002) and (101) planes were identified with d spacing of 0.34 and 0.303 nm, respectively, on the RGO sheet in Figure 1e. Some unreacted Cu0 remained on the deeper RGO layers, as evidenced by the identified Cu0 (111) planes with d = 0.208 nm. XRD confirmed the Cu2S and Cu0 findings (Figure S3 of the Supporting Information). The RGO thus provides the platform for attaining a high surface area Cu2S electrode, clearing the way for a stable and conductive composite counter electrode material with rapid shuttling of charges to the S2/Sn2 redox couple. The photoelectrochemical behavior of the QDSC was evaluated by employing RGO-Cu2S composite film as a counter electrode along with TiO2/CdS/CdSe photoanode in a photoelectrochemical cell containing polysulfide electrolyte (1 M Na2S and 1 M S in aqueous medium). The CdS/CdSe junction was initially studied by Kandilar40 and has recently been shown to exhibit improved performance as cosensitizers for TiO2 electrodes.4145 The CdS-CdSe cosensitized electrode undergoes Fermi level alignment, which facilitates rapid charge transfer to TiO2 as well as broadening of the visible spectrum response. A major problem in some recent research reports is the lack of long-term regeneration feature. For example, Lee et al.45 use a sacrificial electron donor, methanol, in the electrolyte, and the others use a brass counter electrode. Both approaches include 2454
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Scheme 2. Synthetic Strategy for Preparing RGO-Cu2S Composite Material: Cu+ Disproportionates in Ethanol to Yield a Graphene Oxide-Copper Compositea
a
Reduction of graphene oxide provides necessary electronic conductivity. Further exposure to polysulfide solution converts all complexed copper species to the electroactive Cu2S. Alternatively, a 95%/5% N2/H2 stream at 250 °C produced a RGO-Cu0 powder that could also subsequently be applied to the substrate and immersed in polysulfide.
sacrificial components involving electron transfer reactions that are not sustainable with respect to long-term operation of the cell. To the best of our knowledge, only the approach by Deng et al. represents the use of CdS/CdSe cosensitized electrodes with high performance (η = 3.08%) in a regenerative design.46 The consistent high performance reported by many regarding the CdS/CdSe system shows how proper energy band alignment can benefit the design of next generation QDSCs. Exceptionally high coverage of the TiO2 transport layer is obtained with the cosensitization route. The bright-field TEM micrograph (Figure 2) shows a cluster of TiO2 particles covered in CdS, CdSe, and ZnS nanocrystallites, as confirmed by EDX (Figure S4 of the Supporting Information). The photoelectrochemical performance under AM 1.5 conditions using Pt and RGO-Cu2S counter electrodes was compared. Initially, an open cell configuration was used to facilitate optimization of the composite through testing of multiple counter electrodes with a single photoanode. Then, a sandwich cell configuration was used to evaluate the performance of the best candidate in a fully optimized cell design. (Both cell designs are shown in Figure S5 of the Supporting Information.) A Cu/GO ratio of 4 (Figure 3A) provided the best performance as the conductivity and reactivity balance of the RGOCu2S composite was optimized. The IPCE and diffuse reflectance absorption spectrum of the photoanode (Figure 3B) shows the excitation of CdSe/CdS as the origin of the photocurrent. A sandwich solar cell constructed using a RGO-Cu2S counter electrode at the optimized Cu/RGO ratio of 4 was also compared with a Pt counter electrode. The composite film of RGO-Cu2S deposited on the FTO electrode significantly outperforms the Pt counter electrode as shown in Figure 3C. However, this superior performance was short-lived because the RGO-Cu2S composite layer showed poor mechanical stability. The composite film flaked off from the electrode surface when in contact with polysulfide solution for a few hours. We overcame the stability issue by using a polymer binder. The composite film prepared with polymer binder remained intact during the operation of the QDSC. The RGO-Cu0 powder was mixed with poly(vinylidene) fluoride (PVDF) binder before casting as a film on the FTO electrode surface. (See the Experimental Section for further details.) As observed (Figure 3C), the polymer binder not only serves to stabilize the composite film but also affords superior photoelectrochemical performance.
A current density of 18.2 mA/cm2 and open circuit voltage of 0.52 V was obtained with RGO-Cu2S-binder counter electrode in the QDSC. These values were significantly greater than those obtained using RGO-Cu2S without the binder (Isc = 14.5 mA/ cm2 and Voc = 0.43 V) and Pt (Isc = 11.3 mA/cm2 ; Voc = 0.46) counter electrodes. The fill factors for both RGO-Cu2S counter electrodes (with and without binder) were similar, 0.43 and 0.46, respectively, but greater than the one obtained with Pt counter electrode (ff = 0.31). The effectiveness of the RGO-Cu2S-binder as the superior counter electrode is evident when we compare the power conversion efficiencies. The power conversion efficiency for QDSC under AM1.5 illumination gave a value of 4.4% with RGO-Cu2S-binder counter electrode as compared with 2.5% RGO-Cu2S without binder and 1.6% with Pt counter electrodes. IPCE evaluation of the composite- and Pt-based sandwich cells in Figure 3D yielded a maximum value of ∼90% obtained at 400 nm. The relatively high IPCE value shows nearly ideal operation of QDSC for conversion of incident photons to current at low incident light intensities. These results also show the usefulness of the binder in providing a better physical contact between RGO-Cu2S sheets and the conducting substrate along with long-term stability. (Figure S6 of the Supporting Information illustrates photostability for 1 h.) Table 1 summarizes the various reports of CdS/CdSe photoanodes in comparison with ours. To probe the redox processes at the electrodepolysulfide interface, we separately carried out cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements using a three-electrode cell consisting of Pt and RGO composite as working electrodes, Pt as counter electrode, and SCE as reference electrode. Figure 4A shows the CVs of 1M/1 M Na2S/S using RGO-Cu2S working electrode yielded currents that are 510 times greater than those observed with Pt electrode. Further investigation of the redox processes using a diluted polysulfide (0.1 M Na2S + 0.1 M S) solution illustrates the rapid kinetics afforded by the composite relative to the Pt electrode. The surface concentrations of the reduced and oxidized species are depleted with the composite to the extent that Nerstian shifts were observed in the equilibrium potentials with RGO-Cu2S (Figure 4B). The Pt electrode irreversibility and overpotential are even more pronounced in the dilute polysulfide solution (Figure 4C). 2455
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Figure 1. SEM and TEM micrographs of the composite material during different stages of synthesis. (a) SEM image of pristine RGO reduced via UV irradiation. (b) Image of the GO-copper composite prior to immersion in polysulfide. (c) 100 kx SEM image of the RGO-Cu2S composite. (d) 250 kx SEM image of the RGO-Cu2S composite. (e) TEM image (620 kx) of the Cu2S nanoparticles embedded into the RGO matrix along with some residual Cu0. (f) TEM image (340 kx) defocused to show the RGO sheet within the composite.
The differences in the impedance spectroscopy data are striking. When the charge-transfer resistance, Rct, was extracted from the modeled data (equivalent circuit models in Figure S7 of the Supporting Information) shown in Figure 4A, the value for Pt was 998 Ω-cm2, whereas that of the RGO-Cu2S composite was 1.61 Ω-cm2. Rct for the RGO-Cu2S composite was used to extract the exchange current density, J0, using the relationship in eq 3m where R is the gas constant, T is temperature in Kelvins, F is Faraday’s constant, and Rct is the charge transfer resistance in Ω-cm2. J0 ¼
RT FRct
ð3Þ
A value of 16 mA/cm2 for J0 was obtained, much lower than that obtained by Deng et al.46 with Cu2S anchored to carbon paper. The values obtained in this study are more than one order of magnitude lower than those reported by Deng and coworkers (e.g., 205 mA/cm2 for the Cu2SC system). Our approach was to segregate the Pt or RGO-Cu2S as the working electrode in a three-electrode system, allowing us to decrease the complexity of the modeling through deconvolution of a two-electrode response. The Nyquist plot for the RGO-Cu2S electrode is indicative of two impedances in series, each with a resistive and capacitive effect. The charge-transfer resistance was assigned nearest the solution interface and the other resistance to the binder (Figure S6 of the Supporting Information). The platinum exhibited a Randle’s cell response with some nonideality represented with a constant phase element. No diffusion component was evident, attesting to the sluggish kinetics with this electrode. The RGO-Cu2S composite, even in the 1M/1 M solution used in the EIS experiments, did exhibit slight Warburg diffusion impedance at
Figure 2. BFTEM image of a TiO2 particle network covered by sensitizing materials such as CdS, CdSe, and ZnS. The elemental analysis of the network confirms the respective materials. (See Figure S3 of the Supporting Information.).
very low frequencies, further supporting the facile charge-transfer dynamics associated with the composite. The integration of Cu+ species with the exfoliated GO sheets yielded a simple and facile route to design an RGO-Cu2S composite electrode. The disproportionation of Cu+ to generate Cu0 and Cu2+ has allowed us to extract Cu2+ from solution and quantify the Cu/GO ratio in the composite. The RGO-Cu2S composite exhibits enhanced electrochemical performance and mechanical stability in aqueous sulfide/polysulfide medium, thus 2456
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Figure 3. Characterization of the photoelectrochemical cell consisting of CdS-CdSe photoanode and different counter electrodes using regenerative polysulfide (1 M Na2S/1 M S) redox couple: (A) Optimization of Cu/GO ratio for counter electrode performance. (B) IPCE and diffuse reflectance spectra of the photoelectrochemical cell showing the match between photocurrent generation and absorption. (C) Optimized cell performance with and without PVDF binder in a sandwich cell configuration. (D) Comparison of IPCE spectra of the CdS-CdSe photoanode using Pt and RGO-Cu2S (binder) counter electrodes.
Table 1. Comparison of Photoelectrochemical Performance with CdS-CdSe Co-Sensitized Photoanode counter electrode/electrolyte RGO-Cu2S/regenerative
a
a
ISC (mA/cm2)
VOC (mV)
fill factor
efficiency η
reference
18.4
520
0.46
4.40%
present work
platinum/regenerativea
11.3
460
0.31
1.60%
present work
platinum/regenerativea
10.5
660
0.40
2.80%
41
carbon-Cu2S/regenerativea
10.7
497
0.58
3.08%
46
brass/regenerativea
11.1
520
0.60
3.47%
42
brass/regenerativea platinum/with methanol as sacrificial donor
13.7 15.6
575 504
0.63 0.47
4.92% 3.70%
44 45
gold/with methanol as sacrificial donor
16.8
514
0.49
4.22%
45
Regenerative couple usually consists of Na2S/S in aqueous electrolyte
allowing its use in quantum dot sensitized solar cells. An unprecedented power conversion efficiency of 4.4% for liquid junction QDSC was obtained after optimizing the composition of the RGO-Cu2S composite. It is interesting to note that a recent study of PbS-based solid-state QDSC also reported 4.4% power conversion efficiency.47 Whereas we have made a major leap in overcoming the limitations of counter electrode, other factors such as charge recombination processes at the photoanode/ electrolyte interface need to be tackled before we can surpass >5% power conversion efficiency with QDSC.
’ EXPERIMENTAL METHODS Materials Synthesis. Graphene oxide was synthesized via Hummer’s method with slight modifications as reported elsewhere.48,49 GO was complexed with CuAc (STREM Chemicals, 99.9%) by mixing 3050 mg of GO with increasing masses ranging from 30 to 450 mg of CuAc in ethanol and sonicating for 60 min in an ice bath. The resulting composite was washed thoroughly by centrifuging and resuspending in ethanol with a Varian UV/ visible absorption spectrophotometer used to track the evolution of the Cu(Ac)2 (99.9% Alfa). The washed material was dried, 2457
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Figure 4. Electrochemical characterization of the counter electrode in a three-electrode system with SCE used as reference. (A) Cyclic voltammogram of Pt and RGO-Cu2S (binder) in 1 M/1 M polysulfide. (B) Cyclic voltammogram of RGO-Cu2S in 0.1 M/0.1 M polysulfide, which illustrates diffusionlimited shifts in equilibrium potential as a result of the facile kinetics at the electrode surface. (C) Cyclic voltammogram of Pt in 0.1 M/0.1 M polysulfide, indicating significant irreversibility and high overpotential. (D) Electrochemical impedance measurements of Pt and RGO-Cu2S (inset) on FTO substrate. Note that the scales are different in the inset; see Figure S7 of the Supporting Information for additional EIS data.
weighed, and resuspended to yield a composite concentration of 30 mg/mL. Counter electrodes were prepared by spin coating 10 μL of suspension for a total of five coats onto the FTO at 3000 rpm for 30 s. The number of spin-coats was varied early on, and it was found that beyond five coats no enhancements were observed. The electrode was then placed under UV irradiation for 1 h to form RGO, followed by immersion into a 1 M Na2S/1 M S polysulfide solution. Alternatively, the GO-Cu/CuAc composite was treated with 95%/5% N2/H2 to reduce the GO to RGO and any CuAc to Cu0. This powder was recovered and mixed with PVDF binder at 10% w/w. N-Methyl pyrrolidinone solvent was used to create slurry, and the slurry was applied to FTO and placed in vacuum oven for 12 h at 110 °C. UV reduced counter electrodes were subjected to five CV scans to reduce further any remaining available Cu or CuAc within the composite. The open system photoanode was prepared by first sonicating an FTO strip in acetone for 30 min, followed by deposition of a thin-layer (∼5 μm) of TiO2 paste (Dyesol 18-NRT) via doctor blade technique. The electrode was annealed at 400 °C for 1 h. Another layer of TiO2 (Degussa P25 in ethanol + 0.15 mL of titanium isopropoxide) was deposited via doctor blade, followed by another 400 °C anneal for 1 h. Finally, the electrode was immersed in 40 mM TiCl4 for 30 min at 70 °C to enhance the surface area and reduce the number of grain boundaries. Sensitization with
CdS, CdSe, and ZnS using SILAR techniques followed. The CdS SILAR consisted of eight cycles of one 3 min dip into 0.1 M aqueous CdSO4, followed by one 3 min dip into 0.1 M aqueous Na2S with 30 s of deionized water rinses between dips. The CdSe was deposited under a nitrogen atmosphere using 30 mM Cd2+ (Cd(NO3)2) in ethanol and 30 mM Se2 (sodium borohydride-reduced SeO2) as precursors. Cycles consisted of 30 s immersions into each precursor, followed by 30 s of rinsing between dips with a total of five CdSe SILAR cycles. ZnS was deposited using 0.1 M ZnAc2 and 0.1 M Na2S aqueous solutions with 1 min dips, followed by rinsing for a total of two cycles. The electrode was stored in the dark under vacuum while not tested. The compact cell was constructed using an optimized method to obtain the best photoelectrochemical measurements. A TiO2 compact layer, absorbing layer, and scattering layer were prepared as previously described for DSSCs.50 The SILAR depositions for CdS were carried out in the same manner with the exception of using a 1:1 v/v cosolvent of methanol/water in both precursors. A total of five CdS cycles were used on the compact cell as a result of the more rapid adsorption via lower surface tension. The remaining SILAR procedures and cycles were identical to the open system electrode. The electrolyte used in both open and compact systems was 1 M S2/1 M S dissolved in deionized water. To construct the compact cell, we placed a drop of electrolyte onto the photoanode 2458
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The Journal of Physical Chemistry Letters and the counter electrode, and they were sandwiched together with a Surlyn 25 μm spacer. The RGO-Cu0 powder was prepared by treating the GO-Cu0/Cu+ composite (post washing) in a 95%/ 5% N2/H2 atmosphere at 250 °C for 30 min. The resulting powder was mixed with PVDF (10% w/w), and a slurry was formed with 1-methyl 2-pyrrolidinone and bladed onto FTO. The electrode material was dried in a vacuum oven at 110 °C for 12 h. Materials Characterization. The GO and GO-Cuo/Cu+ complexed composite material was characterized using a Shimadzu IRPrestige-21 FTIR, a Bruker D8 X-ray diffractometer, a Magellan 200 SEM, and an FEI 80300 TEM. FTIR measurements were obtained by evaporating 30 μL of suspension onto the scanned surface with an integrating sphere under flow of N2 and were repeated in triplicate. X-ray data were obtained by placing the composite electrode directly under the X-ray beam while the detector angle was scanned from 10° to 80°. SEM images were collected at 10 kV with a working distance of 4 mm directly on conductive FTO substrates. TEM images of the deeper RGOCu2S layers were taken by first scraping the surface layers away, followed by sonication of the remaining layers in 1 mL of ethanol for 1 min. A drop of the suspension was used to coat the TEM copper grid. TEM images were collected at 300 kV. Energydispersive X-ray analysis was performed using the EDX detector coupled to the FEI TEM. Electrochemical and Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out using a Princeton Applied Research 2273 (PARstat) potentiostat in a twoelectrode configuration. Platinum gauze was used as the counter electrode for the open system. The active area of the open system photoanode was 0.28 cm2, and the active area for the compact cells ranged from 0.19 to 0.25 cm2. A 300 W Xe lamp with an AM 1.5 filter was used to illuminate at 100 mW/cm2. IV measurements were recorded by sweeping from 0.1 to 0.55 V at 10 mV/s. Electrochemical measurements were performed with a Gamry PCI4750 potentiostat. CV scans were performed at 50 mV/s starting at the equilibrium potential and scanning in the cathodic direction initially. EIS measurements were collected in potentiostatic mode at the equilibrium potential of the polysulfide. A 5 mV rms amplitude was used, and the scans were performed in the range of 10 mHz to 100 kHz. Gamry E-Chem Analyst was used to model the EIS data.
’ ASSOCIATED CONTENT
bS
Supporting Information. FTIR spectra, XRD, EDX, photoelectrochemical cell design, photocurrent stability measurement, and equivalent circuit model for EIS analysis. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through grant DE-FC02-04ER15533. This is contribution number NDRL 4895 from the Notre Dame Radiation Laboratory.
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We acknowledge Kevin Tvrdy, David Baker, and Pralay Santra for fruitful scientific discussions and suggestions during the design of counter electrodes for QDSC.
’ REFERENCES (1) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834–2860. (2) Tvrdy, K.; Kamat, P. V. Substrate Driven Photochemistry of CdSe Quantum Dot Films: Charge Injection and Irreversible Transformation on Oxide Surfaces. J. Phys. Chem. A. 2009, 113, 3765–3772. (3) Mora-Sero, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046–3052. (4) Gonzalez-Pedro, V.; Xu, X.; Mora-Sero, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783–5790. (5) Liu, L.; Hensel, J.; Fitzmorris, R. C.; Li, Y.; Zhang, J. Z. Preparation and Photoelectrochemical Properties of CdSe/TiO2 Hybrid Mesoporous Structures. J. Phys. Chem. Lett. 2010, 1, 155–160. (6) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854–2860. (7) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry-Baker, R.; Comte, P.; Aranyos, V.; Hagfeldt, A.; Nazeeruddin, M. K.; Gratzel, M. Stable New Sensitizer with Improved Light Harvesting for Nanocrystalline Dye-Sensitized Solar Cells. Adv. Mater. 2004, 16, 1806–1811. (8) Pandey, A.; Guyot-Sionnest, P. Hot Electron Extraction from Colloidal Quantum Dots. J. Phys. Chem. Lett. 2010, 1, 45–47. (9) Vogel, R.; Pohl, K.; Weller, H. Sensitization of Highly Porous, Polycrystalline TiO2 Electrodes by Quantum Sized CdS. Chem. Phys. Lett. 1990, 174, 241–6. (10) Vogel, R.; Hoyer, P.; Weller, H. Quantum-Sized PbS, CdS, Ag2S, Sb2S3 and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors. J. Phys. Chem. 1994, 98, 3183–3188. (11) Graetzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338. (12) Nikumbh, M.; Gore, V.; Gore, R. B. Structural, Optical and Photoelectrochemical Studies of the Cadmium Sulphide Films Grown by Chemical Bath Deposition. Renewable Energy 1997, 11, 459–467. (13) Mane, R. S.; Lokhande, C. D. Chemical Deposition Method for Metal Chalcogenide Thin Films. Mater. Chem. Phys. 2000, 65, 1–31. (14) Mane, R. S.; Roh, S. J.; Joo, O. S.; Lokhande, C. D.; Han, S. H. Improved Performance of Dense TiO2/CdSe Coupled Thin Films by Low Temperature Process. Electrochim. Acta 2005, 50, 2453–2459. (15) Hankare, P. P.; Chate, P. A.; Chavan, P. A.; Sathe, D. J. Chemical Deposition of ZnSe Thin Films: Photoelectrochemical Applications. J. Alloys Comp. 2008, 461, 623–627. (16) Hossain, M. F.; Biswas, S.; Takahashi, T. Study of CdSSensitized Solar Cells, Prepared by Ammonia-Free Chemical Bath Technique. Thin Solid Films 2009, 518, 1599–1602. (17) Patil, S. B.; Singh, A. K. Effect of Complexing Agent on the Photoelectrochemical Properties of Bath Deposited CdS Thin Films. Appl. Surf. Sci. 2010, 256, 2884–2889. (18) Kanniainen, T.; Lindroos, S.; Ihanus, J.; Leskela, M. Growth of Lead Selenide Thin films by the Successive Ionic Layer Adsorption and Reaction (SILAR) Technique. J. Mater. Chem. 1996, 6, 983–986. (19) Sankapal, B. R.; Mane, R. S.; Lokhande, C. D. Deposition of CdS Thin Films by the Successive Ionic Layer Adsorption and Reaction (SILAR) Method. MRS Bull. 2000, 35, 177–184. (20) Pathan, H. M.; Sankapal, B. R.; Desai, J. D.; Lokhande, C. D. Preparation and Characterization of Nanocrystalline CdSe Thin Films Deposited by SILAR Method. Mater. Chem. Phys. 2003, 78, 11–14. (21) Kale, R. B.; Sartale, S. D.; Chougule, B. K.; Lokhande, C. D. Growth and Characterization of Nanocrystalline CdSe Thin Films Deposited by the Successive Ionic Layer Adsorption and Reaction Method. Semicond. Sci. Technol. 2004, 19, 980–986. 2459
dx.doi.org/10.1021/jz201064k |J. Phys. Chem. Lett. 2011, 2, 2453–2460
The Journal of Physical Chemistry Letters (22) Liu, X. X.; Jin, Z. G.; Bu, S. J.; Zhao, J.; Cheng, Z. J. Preparation of CdS Thin Films by SILAR Method. J. Inorg. Mater. 2004, 19, 691–695. (23) Sun, H.; Mu, J. SILAR Deposition of CdS Thin Films on Glass Substrates Modified with 3-Mercaptopropyltrimethoxysilane. J. Dispersion Sci. Technol. 2005, 26, 719–722. (24) Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; N€uesch, F.; Geiger, T.; Zakeeruddin, S. M.; et al. PbS and US Quantum Dot-Sensitized Solid-State Solar Cells: “Old Concepts, New Results. Adv. Funct. Mater. 2009, 19, 2735–2742. (25) Lee, H. J.; Chen, P.; Moon, S.-J.; Sauvage, F.; Sivula, K.; Bessho, T.; Gamelin, D. R.; Comte, P.; Zakeeruddin, S. M.; Seok, S. I.; et al. Regenerative PbS and CdS Quantum Dot Sensitized Solar Cells with a Cobalt Complex as Hole Mediator. Langmuir 2009, 25, 7602–7608. (26) Prabakar, K.; Seo, H.; Son, M.; Kim, H. CdS Quantum Dots Sensitized TiO2 Photoelectrodes. Mater. Chem. Phys. 2009, 117, 26–28. (27) Chen, H. N.; Zhu, L. Q.; Li, W. P.; Liu, H. C. A ThreeDimensional Electrode for Photoelectrochemical Cell: TiO2 Coated ITO Mesoporous Film. Mater. Lett. 2010, 64, 781–784. (28) Liu, Z. F.; Miyauchi, M.; Uemura, Y.; Cui, Y.; Hara, K.; Zhao, Z. G.; Sunahara, K.; Furube, A. Enhancing the Performance of Quantum Dots Sensitized Solar Cell by SiO2 Surface Coating. Appl. Phys. Lett. 2010, 96 (29) Zhang, Q. X.; Zhang, Y. D.; Huang, S. Q.; Huang, X. M.; Luo, Y. H.; Meng, Q. B.; Li, D. M. Application of Carbon Counter Electrode on CdS Quantum Dot-Sensitized Solar Cells (QDSSCs). Electrochem. Commun. 2010, 12, 327–330. (30) Lee, H.; Wang, M. K.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Gratzel, M.; Nazeeruddin, M. K. Efficient CdSe Quantum DotSensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221–4227. (31) Dibbell, R. S.; Watson, D. F. Distance-Dependent Electron Transfer in Tethered Assemblies of CdS Quantum Dots and TiO2 Nanoparticles. J. Phys. Chem. C 2009, 113, 3139–3149. (32) Pernik, D.; Tvrdy, K.; Radich, J. G.; Kamat, P. V. Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked Versus Direct Attachment. J. Phys. Chem. C 2011, 115, 13511–13519. (33) Chakrapani, V.; Baker, D.; Kamat, P. V. Understanding the Role of the Sulfide Redox Couple (S2-/Sn2-) in Quantum Dot Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 9607–9615. (34) Loucka, T. Adsorption and Oxidation of Organic Compounds on a Platinum-Electrode Partly Covered by Adsorbed Sulfur. J. Electroanal. Chem. 1972, 36, 355–367. (35) Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848–1857. (36) Hodes, G.; Manassen, J.; Cahen, D. Electrocatalytic Electrodes for the Polysulfide Redox System. J. Electrochem. Soc. 1980, 127, 544–549. (37) Muszynski, R.; Seger, B.; Kamat, P. Decorating Graphene Sheets with Gold Nanoparticles. J. Phys. Chem. C 2008, 112, 5263–5266. (38) Jasuja, K.; Linn, J.; Melton, S.; Berry, V. Uncapped CatalyticallyEnhanced, Solution-Dispersed Graphene Supported Metal Nanoparticles Grown via Microwave Reduction: Tuning Catalytic, Electrical and Raman Properties. J. Phys. Chem. Lett. 2010, 1, 1853–1860. (39) Vinodgopal, K.; Neppolian, B.; Lightcap, I. V.; Grieser, F.; Ashokkumar, M.; Kamat, P. V. Sonolytic Design of Graphene Au Nanocomposites. Simultaneous and Sequential Reduction of Graphene Oxide and Au(III). J. Phys. Chem. Lett. 2010, 1, 1987–1993. (40) Kandilar, B.; Andreytc, R. Photovoltaic Effect in CdS-CdSe Heterojunctions. Phys. Status Solidi 1965, 8, 897–901. (41) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Ruhle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol., A 2006, 181, 306–313. (42) Huang, X. M.; Huang, S. Q.; Zhang, Q. X.; Guo, X. Z.; Li, D. M.; Luo, Y. H.; Shen, Q.; Toyoda, T.; Meng, Q. B. A Flexible Photoelectrode for CdS/CdSe Quantum Dot-Sensitized Solar Cells (QDSSCs). Chem. Commun. 2011, 47, 2664–2666.
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(43) Chi, C. F.; Cho, H. W.; Teng, H. S.; Chuang, C. Y.; Chang, Y. M.; Hsu, Y. J.; Lee, Y. L. Energy Level Alignment, Electron Injection, and Charge Recombination Characteristics in CdS/CdSe Cosensitized TiO2 Photoelectrode. Appl. Phys. Lett. 2011, 98, 012101. (44) Zhang, Q. X.; Guo, X. Z.; Huang, X. M.; Huang, S. Q.; Luo, Y. H.; Shen, Q.; Toyoda, T.; Meng, Q. B.; Li, D. M. Highly Efficient CdS/CdSe-Sensitized Solar Cells Controlled by the Structural Properties of Compact Porous TiO2 Photoelectrodes. Phys. Chem. Chem. Phys. 2011, 13, 4659–4667. (45) Lee, Y. L.; Lo, Y. S. Highly Efficient Quantum-Dot-Sensitized Solar Cell Based on Co-Sensitization of CdS/CdSe. Adv. Funct. Mater. 2009, 19, 604–609. (46) Deng, M. H.; Zhang, Q. X.; Huang, S. Q.; Li, D. M.; Luo, Y. H.; Shen, Q.; Toyoda, T.; Meng, Q. B. Low-Cost Flexible Nano-Sulfide/ Carbon Composite Counter Electrode for Quantum-Dot-Sensitized Solar Cell. Nanoscale Res. Lett. 2010, 5, 986–990. (47) Gao, J.; Perkins, C. L.; Luther, J. M.; Hanna, M. C.; Chen, H.-Y.; Semonin, O. E.; Nozik, A. J.; Ellingson, R. J.; Beard, M. C. n-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 3263–3266. (48) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Anchoring Semiconductor and Metal Nanoparticles on a 2-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide. Nano Lett. 2010, 10, 577–583. (49) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339. (50) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M. S.; Nazeeruddin, M. K.; Gratzel, M. Highly Efficient and Thermally Stable Organic Sensitizers for Solvent-Free Dye-Sensitized Solar Cells. Angew. Chem.Int. Ed. 2008, 47, 327–330.
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