Reduced Graphene Oxide Nanocomposites for

Article pubs.acs.org/JPCC

NiS2/Reduced Graphene Oxide Nanocomposites for Efficient DyeSensitized Solar Cells Zhuoqun Li, Feng Gong, Gang Zhou, and Zhong-Sheng Wang* Department of Chemistry, Lab of Advanced Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China S Supporting Information *

ABSTRACT: NiS2 nanoparticles and nanocomposites of NiS2 with reduced graphene oxide (NiS2@RGO) have been successfully prepared via a facile hydrothermal reaction of nickel ions and sulfur source in the absence or presence of graphene oxide. NiS2@RGO nanocomposites exhibit excellent electrocatalytic performance for reduction of triiodide, owing to the improved conductivity and positive synergetic effect between NiS2 and RGO. As a consequence, the dyesensitized solar cell with the NiS2@RGO counter electrode (CE) produces a power conversion efficiency of 8.55%, which is higher than that (7.02%) for the DSSC with the NiS2 CE, higher than that (3.14%) for the DSSC with the RGO CE, and also higher than that (8.15%) for the DSSC with the reference Pt CE under the same conditions.

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INTRODUCTION Dye-sensitized solar cell (DSSC) has attracted immense attention as a promising renewable energy device compared to the conventional silicon solar cell because of its low cost and relatively high conversion effieiency.1,2 Recently, the best power conversion efficiency (PCE), 12.3%, has been obtained at AM1.5G (100 mW cm−2) by Yella and co-workers.3 In general, a typical DSSC usually comprises three main components: a dye sensitized TiO2 nanocrystalline film as the working electrode, a platinized conductive glass as the counter electrode (CE), and an electrolyte traditionally containing an iodide/ triiodide (I−/I3−) redox couple between the two electrodes. The working mechanism includes the photoexcitation of the sensitizer followed by ultrafast electron injection into the conduction band of the semiconductor. The oxidized dye is then regenerated by I− in the electrolyte, and the resultant I3− is reduced to I− at the CE interface.4,5 As a crucial component, CE plays an important role in the performance of DSSC in that it collects electrons from external circuit and fulfills electron transfer from the CE interface to the electrolyte by electrocatalyzing the reduction of I3−. For the CE of DSSCs, Pt is still the prior choice owing to its outstanding electrocatalytic activity for reducing triiodide and excellent chemical stability. However, as a noble metal, the abundance of Pt seriously hinders the large-scale production of DSSCs. Therefore, finding more abundant and cheaper materials to replace Pt is a significant research area of DSSCs. In previous studies, some materials, including conductive polymers6 and carbonaceous materials,7 have been proposed as CEs to substitute for Pt in DSSCs. Recently, some inorganic compounds, such as nitrides,8 carbides,9 sulfides,10 oxides,11 and selenides,12 were introduced into DSSCs as electrocatalysts that have good catalytic activity for the reduction of I3−. Among these inorganic compounds, nickel sulfides, which have various © 2013 American Chemical Society

atomic ratios depending on the synthetic conditions, exhibit great potential as the CE of DSSCs due to their low cost and good electrocatalytic activity for reducing triiodide.10 According to the reported data,10 6−7% of PCE was obtained using the nickel sulfide (Ni3S2, NiS) as the CE. In this work, we prepared another compound (NiS2) in the nickel sulfide series as the CE for DSSCs, which has not yet been studied before, to the best of our knowledge. However, the low charge mobility in nickel sulfide nanoparticles due to the random boundaries between particles may limit the electrocatalytic activity and hence solar cell performance. To solve this drawback, a composite of nickel sulfide with an excellent conductive material may be a good choice, which is expected to act as a high-performance CE for efficient DSSCs. Graphene, a two-dimensional carbon material, was lately demonstrated to be a competitive photoelectric material because of its ballistic conduction of charge carriers and charge transfer interactions with other molecules.13 Herein, we propose a facile two-step strategy to prepare nanocomposites of NiS2 nanaoparticles with reduced graphene oxide (NiS2@ RGO) and use them as the CE of DSSCs. To the best of our knowledge, this is the first time to explore NiS2@RGO nanocomposites for application in DSSCs. Although RGO has much lower electrocatalytic activity than NiS2, the nanocomposites of NiS2@RGO exhibit much higher catalytic activity than NiS2 alone due to the synergetic catalysis of NiS2 and RGO. The cell with NiS2@RGO CE produces a PCE of 8.55%, which is much higher than that of the cell with pure NiS2 CE (7.05%). Received: January 29, 2013 Revised: March 8, 2013 Published: March 9, 2013 6561

dx.doi.org/10.1021/jp401032c | J. Phys. Chem. C 2013, 117, 6561−6566

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Synthesis of Graphene Oxide. Graphene oxide (GO) was synthesized according to the modified Hummers method from natural graphite powder.14 The graphite powder (2 g) was added into an 80 °C solution containing concentrated H2SO4 (3 mL), K2S2O8 (1 g), and P2O5 (1 g) and stirred for 6 h. The reaction system was then thermally isolated and cooled down to room temperature. The mixture was carefully washed with distilled water until the pH of the filtrate became neutral. The obtained oxidized graphite powder was dried in air at room temperature overnight. The oxidized graphite powder (2 g) was then added into cold (0 °C) concentrated H2SO4 (46 mL), followed by slow addition of 6 g of KMnO4, keeping stirring at 20 °C. Next, the mixture was stirred at 35 °C for 2 h, to which 92 mL of distilled water was added. In 15 min, a large amount of distilled water (280 mL) and 30% H2O2 solution (5 mL) were put into the above mixture. The obtained GO powder was filtered and washed with 1:10 HCl solution (500 mL) and copious water successively. The purified GO powder was dried in a vacuum oven. Preparation of NiS2@RGO Nanocomposites. A typical synthesis of NiS2@RGO nanocomposites involves the following processes.15 First, 1.0, 3.0, or 6.0 mg of GO powders was dispersed in a 30 mL of ethylene glycol in a beaker under ultrasonication for 60 min (solution A). Concomitantly, 0.13 mL of carbon disulfide (CS2, Alading Reagent Inc.) was added dropwise to 26 mL of 0.187 mol L−1 ethylenediamine aqueous solution, followed by stirring for half an hour (solution B). Solution A was then added to solution B, and the mixture was stirred vigorously for 30 min, followed by dropping 13 mL of 20 mmol NiCl2 aqueous solution into the above solution. The mixture solution was stirred for half an hour and then transferred to a 100 mL autoclave for hydrothermal reaction at 180 °C for 12 h. After the hydrothermal reaction, GO was reduced to RGO, and NiS2 was formed on the surface of RGO, forming the NiS2@RGO nanocomposites. The precipitate of NiS2@RGO was filtered, washed with ethanol, and distilled water three times each and dried in a vacuum oven at 60 °C. The NiS2@RGO sample obtained from 3.0 mg of GO powder gave the best solar cell performance, so we chose this sample to conduct the experiments through this work. NiS2 was synthesized with the same method as the NiS2@ RGO composite without introduction of GO to the reaction system while RGO was prepared with the same method without introduction of sulfur source and NiCl2. Fabrication of Electrodes and DSSCs. The CEs with various catalysts, viz. RGO, NiS2, and NiS2@RGO, were prepared through a simple drop-casting method. First, 1 mg of catalyst was dispersed in 9 mL of distilled water by ultrasonication for 1 h. 100 μL of this solution was dropcasted on FTO-coated glass (fluorine-doped SnO2, 15 Ω/ square, Nippon Sheet Glass Co., Ltd., Japan), which was masked by a 3M Scotch tape with an exposed area of 0.6 × 0.6 cm2. Then the films were dried in an oven at 70 °C. For comparison, pyrolytic Pt CE was prepared by drop-casting 50 μL of H2PtCl6 in ethanol (5 mM) on a 1.5 × 1.5 cm2 FTO glass followed by sintering at 400 °C for 30 min. The loading amount of the NiS2@RGO nanocomposite, pure NiS2, and RGO on the substrate was 30.7 μg cm−2 while the loading amount of Pt on the substrate was 23.0 μg cm−2. TiO2 films (15 μm thick)16 were coated on FTO glass by a screen-printing technique. TiO2 films were soaked overnight in

the N719 solution (Lumtec Corp., 0.3 mM in a mixed solvent of acetonitrile and tert-butanol by a volume ratio of 1:1). The dye-loaded TiO2 film as the working electrode and the CE were separated by a hot-melt Surlyn film (30 μm thick) and sealed together by hot-pressing. The redox electrolyte (0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine using anhydrous acetonitrile as a solvent) was injected into the interspace between the photoanode and CE through a predrilled hole. Finally, the hole was sealed with a Surlyn film covered with a thin glass slide under heat. Characterizations. The samples were characterized on an X-ray powder diffractometer (D8 Advance, Bruker) with Cu Kα radiation (λ = 0.154 nm). The Raman scattering measurements were performed on a Renishaw spectrometer at 633 nm with a 100× objective. The morphology analysis of the NiS2 and NiS2@RGO was carried out by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL) equipped with EDS to measure the elements in the samples. The Brunauer−Emmett−Teller (BET) specific surface area was analyzed by the BET equation using a surface area and porosity analyzer (ASAP2020, Micromeritics). The RGO content in the nanocomposites was determined by thermogravimetric analysis (TGA) on a TGA/DTA analyzer (DTG-60H, Shimadzu) in air with a heating rate of 10 °C min−1. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were executed on an electrochemical workstation (ZAHNER ZENNIUM CIMPS-1, Germany). CV was carried out in a three-electrode system with different CEs as the working electrode, a platinum wire as the counter electrode, and Ag/Ag+ electrode as the pseudoreference electrode, which was calibrated with a ferrocene solution after the CV measurements, at a scan rate of 50 mV s−1. The electrode was dipped in an anhydrous acetonitrile solution containing 0.1 M LiClO4, 10 mM LiI, and 1 mM I2. EIS spectra were measured in a symmetrical sandwich cells assembled with two identical CEs filled with the redox electrolyte to be used in DSSCs. A photocurrent density−voltage (J−V) curve was measured using a Keithley 2400 source meter under the illumination of simulated AM1.5G solar light coming from a solar simulator (Oriel-91193 equipped with a 1000 W Xe lamp and an AM1.5 filter); the light intensity was calibrated using a reference Si solar cell (Oriel-91150). A black mask with an aperture area of 0.2304 cm2 that fully covered on the surface of DSSCs was used to avoid stray light completely.

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RESULTS AND DISCUSSION Characterizations of NiS2 and NiS2@RGO. Figure 1 shows the XRD patterns for NiS2, NiS2@RGO, RGO, and GO samples. GO exhibited a typical XRD peak at 10.5°, corresponding to an interplanar spacing of 0.84 nm. Upon reduction of GO during hydrothermal reaction,17 this peak became broadened and shifted to 24.6° for RGO, corresponding to an interplanar spacing of 0.36 nm. All the diffraction peaks for the as-prepared nickel sulfide sample could be well indexed to NiS2 (JCPDS No. 65-3325). The nanocomposites also exhibited the XRD peaks for NiS2, but the signal for RGO was not observed. The good crystallinity and relatively high content of NiS2 (91 wt %) in the composite gave strong diffraction peaks which might overlap the diffraction peak of RGO.18,19 In addition, NiS2 nanoparticles attached onto the surface of RGO during the reaction process, which prevented 6562



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Figure 1. XRD patterns of NiS2, NiS2@RGO, RGO, and GO.

the aggregation and restacking of RGO sheets and thus weakened the diffraction intensity of RGO sheets.20 The atomic ratio of Ni:S was 1.00:2.16 revealed by EDS, which is close to the stoichiometric ratio of NiS2. The BET surface area was measured of 11.4, 9.4, 8.6, and 5.8 m2 g−1 for NiS2@RGO, NiS2, RGO, and Pt, respectively. The TGA curves of the NiS2@RGO and RGO shown in Figure S1 gave 9 wt % RGO and 91 wt % NiS2 in NiS2@RGO. Figure 2 shows the Raman spectra of NiS2, NiS2@RGO, RGO, and GO. The D band and G band are located at 1329

Figure 3. HRTEM images of as-synthesized NiS2@RGO (a, c) and NiS2 (b, d).

samples, which is in good coincidence with the theoretical value (d200 = 2.83 Å for NiS2). Electrochemical Behaviors of CEs. To characterize the electrocatalytic activity of various catalysts, CV experiments were carried out under the same conditions, as shown in Figure 4. Two typical pairs of oxidation and reduction peaks (Ox-1/

Figure 2. Raman spectra of NiS2@RGO, RGO, GO, and NiS2. Figure 4. CV curves of iodide/triiodide redox species for NiS2@RGO, NiS2, Pt, and RGO CE.

and 1585 cm−1 in the Raman spectra for NiS2@RGO, RGO, and GO. The intensity ratio of D band to G band (ID/IG) is 1.18 for GO, which increases to 1.30 for RGO, confirming the reduction of GO.21 The ID/IG ratio further increases to 1.94 for NiS2@RGO, suggesting interactions between NiS2 and RGO. NiS2 shows a stretching vibration peak of the S−S pair with Ag mode at 480 cm−1.22 The presence of the Raman peak for NiS2 and the Raman peaks for RGO in the spectrum of NiS2@RGO suggests the formation of NiS2@RGO nanocomposites. The morphologies of the formed NiS2@RGO and NiS2 are characterized with HRTEM, as depicted in Figure 3. For NiS2@ RGO, NiS2 nanoparticles were deposited and well distributed on the RGO nanosheet surface with an average particle size of about 200−300 nm (Figure 3a). However, NiS2 nanoparticles aggregated when GO was not present in the reaction system (Figure 3b). GO with oxygen functional groups could serve as a substrate for nucleation of nanomaterials, and the interactions between functional groups on GO and NiS2 nanoparticles during nucleation and growth led to well-dispersed nanoparticles.20 The clear observation of lattice fringes (Figure 3c,d) for NiS2 nanoparticles indicates that the as-prepared NiS2@ RGO and NiS2 have good crystallinity. The interplanar spacing of d200 derived from the lattice fringes is 2.7 ± 0.1 Å for both

Red-1 and Ox-2/Red-2, as labeled in Figure 4) were observed for all the samples except the RGO CE. The left pair was assigned to eq 1, and the right pair was assigned to eq 2. The characteristics of peaks Ox-1 and Red-1 are at the focus of our analysis because the CE is responsible for catalyzing the reduction of I3− to I− in a DSSC. Moreover, the peak-to-peak separation (Epp, Table 1), which is inversely correlated with the standard electrochemical rate constant of a redox reaction, and the peak current are two important parameters for comparing catalytic performance of different CEs.23 The CV curve for NiS2 electrode exhibits an obvious Red-1 peak at −0.49 V (vs Ag/ Ag+) which is absent for the RGO CE in the scan range. This indicates that NiS2 CE has better catalytic activity than RGO CE. The peak currents of Ox-1/Red-1 for NiS2@RGO are higher than those for NiS2 and those for RGO as well. In addition, the Epp for Ox-1 and Red-1 of the NiS2@RGO CE is 0.50 V, which is smaller than that for the NiS2 CE (0.65 V). These results demonstrate a positive synergetic effect between the NiS2 and RGO sheets for reducing I3− to I−, which can efficiently improve interfacial charge transfer and provide more active catalytic sites.24 Moreover, the improved conductivity 6563



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Table 1. Photovoltaic and Electrochemical Parameters for Different CEsa CEs

Voc (mV)

Jsc (mA cm−2)

FF

PCE (%)

Rs (Ω cm2)

Rct (Ω cm2)

Epp (V)

NiS2 NiS2@RGO RGO Pt

738 749 716 739

14.42 16.55 10.98 15.75

0.66 0.69 0.40 0.70

7.02 8.55 3.14 8.15

5.1 6.4 14.2 2.2

8.8 2.9 100.2 0.5

0.65 0.50 0.44

a

The Epp value for the RGO CE could not be determined because no Red-1 peak was observed in the scan range. The deviation error for these parameters was less than 5%.

according to the equivalent circuit shown in the inset of Figure 6b.25 It is noted that for a porous electrode material, besides the Nernst diffusion impedance determined by the bulk electrolyte diffusion appearing in the low-frequency region, a second Nernst diffusion impedance resulting from the electrolyte diffusion in electrode pores should appear in the high-frequency region.23 However, the Nernst diffusion impedance in electrode pores can be omitted in our case due to the small amount of catalyst on FTO. This can be verified by the EIS changes through varying the catalyst amount and applied potential. As the loading amount of the catalyst increases (Figure S2a) or the bias is applied (Figure S2b), the left (high-frequency) arc shrinks, indicating that the observed high-frequency arc is attributed to the charge transfer at the electrode/electrolyte interface.23 The resistance per unit area (Table 1), which correlates inversely with the electrocatalytic activity, was calculated using the total surface area of catalysts on the electrode obtained by the production of the loading amount, the geometric area (0.36 cm2), and the BET surface area of the sample. The total surface area was calculated to be 1.26, 1.04, 0.95, and 0.48 cm2 for the NiS2@RGO, NiS2, RGO, and Pt electrode, respectively. As shown in Table 1, the Rct for RGO, NiS2, NiS2@RGO, and Pt was 100.2, 8.8, 2.9, and 0.5 Ω cm2, respectively. The largest Rct for RGO CE indicated its lowest catalytic ability among these CEs. As compared to RGO, NiS2 gave much lower Rct, indicating that the latter had much higher catalytic activity than the former. The NiS2@RGO composite gave much lower Rct than NiS2, indicating that the nanocomposites had much higher catalytic activity than each component in the composite. This is attributed to the presence of RGO sheets that serve as the conduction pathway (Figure 5) and the synergetic effect of NiS2 and RGO.24 Figure 7 gives the Tafel polarization curves carried out on the symmetrical cells. A larger slope in the anodic or cathodic branch implies a higher exchange current density (J0) on the electrode.7f The anodic and cathodic branches of RGO showed

(Figure 5) from NiS2 to NiS2@RGO is another plausible cause for the improved electrocatalytic performance. As compared to

Figure 5. Tafel polarization curves for the NiS2 and NiS2@RGO pressed samples at a scan rate of 50 mV s−1. The area is 0.25 cm2. The loading amount is 2.88 mg cm−2.

the reference Pt electrode, both NiS2 and NiS2@RGO showed higher peak currents due to the higher loading but exhibited slightly larger Epp. I3− + 2e− ↔ 3I− −

3I 2 + 2e ↔

2I3−

(1) (2)

To evaluate the electrocatalytic activity of the as-prepared CEs for the reduction of triiodide, EIS tests were performed using symmetric cells fabricated with two identical CEs (CE/ electrolyte/CE), as shown in Figure 6. The high-frequency intercept on the real axis (Z′-axis) represents the series resistance (Rs), and the semicircle in the high-frequency region (left) arises from the charge-transfer resistance (Rct, the radius of the arc on the real axis) and the constant phase element (CPE) at the CE/electrolyte interface, while the one in the lowfrequency range (right) stems from Nernst diffusion impedance (ZN) of the triiodide/iodide redox couple in the electrolyte,

Figure 6. Nyquist plots of EIS for symmetric cells fabricated with NiS2@RGO, NiS2, RGO, and Pt CEs at (a) reduced and (b) full impedance ranges. The inset of (b) gives the equivalent circuit diagram. 6564



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a PCE of 8.55 ± 0.13% (Voc = 749 mV, Jsc = 16.55 mA cm−2, FF = 0.69), which is much higher than the efficiency for each component of the nanocomposites as the CE. The improved FF is attributed to the increased electrocatalytic performance of cathode materials.7g However, it is interesting that the Jsc is also improved significantly when the cathode changes from RGO or NiS2 to NiS2@RGO. As the charge transfer resistance influences the J−V curve near Voc,7g the photocurrent is usually similar for various cathodes as reported previously7g because it is mostly controlled by the photoanode. Although we do not know the reason why the photocurrent for various cathodes is that different so far, the significant increase in Jsc by varying the cathode materials is very interesting, which deserves detailed investigation in the future. Under the same conditions, the cell with the Pt CE produces a PCE of 8.15 ± 0.17% (Voc = 739 mV, Jsc = 15.75 mA cm−2, FF = 0.70). This result indicates that the NiS2@RGO nanocomposites are even outperforming Pt when applied as the CE of DSSCs.

Figure 7. Tafel polarization curves of different CEs that are same as the ones used in EIS experiments.

smaller slopes than NiS2, which showed smaller slopes than NiS2@RGO and Pt. This indicates that J0 increases in the order of RGO < NiS2 < NiS2@RGO ≈ Pt. In addition, J0 can be obtained by eq 3, where R is the gas constant, T is the absolute temperature, Rct is the charge-transfer resistance extracted from the EIS spectra (Figure 6), and F is the Faraday’s constant. As the Rct for as-prepared CEs are in the order of Pt ≈ NiS2@ RGO < NiS2 < RGO, the tendency of J0 for different CEs calculated with eq 3 is in agreement with that derived from the Tafel curves. J0 = (RT )/(nFR ct)

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CONCLUSIONS In summary, we have successfully prepared NiS2@RGO nanocomposites via a facile in situ hydrothermal reaction and applied the composite as the novel CE material for DSSCs by a drop-casting technique. The electrochemical results reveal that the positive synergetic effect between NiS2 and RGO sheets on the reduction of I3− can greatly improve the catalytic activity, resulting in a significant enhancement of PCE. The PCE for the DSSC with NiS 2 @RGO CE reaches 8.55%, which is significantly higher than that (7.02%) for the DSSC with the NiS2 CE and that (3.14%) for the DSSC with the RGO CE. The nanocomposites are even better than the Pt electrode in solar cell performance, suggesting NiS2@RGO is a promising alternative to Pt electrode for DSSCs.

(3)

The electrochemical stability of NiS2, NiS2@RGO, and Pt was investigated using the dummy cells subjected to sequential scan of CV and EIS for 10 cycles (Figure S3). After 10 cycles of scanning, the Rct increased from 8.8 to 16.1 Ω cm2 by 83% for NiS2, from 2.9 to 3.7 Ω cm2 by 28% for NiS2@RGO, and from 0.5 to 0.7 Ω cm2 by 40% for Pt. This result revealed that the NiS2@RGO composite had better electrochemical stability than NiS2. Photovoltaic Performance. Figure 8 shows the J−V curves of the DSSCs with NiS2@RGO, NiS2, RGO, and Pt CEs,

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ASSOCIATED CONTENT

S Supporting Information *

TGA curves, EIS for the composite with different loading amount or at different bias, and electrochemical stability for the CEs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +86-21-5163-0345. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Basic Research Program (2011CB933302) of China, the National Natural Science Foundation of China (20971025, 50903020 and 90922004), STCSM (12JC1401500), Shanghai Pujiang Project (11PJ1401700), Shanghai Leading Academic Discipline Project (B108), and Jiangsu Major Program (BY2010147) for the financial support.

Figure 8. J−V characteristics of DSSCs with different CEs under simulated AM1.5G solar light (100 mW cm−2).

and the photovoltaic parameters are summarized in Table 1. The DSSC with the RGO CE gives a power conversion efficiency (PCE) of 3.14% (open-circuit photovoltage (Voc) = 716 mV, short-circuit photocurrent density (Jsc) =10.98 mA cm−2, fill factor (FF) = 0.40), while the DSSC with the NiS2 GE produces a PCE of 7.02% (Voc = 738 mV, Jsc =14.42 mA cm−2, FF = 0.66). The device with the NiS2 CE is much better than that with the RGO CE, particularly in Jsc, FF, and PCE. Interestingly, the nanocomposites of NiS2 with RGO generates

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REFERENCES

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