Multiwalled Carbon Nanotube@Reduced Graphene Oxide

Article pubs.acs.org/JPCC

Multiwalled Carbon Nanotube@Reduced Graphene Oxide Nanoribbon as the Counter Electrode for Dye-Sensitized Solar Cells Min-Hsin Yeh,†,# Lu-Yin Lin,†,# Chia-Liang Sun,*,‡ Yow-An Leu,† Jin-Ting Tsai,‡ Chen-Yu Yeh,*,§ R. Vittal,† and Kuo-Chuan Ho*,†,∥ †

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Chemical and Materials Engineering, Chang Gung University, Tao-Yuan 333, Taiwan § Department of Chemistry and Center of Nanoscience and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan ∥ Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: In this study, a new core−shell heterostructure of multiwalled carbon nanotube@reduced graphene oxide nanoribbon (MWCNT@rGONR) was prepared by modified microwave-assisted synthesis step and a chemical reduction. The core−shell heterostructure of MWCNT@rGONR was used as the catalytic film of the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The chemical state and the degree of defects on the surface of MWCNT@rGONR were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectra, respectively. Transmission electron microscopy (TEM) image of the film of MWCNT@rGONR shows graphene sheet, covering on a MWCNT, indicating a core of the carbon nanotube and its shell of graphene. Photocurrent density−voltage characteristics of the DSSCs, using commercial graphene nanopowder (GNP), MWCNT, and MWCNT@rGONR as the CE materials were obtained at 100 mW cm−2. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrocatalytic abilities of the films of GNP, MWCNT, and MWCNT@ rGONR. Owing to the excellent electrocatalytic ability of the MWCNT@rGONR for the reduction of triiodide ions (I3−), a solar-to-electricity conversion efficiency (η) of 6.91% was achieved for its DSSC, using our synthesized YD2-o-C8 porphyrin dye, while efficiencies of 4.48% and 5.93% were obtained for the DSSCs with the bare GNP and pristine MWCNT, respectively. The performance of the cell with the MWCNT@rGONR is comparable to that of the cell with a sputtered Pt (s-Pt) on its CE (7.26%).

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INTRODUCTION Carbonaceous materials, such as graphite, diamond, fullerene, amorphous carbon, active carbon, carbon black (CB), carbon nanotubes, and graphene (GN) have been widely studied in different fields, owing to the fact that the physical and chemical properties of carbon vary greatly with its form. Some allotropic forms of carbon attract a lot of attention for electrochemical applications, such as in fuel cells,1 lithium-ions batteries,2 hydrogen storages,3 sensors,4 and supercapacitors,5 mainly because of their outstanding electrical conductivity, chemical durability, and electrocatalytic ability. These materials possess a high potential to replace expensive materials and thereby to reduce the cost of the pertinent devices and also to enhance the performance and durability of such devices. A counter electrode (CE) in a dye-sensitized solar cell (DSSC) has the roles of collection of electrons from the photoanode and reduction of triiodide ions (I3−) to iodide ions (I−) in the electrolyte. Typically, a conducting glass with Pt catalytic layer is used as the CE of a DSSC, due to its proven chemical durability6 and outstanding electrocatalytic ability for I3− reduction. However, Pt is very expensive and is thus a factor © 2014 American Chemical Society

to be considered for cost-effective fabrication of DSSCs, although it shows a high catalytic ability for I3− reduction and an excellent electronic conductivity. Replacement of Pt with other cheaper materials, such as conducting polymers,7 transition metallic compounds,8 and carbonaceous allotropes,9 is required for the reduction of production cost of the cells, especially when the production is in mass scale. Several carbonaceous materials, e.g., activated carbon,10 CB,11 acetylene-black spheres,12 hard carbon spherule,9a graphite,13 and multiwalled carbon nanotube (MWCNT) or single-walled carbon nanotube (SWCNT)14 have become potential materials to substitute Pt, because of their low cost, high conductivity, and good catalytic ability for the reduction of I3− ions. Recently, Wu et al. proposed nine kinds of carbonaceous materials as the catalysts of CEs for DSSCs.15 Special Issue: Michael Grätzel Festschrift Received: December 22, 2013 Revised: March 16, 2014 Published: March 27, 2014 16626

dx.doi.org/10.1021/jp412542d | J. Phys. Chem. C 2014, 118, 16626−16634

The Journal of Physical Chemistry C

Article

hydroxide (NH 4 OH, 25% solution in water), sodium borohydride (NaBH4, 99%), hydrazine monohydrate (N2H4· H2O, 99%), and hexachloroplatinic (IV) acid hexahydrate (H2PtCl6·6H2O, 99.9%) were obtained from Acros. Ethanol (EtOH, absolute), lithium perchlorate (LiClO4, ≥98.0%), 2methoxyethanol (≥99.5%), neutral cleaner (SODOSIL RM 02), Nafion 117 solution (∼5 wt % in a mixture of lower aliphatic alcohols and water), and titanium (IV) tetraisoproproxide (TTIP, >98%) were obtained from Sigma-Aldrich. Nitric acid (HNO3, ca. 65% solution in water), ethylene glycol (EG, 99%), and acetonitrile (ACN, 99.99%) were obtained from J. T. Baker. 3-Methoxypropionitrile (MPN, 99%) was obtained from Fluka. Graphene nanopowder (GNP, UR-GRAPHENE12, 99.2%, average flake thickness: ∼12 nm with 30−50 monolayers, average particle (lateral) size: ∼4.5 μm) was obtained from Uni-Region (Taipei, Taiwan). Multiwalled carbon nanotubes (MWCNTs, > 90%, diameter: 10 nm, length: 5∼15 μm) were supplied by Scientech Corporation (Taipei, Taiwan). 1,2-Dimethyl-3-propylimidazolium iodide (DMPII) was obtained from Solaronix (S. A., Aubonne, Switzerland). The novel porphyrin photosensitizer with high molar extinction coefficient, namely, YD2-o-C8, was synthesized by our group, as reported previously.29 Scheme 1 shows the chemical structure of the synthesized YD2-o-C8.

Ever since Nobel laureates Novoselov and Geim discovered two-dimensional GN in 2004,16 it has been introduced in various electrochemical devices, such as sensors,17 lithium-ion batteries,18 and capacitors,19 owing to its remarkable electrical, optical, thermal, and mechanical properties as well as its extraordinarily high surface area (∼2630 m2 g−1).20 Encouraged by these outstanding advantages of GN, a few researchers have introduced it as the catalytic film on the CEs of DSSCs. Wang et al. have demonstrated the electrode with the GN film as an alternative to a metal oxide transparent electrode (e.g., tindoped In2O3 (ITO) and fluorine-doped SnO2 (FTO) electrodes) in a solid-state DSSC.21 Kavan et al. have reported that graphene nanoplatelets can be used as a catalytic film on the CE of a DSSC.22 Aksay et al. have fabricated a DSSC with functionalized GN on its CE and achieved an efficiency of 4.99%, which is comparable to that of a cell with a typical Pt-CE (5.48%).23 Wang et al. designed 3D honeycomb-like structured graphene as a catalyst of CE for DSSCs and achieved an efficiency of 7.80%.24 We also demonstrated the feasibility of using GN-based composites as the CEs in DSSCs.25 However, in order to fully realize these properties and applications of GN, consistent, reliable, and inexpensive methods are crucial for growing high-quality graphene layers in excellent yields. Recently, graphene nanoribbons (GNRs) have become attractive candidates for nanoelectronics, spintronics, and nanoelectromechanical systems.26 GNRs were predicted theoretically for their existence by Nakada et al. in 1996.27 Although GNRs have been fabricated by a variety of methods until recently,28 there is still a lack of a facile and large-scale unzipping process for its production. Microwave heating has been successfully used to reduce the reaction time, increase the product yield, and enhance the product purity; this heating method reduces unwanted side reactions, with reference to conventional heating methods. In this study, a new core−shell heterostructure of multiwalled carbon nanotube@reduced graphene oxide nanoribbon (MWCNT@rGONR) was fabricated by a modified microwaveassisted synthetic method and used as the CE material in a DSSC. Further, a novel porphyrin dye, coded as YD2-o-C8, synthesized previously by our group, was introduced in this research.29 The photocurrent density−voltage characteristics of the DSSCs, using commercial graphene nanopowder (GNP), MWCNT, and the MWCNT@rGONR as the CE materials were obtained at 100 mW cm−2. Owing to the excellent electrocatalytic ability of the MWCNT@rGONR for the reduction of triiodide ions (I 3 −), a solar-to-electricity conversion efficiency (η) of 6.91% was achieved for the DSSC with MWCNT@rGONR, sensitized by our YD2-o-C8 porphyrin dye, while efficiencies of 4.48% and 5.93% were obtained for the DSSCs with the bare GNP and pristine MWCNT, respectively. The efficiency of the cell with MWCNT@rGONR (6.91%) in this study is comparable to the previous report of cell efficiency (7.6%) for a cell with the same dye, namely, YD2-o-C8,29 and with the same conventional iodide/triiodide (I−/I3−) redox electrolyte.

Scheme 1. Chemical Structure of the YD2-o-C8 Dye

Synthesis of the MWCNT@rGONR by the Modified Microwave-Assisted Process. The process of synthesizing the core−shell heterostructure of MWCNT@rGONR is based on the following steps. First, the core−shell heterostructure of multiwalled carbon nanotube/graphene oxide nanoribbons (MWCNT@GONR) is prepared according to our previous work.30 In brief, the MWCNT (0.05 g) was suspended in a mixing solution of H2SO4/H3PO4 (volume ratio: 9/1) and treated in a microwave reactor (CEM-Discover, North Carolina, USA) at 200 W for 2 min. After the addition of 0.25 g of KMnO4 to the solution, the solution was treated with the same microwave power at 65 °C for 4 min. This method introduces a second, weak acid such as H3PO4 into the system, so as to improve the selectivity of the oxidative unzipping, presumably by an in situ protection of the vicinal diols formed on the basal plane of the graphene during the oxidation, and thereby prevents the overoxidation and subsequent hole generation. The solution was filtered through a Millipore membrane (0.1 μm pore size); a core−shell structure of the MWCNT@GONR was thus obtained. The MWCNT@GONR was then washed with water several times and was further reduced to MWCNT@rGONR by a chemical conversion method. In a typical experiment, 0.1 g of the MWCNT@ GONR was dispersed in 50 mL of distilled water. Then 0.1 mL

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EXPERIMENTAL SECTION Materials. Lithium iodide (LiI, synthetical grade), iodine (I2, synthetical grade), and poly(ethylene glycol) (PEG, MW = 20,000) were obtained from Merck. Guanidinium thiocyanate (GuSCN, 99%), sodium hydroxide (NaOH, extra pure, pellets), 4-tert-butylpyridine (tBP, 96%), tert-butyl alcohol (tBA, 96%), tetrabutylammonium triiodide (TBAI3, >97%), ammonium 16627



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Figure 1. XPS of (a) MWCNT@GONR and (b) MWCNT@rGONR.

YD2-o-C8 dye solution in a mixed solvent of ACN/tBA (volume ratio: 1/1). The CE with the sputtered-Pt (s-Pt) was prepared by sputtering Pt with a thickness of 50 nm on the ITO substrate. EtOH-based solution with Nafion (0.5 wt %) containing 1 wt% of GNP, or MWCNT, or MWCNT@rGONR was individually prepared for depositing the corresponding film on the ITO glasses by drop coating. The role of Nafion is to act not only as a dispersant for inhibiting the aggregation of carbonaceous materials, but also as a binder to enhance the connection between the catalyst and the substrate. These films deposited on ITO glasses were designated as GNP, MWCNT, and MWCNT@rGONR; the thus coated ITO glasses were used as CEs for DSSCs. The particle size and structure of GNP, MWCNT, and MWCNT@rGONR were investigated by transmission electron microscopy (TEM, JEM-1230, JEOL, Tokyo, Japan). Surface morphologies of GNP, MWCNT, and MWCNT@rGONR films were observed by using fieldemission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA). Cyclic voltammetry (CV) was performed to investigate the electrocatalytic abilities of the CEs. The CV was carried out in an ACN solution, containing 10.0 mM I−, 1.0 mM I2, and 0.1 M LiClO4 with a three-electrode electrochemical system, using GNP, or MWCNT, or MWCNT@rGONR as the working electrode, a Pt foil as the CE, and an Ag/Ag+ as the reference electrode. Cell Assembly and Measurements. The above prepared TiO2 electrode was coupled with the CE; these two electrodes were separated by a 60 μm-thick Surlyn (SX1170-60, Solaronix S.A., Aubonne, Switzerland) and sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M TBP in MPN/ ACN (volume ratio = 1/1) was used as the electrolyte. The electrolyte was injected into the gap between the two electrodes by capillarity, and the hole was sealed with hot-melt glue after the electrolyte injection. The assembled device was illuminated by a class A quality solar simulator (XES-301S, AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan), and the incident light intensity (100 mW cm−2) was calibrated with a standard Si Cell (91150KG1, Oriel Instrument, California, USA). Photoelectrochemical characteristics of the DSSCs were recorded with a potentiostat/ galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, The Netherlands). Electrochemical impedance spectroscopy (EIS) was obtained by the above-mentioned potentiostat/galvanostat equipped with an FRA2 module, under a constant light illumination of 100 mW cm−2. The frequency range explored

of hydrazine monohydrate was added, and the mixture was heated at 95 °C for 1 h. After the completion of the reaction, the MWCNT@rGONR was collected by filtration as a black powder. The obtained cake was washed with distilled water several times to remove excess hydrazine, and the final product was dried in a vacuum oven at 80 °C. X-ray photoelectron spectroscopy (XPS, Theta Probe, Thermo Fisher Scientific, UK) was employed in the investigation of the MWCNT@ GONR and MWCNT@rGONR. The degrees of defects of the MWCNT@GONR and MWCNT@rGONR were estimated by Raman spectra, using a Dimension Raman system with 532 nm laser source (P2, Lambda Solution, Inc., USA). Preparation and Characterization of the Photoanode and Counter Electrode. FTO (TEC-7, 7 Ω sq.−1, NSG America, Inc., New Jersey, USA) and ITO (UR-ITO007-0.7 mm, 10 Ω sq.−1, Unionward Corp., Taipei, Taiwan) conducting glasses were first cleaned with a neutral cleaner and then washed with deionized water, acetone, and IPA sequentially. The TiO2 paste and the corresponding photoanode were prepared based on the procedure mentioned in the literature.31 Briefly, 72 mL of TTIP was added to 430 mL of 0.1 M HNO3 with constant stirring, and the content was heated to 88 °C for 8 h; the mixture was cooled down to room temperature, and the resultant colloid was filtered and heated in an autoclave at 240 °C for 12 h. The TiO2 colloid was thus concentrated to 8 wt%, and an amount of PEG (25 wt% with respect to TiO2) was added to the colloid. The conducting surface of the FTO was treated with a solution of TTIP in 2-methoxyethanol (weight ratio of 1:3). A first layer of 10 μm-thick mesoscopic TiO2 film consisting of TiO2 nanoparticles (particle size, 20 nm) was coated from the above obtained colloidal solution on the treated FTO glass by a “doctor blade” technique; a second 5 μm-thick scattering layer, made up with TiO2 particles of 100 and 20 nm (weight ratio of 1:1) was then coated on the first layer. The PEG was intended to prevent the film from being cracked during drying; the treatment with the solution of TTIP was intended for obtaining a good mechanical contact between the conducting surface and the TiO2 film on it. A portion of 0.4 × 0.4 cm2 was selected from the composite film as the active area by removing the side portions by scrapping. The FTO glass with the TiO2 film was gradually heated to 450 °C (rate = 10 °C/min) in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 450 °C and cooling to 80 °C, it was immersed for 4 h in a 0.2 mM 16628



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Table 1. C1s Peak Positions and Atomic Percentages of MWCNT@GONR and MWCNT@rGONR

a

samplea

CC

C−C

C−O

CO

C(O)O

MWCNT@GONR MWCNT@rGONR

284.3(27) 284.3(31)

284.8(41) 284.8(51)

286.1(15) 286.1(13)

287.5(2) 287.5(3)

288.7(15) 288.7(2)

Fitting of the C1s peak binding energy (eV) (atomic percentage (%))

Figure 2. TEM images of (a) MWCNT@GONR and (b) MWCNT@rGONR.

Figure 2a and b shows transmission electron microscopy (TEM) images of MWCNT@GONR and MWCNT@rGONR, respectively. Nanoribbons of graphene oxide can be seen on both sides of the nanotubes in Figure 2a. Figure 2b clearly shows that the basic structure in Figure 2a is retained here too, i.e., after the hydrazine monohydrate reduction of the MWCNT@GONR to MWCNT@rGONR. Moreover, TEM image of MWCNT@rGONR also reveals that the diameter of MWCNTs core and the thickness of rGONR shell are 15 and 25 nm, respectively, as shown in Figure S1. Thus it can be said that the MWCNT@GONR was entirely reduced to the MWCNT@rGONR by the hydrazine monohydrate reduction without any deformation in its nanostructure. Raman spectroscopy is most sensitive to highly symmetric covalent bonds with little or no natural dipole moment and can provide a wealth of information on the structures of carbonaceous materials.33 Figure 3 shows the positions of D band (1355 cm−1, ring breathing mode from sp2 carbon rings, A1g mode) and G band (1579 cm−1, planar configuration sp2 bonded carbon with bond-stretching motion, E2g mode) of the

was 65 kHz to 10 mHz. The applied bias voltage was set at the open-circuit voltage (VOC) of the DSSC, between the CE and the FTO/TiO2/dye working electrode, starting from the shortcircuit condition; the corresponding ac amplitude was set to be 10 mV. Impedance spectroscopy data were obtained, optimized by fitting to an equivalent circuit, using a Z-View software.35,36 Moreover, a symmetric cell was used to investigate the electrocatalytic ability of the CE by EIS with the frequency ranging from 10 mHz to 100 kHz;32 the cell consisted of an iodide/triiodide (I−/I3−) electrolyte and two identical electrodes with an area of 1 cm2, separated by a Surlyn spacer film of 60 μm thickness.

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RESULTS AND DISCUSSION Characteristics of the MWCNT@GONR and MWCNT@ rGONR. In this study, the core−shell heterostructure of MWCNT@rGONR was created by modified microwaveassisted process with two steps, i.e., the multiwalled carbon nanotubes/graphene oxide nanoribbons (MWCNT@GONR) was first synthsized by a microwave-assisted method and then was further reduced to the desired MWCNT@rGONR by a chemical method. The chemical properties on the surfaces of MWCNT@GONR and MWCNT@rGONR were studied by using X-ray photoelectron spectroscopy (XPS). Figure 1a,b shows detailed C1s XPS spectra of the MWCNT@GONR and MWCNT@rGONR, respectively, which include the groups of aromatic (CC), aliphatic (C−C), hydroxyl carbon (C−O), carbonyl carbon (CO), and carboxyl carbon (C(O)O). The related peak positions and atomic percentages are listed in Table 1. The C1s spectra of the MWCNT@GONR show a high degree of oxidation, which can be observed from the broad peak of the C(O)O group (carboxyl, 288.7 eV). Abundant oxygen-containing functionalities may be present in the shell structure of GONR of the heterostructure of MWCNT@ GONR. The spectra of the MWCNT@rGONR reveal that the carboxyl carbon C(O)O groups were almost entirely transformed into aromatic CC and aliphatic C−C groups.

Figure 3. Raman spectra of MWCNT@GONR and MWCNT@ rGONR. 16629



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Figure 4. SEM images of the films of (a) GNP, (b) MWCNT, and (c) MWCNT@rGONR.

films of MWCNT@GONR and MWCNT@rGONR and also the related ratios of D peak intensity to G peak intensity (D/G ratio) for the two films. An increase in the D/G ratio for the MWCNT@GONR may be due to a high density of defects on the shell of GONR, which is consistent with the C/O ratio seen in the XPS results. Encouraged by these observations of XPS and Raman spectroscopy, the MWCNT@rGONR was employed as the catalytic material in a DSSC. Surface Morphologies and Characteristics of GNP, MWCNT, and MWCNT@rGONR. In order to verify the effect of core−shell heterostructure of MWCNT@rGONR, the commercial graphene nanopowder (GNP) and multiwalled carbon nanotubes (MWCNTs) will be used separately as the catalytic materials for comparison. Figure S2a (Supporting Information) shows TEM image of a commercial GNP sheet and S2b shows that of a MWCNT. The GNP seems to have a wrinkled and rough surface, which may be attributed to the stacking of several pieces of GN through strong π−π interactions on its surface.41 The MWCNT has a tubular structure, as expected, and its average diameter and length were estimated to be around 10 nm and 5 μm, respectively. The surface morphologies of the films of GNP, MWCNT, and MWCNT@rGONR were obtained using a field-emission scanning electron microscopy (FE-SEM). Figure 4 shows FESEM images of the films of GNP, MWCNT, and MWCNT@ rGONR. Figure 4a shows the surface of the GNP, which appears to be smoother than those of the MWCNT (Figure 4b) and MWCNT@rGONR (Figure 4c), i.e., without apparent pores as in the cases of MWCNT and MWCNT@rGONR. Unlike the heaped structure of GNP, a porous surface morphology can be seen in the cases of MWCNT and MWCNT@rGONR. Yen et al. reported, owing to the steric hindrances, that the adsorption of MWCNT on GN reduces the π−π interactions between the GN sheets, thereby reducing the aggregation of GN. 34 Hybrid heterostructure of MWCNT@rGONR, namely, central tube-like core of MWCNT with the plane structure of GN sheets surrounded outside, had an outstanding intrinsic property for avoiding aggregation and further exhibiting the porous and rough characteristics. The specific surface characteristics of the eletrodes with GNP, MWCNT, and MWCNT@rGONR were further characterized by cyclic voltammotry (CV) in a three-electrode system with 1.0 M LiClO4 aqueous solution as the electrolyte. Figure 5 shows CVs of the three electrodes with GNP, MWCNT, and MWCNT@rGONR. It can be seen that the nonfaradaic current density, i.e., capacitive current density of

Figure 5. Cyclic voltammograms of the electrodes with GNP, MWCNT, and MWCNT@rGONR, at a scan rate of 100 mV s−1, obtained in an aqueous solution of 1.0 M LiClO4.

MWCNT@rGONR is much higher than either GNP or MWCNT, which implies that the MWCNT@rGONR possesses much higher surface area for the accumulation of electrical charges at its interface with the electrolyte.35 Some reports suggest that the tortuous CNT can be used to inhibit the aggregation of GN, which will significantly raise the electrolyte-accessible surface area for the composite, with reference to that of pristine GN.36 The high surface area of MWCNT@rGONR was not only attributed to its hetrosturcture with the plane structure of GN sheet, but also ascribed to its hetrosturcture with the tube stuctruce of MWCNT for avoiding aggregation. It is worth mentioning that this phenomenon was also found in our recent study on MWCNT@GONR for supercapacitor application.37 Photovoltaic Performance of the DSSCs with the CEs Containing GNP, MWCNT, and MWCNT@rGONR. Photocurrent density−voltage (J−V) characteristics of the DSSCs with the CEs containing the films of GNP, MWCNT, and MWCNT@rGONR are shown in Figure 6, and the corresponding photovoltaic parameters are summarized in Table 2. Among the DSSCs with CEs containing three kinds of carbonaceous materials, the cell with MWCNT@rGONR-CE shows the best performance with a cell efficiency (η) of 6.91%, an open-circuit voltage (VOC) of 0.70 V, a fill factor (FF) of 0.59, and a short-circuit current density (JSC) of 16.87 mA cm−2; this η is far higher, compared to those of the cells with GNP-CE (4.86%) and MWCNT-CE (5.93%). Higher efficiency of the DSSC with MWCNT@rGONR is due to its 16630



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verified in further discussions through CV and EIS, by using a three-electrode electrochemical system and a symmetric cell with two identical electrodes, respectively. Cyclic Voltammetric Analysis of the Electrocatalytic Ability for I3− Reduction at the Electrodes with GNP, MWCNT, and MWCNT@rGONR. CV was performed, using a three-electrode electrochemical system, to understand the reaction kinetics and electrocatalytic abilities of the CEs with GNP, MWCNT, and MWCNT@rGONR for I−/I3− redox reaction. Figure 7 displays the CVs of the electrodes with GNP,

Figure 6. Photocurrent density−voltage curves of the DSSCs with GNP, MWCNT, and MWCNT@rGONR on their CEs, measured at 100 mW cm−2 (AM 1.5G).

Table 2. Photovoltaic Parameters of the DSSCs with GNP, MWCNT, and MWCNT@rGONR on Their CEs, Measured at 100 mW cm−2 (AM 1.5G)a CE GNP MWCNT MWCNT@ rGONRs

η (%)

VOC (V)

JSC (mA cm−2)

FF

Rs (Ω cm2)

Rct (Ω cm2)

4.86 5.93 6.91

0.68 0.66 0.70

13.00 15.71 16.87

0.55 0.57 0.59

18.92 16.07 14.97

2.13 1.63 1.27

a

Figure 7. Cyclic voltammograms of the electrodes with GNP, MWCNT, and MWCNT@rGONR, obtained in an electrolyte containing 10 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in ACN.

The table also shows the corresponding values of the ohmic series resistances (Rs) and of the charge transfer resistances (Rct) at the interfaces of the CEs, obtained by EIS using a symmetric cell.

MWCNT, and MWCNT@rGONR. In a DSSC, the photoexcited electrons from the dye are injected into the TiO2 conduction band. The oxidized dye is then reduced by the I− ions in the electrolyte, and the resulting I3− ions are reduced at the CE. The redox reactions at the photoanode and the CE are shown in eqs 1 and 2, respectively:39

higher VOC, JSC, and FF, with references to those of the cells with GNP and MWCNT. The higher JSC of the cell with MWCNT@rGONR may be attributed to the higher surface area of MWCNT@rGONR and thereby to its higher number of electroactive sites for the reduction of triiodide ions (I3−), with reference to these two parameters in the cases of GNP and MWCNT. High electrocatalytic ability of the MWCNT@rGONR could accelerate the generation of iodide ions (I−), thereby leading to faster dye regeneration, faster electron injection, and ultimately to a higher JSC for its cell. Although GNP had extraordinarily high surface area and outstanding conductivity, due to the fact that the physicochemical properties of aggregated GN are similar to graphite, the performance of its cell is significantly worse than that expected. Electrocatalytic sites of a crystalline carbon material are usually located on the edges of the crystals;38 this implies that MWCNT has fewer electrocatalytic sites for the reduction of I3− ions, owing to their highly oriented stacking structures. The hybrid heterostructure of MWCNT@rGONR with the central cores of nanotubes remained tube-like structure, which is able to bridge the gaps to the shell of GN sheets and form a continuously conductive network. This hybrid heterostructure is supposed to have smoothly conductive pathways for electron conduction and hopping.36 The smooth electron pathways in the CE of the cell with MWCNT@rGONR are expected to induce the fast movement of redox couple in the electrolyte of the cell, which in turn can lead to fast electron transfer kinetics in the cell and thereby to a high FF to the cell (0.59), compared to that of the cell with GNP (0.55) and MWCNT (0.57). The advantage of the hybrid heterostructure of MWCNT@rGONR will be

3I− → I3− + 2e−

(1)

I3−

(2)

−

−

+ 2e → 3I

Among the electrodes containing different carbonaceous materials, the electrode with MWCNT@rGONR shows a higher peak cathodic current density (2.62 mA cm−2), corresponding to the reaction in eq 2, than those of the electrodes with GNP (1.81 mA cm−2) and MWCNT (1.06 mA cm−2). In general, a higher redox peak current density of the CE with MWCNT@rGONR implies a higher electrocatalytic ability for this CE for I3− reduction.40 Furthermore, the peak separation for the CE with MWCNT@rGONR (304 mV) is smaller than those for the CEs with GNP (590 mV) and MWCNT (472 mV). These results also indicate that the electrocatalytic ability of MWCNT@rGONR is far superior to those of GNP and MWCNT; this may be due to higher concentration of defects on the MWCNT@rGONR surface with GN sheets.23 This higher electrocatalytic ability of the CE with MWCNT@rGONR enables better performance to its DSSC. In order to investigate the charge transfer kinetics and diffusion resistances of the CEs with GNP, MWCNT, and MWCNT@rGONR, EIS was carried out using a symmetric-cell having two identical electrodes. EIS Analysis of the Electrocatalytic Ability for the Reduction of I3− ions at the CEs with GNP, MWCNT, and 16631



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MWCNT@rGONR. The charge transfer resistances for the reduction of I3− at the interfaces of GNP, MWCNT, and MWCNT@rGONR with the electrolyte were investigated by EIS; the EIS was carried out by using a symmetric cell having two identical electrodes. Figure 8 shows Nyquist plots of the

observed in the case of MWCNT@rGONR, due to its higher electrocatalytic ability for the reduction of I3−. These results show that MWCNT@rGONR can effectively catalyze the reduction of I3− to I−. In addition, a lower Rct value also implies a higher exchange current density (J0).The relationship between these two parameters can be expressed by eq 3:41 J0 =

RT nFR ct

(3)

where J0 is the exchange current density, R is the gas constant, T is the absolute temperature, F is Faraday’s constant, n is the number of electrons involved in the reduction of I3− to I−, and Rct is the charge transfer resistance. The high J0 of the CE with MWCNT@rGONR is consistent with its best performance, among all the electrodes, not only in terms of its outstanding electrocatalytic ability, but also in terms of the photovoltaic performance of its DSSC. Comparative Study of the Photovoltaic Performance of DSSCs with CEs Containing Films of Sputtered Pt and MWCNT@rGONR. The photovoltaic parameters of the cell with MWCNT@rGONR are compared with those of a cell with a sputtered-Pt film (s-Pt). The J−V curves and the corresponding photovoltaic parameters are shown in Figure 9.

Figure 8. Nyquist plots of the symmetric cell with GNP, MWCNT, and MWCNT@rGONR, obtained at zero bias potential; the equivalent circuit is shown in the figure as an inset.

symmetric cell with GNP, MWCNT, and MWCNT@rGONR; the equivalent circuit is shown in the figure as an inset. The EIS of a symmetric cell can be divided into three parts. The ohmic series resistance (Rs), i.e., the total resistance of the substrates, the catalytic layers on these substrates, and the electrolyte, is determined in the high frequency region (106−105 Hz) where the phase is zero. The first semicircle in the middle frequency range (105−10 Hz) represents the resistance against the heterogeneous electron transfer at an electrode/electrolyte interface (Rct). The second semicircle in the low frequency range (10−0.1 Hz) represents the resistance (ZW) in the electrolyte.32 The values of Rs and Rct, evaluated by fitting the impedance spectra with the equivalent circuit, are given in Table 2. The symmetric cells with GNP, MWCNT, and MWCNT@rGONR show Rs values of 18.92, 16.07, and 14.97 Ω cm2, respectively. The Rs of the symmetric cell with MWCNT@rGONR is smaller than those of the cells with GNP and MWCNT. This result is a measure for the higher conductivity of the film of MWCNT@rGONR, as compared to those of the films of pristine GNP and MWCNT. High conductivity of MWCNT@ rGONR is attributed to the heterostructure of tube-like core that bridges the gaps to the shell of GN sheets and forms a continuously conductive network for electron transfer.36 Moreover, the resistances of the films with GNP, MWCNT, and MWCNT@rGONR were also measured by four-point probe analysis. The result shows that the film of MWCNT@ rGONR (14.46 ± 1.08 Ω) gave the best conductivity for electron transfer, as compared to that of GNP (23.27 ± 0.93 Ω) and MWCNT (18.08 ± 1.23 Ω). This result is consistent with the trend obtained from the EIS analysis. The tendency of the Rs and the resistance with these carbonaceous materials is consistent with the tendency of FF values of the DSSCs with these materials. On the other hand, the symmetric cells with GNP, MWCNT, and MWCNT@rGONR show Rct values of 2.13, 1.63, and 1.27 Ω cm2, respectively. A lower value of Rct is

Figure 9. Photocurrent density−voltage curves of the DSSCs with CEs containing s-Pt and MWCNT@rGONR, measured at 100 mW cm−2 (AM 1.5G). The related photovoltaic parameters are listed in the figure.

The η of 6.91% of the DSSC with MWCNT@rGONR is very close to that of the cell with s-Pt (7.26%). One of the reasons for the better performance of the cell with s-Pt lies in the fact that unabsorbed incident light could be reflected back by the semitransparent s-Pt to the TiO2 photoanode for re-absorption; this is one of the reasons for the higher JSC (17.69 mA cm−2) of the DSSC with s-Pt-CE, compared to that of the cell with MWCNT@rGONR-CE (16.87 mA cm−2). 6 Finally, it is worth mentioning that the performance of the DSSC with MWCNT@rGONR is competitive to that of the cell with the conventional s-Pt. Thus, the new MWCNT@rGONR heterostructure can be a potential material for replacing the conventional, expensive Pt in a DSSC.

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CONCLUSIONS A new core−shell heterostructure of MWCNT@rGONR was synthesized and used as the catalyst of CE for a DSSC. A TEM image of MWCNT@rGONR shows GN sheet covered on a MWCNT, indicating a core of the carbon nanotube and its 16632



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Dispersed on Arrayed CNx Nanotubes with High Electrochemical Activity. Chem. Mater. 2005, 17, 3749−3753. (2) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Carbon Nanotubule Membranes for Electrochemical Energy Storage and Production. Nature 1998, 393, 346−349. (3) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of Hydrogen in Single-Walled Carbon Nanotubes. Nature 1997, 386, 377−379. (4) Besteman, K.; Lee, J.-O.; Wiertz, F. G. M.; Heering, H. A.; Dekker, C. Enzyme-Coated Carbon Nanotubes as Single-Molecule Biosensors. Nano Lett. 2003, 3, 727−730. (5) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537−1541. (6) Fang, X.; Ma, T.; Guan, G.; Akiyama, M.; Kida, T.; Abe, E. Effect of the Thickness of the Pt Film Coated on a Counter Electrode on the Performance of a Dye-Sensitized Solar Cell. J. Electroanal. Chem. 2004, 570, 257−263. (7) (a) Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Application of Poly(3,4-ethylenedioxythiophene) to Counter Electrode in DyeSensitized Solar Cells. Chem. Lett. 2002, 31, 1060−1061. (b) Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. I−/I3− Redox Reaction Behavior on Poly (3,4-ethylenedioxythiophene) Counter Electrode in Dye-Sensitized Solar Cells. J. Photochem. Photobiol. A: Chem. 2004, 164, 153−157. (c) Lee, K. M.; Chen, P. Y.; Hsu, C. Y.; Huang, J. H.; Ho, W. H.; Chen, H. C.; Ho, K. C. A High-Performance Counter Electrode Based on Poly(3,4-alkylenedioxythiophene) for Dye-Sensitized Solar Cells. J. Power Sources 2009, 188, 313−318. (8) (a) Wu, M.; Lin, X.; Wang, Y.; Wang, L.; Guo, W.; Qi, D.; Peng, X.; Hagfeldt, A.; Grätzel, M.; Ma, T. Economical Pt-free Catalysts for Counter Electrodes of Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 3419−3428. (b) Wei, W.; Wang, H.; Hu, Y. H. Unusual Particle-size-induced Promoter-to-poison Transition of ZrN in Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 14350−14357. (c) Wang, H.; Wei, W.; Hu, Y. H. Efficient ZnO-Based Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 6622−6628. (d) Chi, W. S.; Han, J. W.; Yang, S.; Roh, D. K.; Lee, H.; Kim, J. H. Employing Electrostatic Selfassembly of Tailored Nickel Sulfide Nanoparticles for Quasi-solid-state Dye-Sensitized Solar Cells with Pt-free Counter Electrodes. Chem. Commun. 2012, 48, 9501−9503. (9) (a) Huang, Z.; Liu, X.; Li, K.; Li, D.; Luo, Y.; Li, H.; Song, W.; Chen, L.; Meng, Q. Application of Carbon Materials as Counter Electrodes of Dye-Sensitized Solar Cells. Electrochem. Commun. 2007, 9, 596−598. (b) Wang, H.; Hu, Y. H. Graphene as a Counter Electrode Material for Dye-Sensitized Solar Cells. Energy Environ. Sci. 2012, 5, 8182−8188. (10) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.; Nakamura, J. I.; Murata, K. High-Performance Carbon Counter Electrode for Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2003, 79, 459−469. (11) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Péchy, P.; et al. Highly Efficient Dye-Sensitized Solar Cells Based on Carbon Black Counter Electrodes. J. Electrochem. Soc. 2006, 153, A2255− A2261. (12) Cai, F.; Chen, J.; Xu, R. Porous Acetylene-Black Spheres as the Cathode Materials of Dye-Sensitized Solar Cells. Chem. Lett. 2006, 35, 1266−1267. (13) Chen, J.; Li, K.; Luo, Y.; Guo, X.; Li, D.; Deng, M.; Huang, S.; Meng, Q. A Flexible Carbon Counter Electrode for Dye-Sensitized Solar Cells. Carbon 2009, 47, 2704−2708. (14) (a) Hino, T.; Ogawa, Y.; Kuramoto, N. Dye-Sensitized Solar Cell with Single-Walled Carbon Nanotube Thin Film Prepared by an Eectrolytic Micelle Disruption Method as the Counterelectrode. Fullerenes, Nanotubes, Carbon Nanostruct. 2006, 14, 607−619. (b) Suzuki, K.; Yamaguchi, M.; Kumagai, M.; Yanagida, S. Application of Carbon Nanotubes to Counter Electrodes of Dye-Sensitized Solar

shell of GN. XPS of MWCNT@GONR and MWCNT@ rGONR demonstrate that the former is almost entirely transformed into the latter. Raman spectroscopy reveals a low density of defects on the shell of rGONR, indicating a high electron transfer in favor of MWCNT@rGONR. Among these carbonaceous materials, i.e., bare GNP, pristine MWCNT, and heterostructure of MWCNT@rGONR, MWCNT@rGONR exhibits the highest surface roughness and surface area, which were verified by SEM and CV, respectively. The DSSC with the catalytic film of MWCNT@rGONR on its CE and sensitized with YD2-o-C8 dye shows the best efficiency (η of 6.91%) with reference to those of the cells with the films of GNP (4.86%) and MWCNT (5.93%). The η of the cell with MWCNT@ rGONR is comparable to that of the cell with s-Pt on its CE (7.26%). The best performance of the cell with the film of MWCNT@rGONR is validated by the highest electrocatalytic ability of the film for the reduction of I3− ions and by its least charge transfer resistance at its interface with the electrolyte. In brief, the low-cost MWCNT@rGONR can be a potential material for replacing the conventional, expensive Pt in a DSSC.

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ASSOCIATED CONTENT

S Supporting Information *

Related characterization of materials is described in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +886-3-2118-800 x5379; Fax: +886-3-2118-668; E-mail: [email protected] (C.-L.S.) *Tel.: +886-4-22840411 x610; Fax: +886-4-2286-2547; E-mail: [email protected] (C.-Y.Y.) *Tel.: +886-2-2366-0739; Fax: +886-2-2362-3040; E-mail: [email protected] (K.-C.H.) Author Contributions #

These authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Council (NSC) of Taiwan, under Grant Numbers NSC 1002221-E-002-242-MY2 and NSC 100-3113-E-008-003. C.L. thanks the Chang Gung Memorial Hospital for financial support through the project CMRPD2C0012. Some of the instruments used in this study were made available through the financial support of the Academia Sinica, Taipei, Taiwan, under Grant Number AS-100-TP-A05.

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REFERENCES

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