Controlled Growth of CuS on Electrospun Carbon Nanofibers as an

Article pubs.acs.org/JPCC

Controlled Growth of CuS on Electrospun Carbon Nanofibers as an Efficient Counter Electrode for Quantum Dot-Sensitized Solar Cells Linlin Li,†,‡ Peining Zhu,†,§ Shengjie Peng,*,‡ Madhavi Srinivasan,‡ Qingyu Yan,*,‡ A. Sreekumaran Nair,∥ Bin Liu,⊥ and Seeram Samakrishna*,§ ‡

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 Centre for Nanofibers and Nanotechnology, Department of Mechanical Engineering, National University of Singapore, Singapore 117576 ∥ Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidhyapeetham University, Cochin 682041, India ⊥ Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576 §

S Supporting Information *

ABSTRACT: One-dimensional CuS/electrospun carbon nanofiber heteroarchitectures (CuS/EC) with high catalytic activity have been successfully fabricated by combining the versatility of the electrospinning technique and following a hydrothermal process. The CuS nanoparticles as the secondary nanostructures were uniformly grown on the primary electrospun carbon nanofibers with good dispersion by optimizing reaction conditions. It was found that the L-cysteine used as the sulfur donor and chelating reagent was favored for the growth of CuS on the carbon fibers. A possible formation mechanism and growth process of the CuS nanoparticles on the carbon fibers is discussed based on the experimental results. The as-prepared CuS/EC composite was then spray-deposited on FTO glass and demonstrated good performance in quantum dot-sensitized solar cells (QDSCs), which was higher than the conventional Pt electrode. The good performance is attributed to its heteroarchitecture. The CuS nanoparticles with high catalytic activity play the main role in the reduction process of the oxidized polysulfide, while the carbon nanofibers with the 3-D mat morphology bridge all the CuS nanoparticles as the framework and facilitate the charge transport during the catalysis process. polysulfide redox system based QDSCs.11 Therefore, it is imperative to investigate the photovoltaic performance of the combination of CuS and carbon. It is known that electrospinning has been widely utilized to fabricate nanostructures for its advantages of simple set up, versatility, and large-scale production. The electrospun products have been widely applied in photocatalysis,21 solar cell,22 lithium ion batteries,23 and supercapacitors.24 In particular, electrospun carbon nanofibers as the counter electrode exhibited high performance in dye-sensitized solar cells, due to the one-dimensional nature and high surface area, which could provide low charge transfer resistance and large reaction sites.25 The deposition of counter electrode film can be carried out by several methods, such as inkjet printing, screen printing, doctor-blading, and spray coating. Among them, spray coating is capable of delivering large-area, uniform thin films through a relatively simple process in a relatively short time, offering ample processing possibilities of engineering the film structure.26,27 Therefore, the spray technology is widely accepted in industrial production. It is expected that the

1. INTRODUCTION Semiconductor quantum dots (QDs) are now frequently used as the light harvester in the sensitized solar cells for their advantages of high molar extinction coefficients, large intrinsic dipole moments, and tunable band gaps.1−5 So far, various semiconductors QD sensitizers such as CdS,4,6 CdSe,5,7 InP,8 and InAs9 have been widely studied for QDSCs. Meanwhile, counter electrodes are also attracting tremendous research attention in the study of the QDSCs.10,11 It was reported that the platinum as a counter electrode was a problem when the polysulfide redox couple was utilized as the electrolyte in QDSCs. The sulfur-containing compounds in the electrolyte will adsorb preferably and strongly to Pt surfaces, which would decrease the surface activity and conductivity of the Pt electrodes and then shorten the lifetime.11,12 Therefore, metal sulfides, such as Cu2S, CoS2, CuS, and PbS are currently studied as the counter electrode for the QDSCs for their properties of low resistance and high electrocatalytic activity for the redox reaction of polysulfide.13−16 It has been demonstrated that copper sulfide surpasses platinum in electrocatalytic activity toward polysulfide reduction.14,17 At the same time, as a promising low-cost replacement of Pt counter electrode, carbon nanofibers have been widely researched as the alternative material for reducing the cost of the solar cells.18−20 Moreover, recent research revealed that a carbon electrode showed much higher performance than Pt in the © XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: November 30, 2013 Revised: April 23, 2014

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dx.doi.org/10.1021/jp4117529 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. SEM images of electrospun carbon nanofibers (EC) (a) and CuS/EC composite (b−d); XRD patterns (e) of the EC (pattern I) and CuS/ EC (pattern II) composite.

(DMF) solution. The solution was kept under stirring for 12 h at room temperature. After that, the solution was then subjected to electrospinning using a commercial machine (NANON, MECC Japan) with an applied voltage of 15 kV, working distance of 10 cm and a flow rate of 1.0 mL/h. The electrospun fibers were collected on an aluminum foil which was wrapped around a rotating drum. The as-obtained PAN polymer nanofibers were then stabilized in the air for 3 h at 280 °C and carbonized at 1000 °C in nitrogen and then cooled down to room temperature. 2.2. Fabrication of the CuS/EC Composite and CuS/EC Film. CuS/EC composites were prepared by a hydrothermal process. In a typical procedure, 0.02 g of the as prepared electrospun carbon nanofibers were dispersed in the aqueous solution premade by dissolving 0.0085 g of CuCl2 and 0.12 g of L-cysteine into 40 mL of deionized water. After sonicating for 30 min, the above mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 30 mL. Subsequently, the autoclave was sealed and maintained at 160 °C for 12 h. After hydrothermal reaction, the autoclave was air-cooled to room temperature. The black product was collected, washed with ethanol and deionized water several times, and finally dried in an electric oven at 80 °C for 4 h. Thus, the CuS/EC composites were formed. In addition, different reaction times were set to investigate the formation process of the films, and the procedures were similar to the above process. The asfabricated CuS/EC Composite was then sonicated in ethanol for 2 h to obtain a 10 mg/mL suspension. The suspension was sprayed with an airbrush (Airbrush Model 150) onto cleaned FTO glass, which had been previously heated up to 80 °C to

electrospun CuS/EC counter electrode obtained by spraydeposition can present high performance in QDSCs. In the present study, the spray-deposited CuS/EC composite is fabricated for the purpose of the high performance counter electrode material in QDSCs. The 3-dimensional (3D) connected nanostructure composed of continuous carbon nanofibers with uniform diameters is fabricated by the easy method of electrospinning. After that, the CuS nanoparticles were grown on the surface of the carbon nanofibers followed with a hydrothermal treatment. The as-grown CuS nanoparticles with a size ranging from 20 to 50 nm are not only uniformly monodispersed, but also closely attached to the EC nanofibers surface. The CuS/EC composite is then spraydeposited on FTO glass and demonstrates a high efficiency of 3.86% in the CdS/CdSe sensitized solar cell, which is much better than the sputtered Pt, bare CuS and EC counter electrodes. The superior photovoltaic performance can be attributed to the novel heteroarchitectures. The well distributed CuS nanoparticles with high catalytic activity functioned as the catalyst for the reaction process at the counter electrode/ electrolyte interface, while the carbon nanofibers with 3-D connected mat structure bridged all the CuS nanoparticles, facilitated the charge transport, and enhanced the overall performance of the solar cell.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Carbon Nanofibers. The carbon nanofibers (EC) were prepared by a typical electrospinning process. First, 1 g polyacrylonitrile (PAN) (Mw = 150000) powders were dissolved in 10 mL of N,N-dimethylformamide B



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Figure 2. TEM images of CuS/EC composite (a) and the CuS nanoparticles grown on the surface of electrospun carbon nanofiber (b). The highresolution lattice pattern of CuS nanoparticle (c). The SAED pattern of the CuS/EC nanocomposite (d). The elemental mapping images of a single carbon nanofiber with CuS nanoparticles grown on the surface (e−h).

at 70 °C and heated at 450 °C for 30 min again. The chemical bath deposition (CBD) technique was utilized to assemble CdS and CdSe QDs on TiO2 photo anodes with a literature reported method.28,29 Briefly, CdS QDs were first in situ grown on the TiO2 photoanode with a mixture solution of 20 mM CdCl2, 66 mM NH4Cl, 230 mM ammonia, and 140 mM thiourea for 30 min. The pH of the solution was around 9.5. After that, the films were removed from the solution and washed with deionized water completely. Then, the CdSe QDs were deposited on the CdS-coated TiO2 films by immerging the latter into an aqueous solution with 26 mM CdSO4, 40 mM N(CH2COONa)3 (NTA), and 26 mM Na2SeSO3 for 5.5 h. Finally, CdS/CdSe cosensitized photoanodes were passivated with ZnS by twice immersing into 100 mM Zn (CH3COO)2 and 100 mM Na2S aqueous solution for 1 min alternately. The

allow the evaporation of ethanol and the fabrication of uniform films. For comparison, bare electrospun carbon nanofibers without CuS nanoparticles (defined as EC electrode) and CuS nanoparticles without the support of electrospun carbon nanofibers (defined as CuS electrode) were also prepared with the same method as the counter electrode. Moreover, traditional Pt counter electrode was prepared for comparison by spin-coating H2PtCl6 (50 mM in isopropyl alcohol) on the FTO substrates with following sintering at 390 °C in air for 30 min. 2.3. Fabrication of QDSCs. TiO2 film was prepared by screen printing the commercial paste (Solarnix) on FTO glass. The thickness of the TiO2 film is controlled to be 15 μm and the working area of 0.25 cm2. Then, the TiO2 film was dried at 80 °C and heated at 450 °C for 30 min. Subsequently, the TiO2 film was treated with 50 mM TiCl4 aqueous solution for 30 min C



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Figure 3. XPS spectra of CuS/EC nanocomposite (a), Cu 2p peak (b), S 2p peak (c), and C 1s peak (d) in the composite.

polysulfide electrolyte (1 M Na2S and 1 M S) was utilized for the cell. 2.4. Characterizations. The morphology of the products were investigated by scanning electron microscopy (SEM; JEOL 7600). The fine structure was investigated by transmission electron microscopy (TEM, JEOL 2100). The The Xray diffraction (XRD) patterns were recorded using a Siemens D5005 X-ray diffractometer employing Ni-filtered Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALab220i-XL electron spectrometer from VG Scientific. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was employed for analysis with an incident angle of 30° with respect to surface normal. The photocurrent−voltage (I− V) curves were measured by XES-151 S solar simulator (San-Ei, Japan) under AM1.5 G condition and an Autolab PGSTAT30 integrated with a potentiostat, respectively. For the EIS tests, two identical CuS/EC or Pt electrodes were placed face-to-face to form symmetric dummy cells filling the S2−/Sn2− electrolyte similar to the one applied in fabricating QDSSCs.

it could be observed that a very thin layer of CuS nanoparticles was successfully grown on the surface of carbon nanofibers, forming the heteroarchitecture. Figure 1d shows the SEM image with an even higher magnification. It could be seen that the diameters of the CuS nanoparticles were ranging from 20 to 50 nm. Moreover, the CuS nanoparticles were uniformly distributed across the surface of the carbon nanofiber without aggregation, which would promote a high level exposure of the nanoparticles’ surface. Also, it could be observed that the CuS nanoparticles were firmly attached on the surface of the carbon nanofibers, promising the good connection between the two, which would be good for the charge transport for the applications in solar cells. The XRD patterns of the pure carbon nanofibers (pattern I) and CuS/EC composite (pattern II) are shown in Figure 1e. In the pattern I and II, the broad peaks centered at around 25 was attributed to the (002) planes of the carbon structure in electrospun carbon nanofibers. Morevoer, compared with the pure carbon nanofibers, the diffraction peaks of CuS were also clearly observed in pattern II, which could be perfectly indexed as pure hexagonal CuS phase (JCPDS: 6−464).30 No characteristic peaks for impurity, such as Cu and CuO were observed, suggesting that the composition was CuS and carbon nanofibers only. Moreover, the morphology and the structure of the composite were further investigated by TEM. The TEM image presented in Figure 2a clearly demonstrated the heterostructure of the CuS/EC composite. The CuS nanoparticles were uniformly distributed on the surface of the carbon nanofibers as the secondary structure, while the carbon nanofibers were functioning as the primary structure supporting the CuS nanoparticles and keeping the nanoparticles connected. It must be noted that the CuS nanoparticles were

3. RESULTS AND DISCUSSION Figure 1 shows the morphology and purity of the as-obtained pure carbon nanofibers and the CuS/EC nanocomposite. Figure 1a shows the typical SEM images of the electrospun carbon nanofibers with random orientations and 3-D connected network. A SEM image with higher magnification is presented as the inset in Figure 1a. The diameters of the carbon nanofibers ranged from 200 to 500 nm. After the growth of the CuS nanoparticles, the nanostructure was also investigated by the SEM and presented in Figure 1b. It could be seen that the morphology and dimensions, as well as the 3-D connected network of the carbon nanofibers, were not changed. However, with a higher magnification SEM image presented in Figure 1c, D



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Figure 4. SEM images of the CuS−carbon nanofiber composites after reaction times of 1 (a), 5 (b), and 16 h (c), as well as the CuS nanostructure obtained in the absence of electrospun carbon (d).

found on the surfaces of the samples, indicating that the asprepared CuS−carbon composite is relatively pure. The peaks for the Cu showed the typical Cu 2p3/2 (932.30 eV) and Cu 2p1/2 (952.80 eV) binding energy, while the peak of S could be deconvoluted to the binding energy of S 2p3/2 (160.63 eV) and S 2p1/2 (163.06 eV). These are in good agreement with the literature reported results.30 At the same time, the peak of C centered at 285 eV could be assigned to the bond of C−C (284.6 eV), C−O (285.6 eV), and O−CO (289 eV), demonstrating the existence of the carboxyl carbon.32 This would be beneficial for the growth of the CuS nanoparticles on the electrospun nanofibers as well as the good connection between the two, as the carboxyl would act as the nucleation site for the CuS nanoparticles on the carbon nanofibers.32 The control experiment of the growth of the CuS nanoparticles was carried out by varying the reaction time of the hydrothermal treatment. The samples obtained with reaction times of 1, 5, and 16 h were presented in Figure 4. After a 1 h hydrothermal treatment, few CuS nanoparticles were starting to grow on the surface of the electrospun carbon nanofibers. When the time increased to 5 h, more CuS nanoparticles were observed growing on the surface of the electrospun carbon nanofibers. However, the CuS nanoparticles were still not enough to cover most of the surface of the carbon nanofibers. On the other hand, when the hydrothermal treatment was over-reacted to 16 h, the CuS nanoparticles were intended to agglomerate into a big block with a diameter around micrometer, which would decrease the surface area as

not peeled off from the carbon nanofibers by the ultrasonic process during the sample preparation for TEM measurements, indicating the strong connection between the CuS nanoparticles and the carbon nanofibers. A TEM image with higher magnification is also presented in Figure 2b, revealing a CuS nanoparticle with a diameter of around 20 nm attached on the surface of the carbon nanofiber. The high magnification TEM image in Figure 2c shows the lattice pattern of the CuS nanoparticle, revealing the lattice spacing of 3.04 Å. This is in a good agreement to the interplanar distance of (102) of hexagonal CuS.30 The electron diffraction pattern in Figure 2d indicated that the CuS nanoparticles are polycrystalline. The TEM element mapping was also carried out on a single nanofiber with the heterostructure, as shown in Figure 2e. The signals of C, S, and Cu were presented in Figure 3f−h, respectively. From the images, it could be seen that the signals of the C were strongly detected and formatted a good shape of fiber, well matched with the fiber structure in Figure 2e. At the same time, the signals of S and Cu were also detected, which is originated from the CuS nanoparticle. It must be noted that the signal of the Cu was strong and more spread than S due to the influence of the copper TEM grid. The TEM element mapping results were unambiguous and confirmed the carbon nanofibers were coated with CuS nanoparticles. The composition of the compound was further confirmed by the XPS spectra. The XPS spectra indicated the presence of Cu and S from the CuS nanoparticles as well as the C from the electrospun carbon nanofibers.31 No obvious impurities were E



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Figure 5. SEM images of the plan (a) and cross-sectional view (b) of the CuS/EC film, respectively. The insets are the magnified part and the photo of the spray-deposited CuS/EC film.

Figure 6. Schematic illustration of the formation of CuS/EC composite and the spray-deposited CuS/EC film on FTO glass.

well as the reaction sites of the CuS. Moreover, the control experiment of the growth of CuS without the support of electrospun carbon nanofibers was also carried out. The result was demonstrated in Figure 4d. It could be seen from the SEM image that, without the support of the electrospun carbon nanofibers, the CuS grew into aggregates with large diameters rather than small nanoparticles attached on the one-dimensional networks. This demonstrates the necessity of the present of electrospun nanofibers to form the well connected 3-D network structure. Furthermore, TAA was employed as the sulfur source as another control experiment (see Figure S1 in Supporting Information). The as-obtained composite showed poor connection between the CuS nanoparticles and electrospun nanofibers. All the control experiments demonstrate that

the formation of the as-obtained CuS/EC nanocomposite with well connected 3-D network and heteroarchitecture was the results of the precise design and control of the experiments and reactions. The CuS/EC film as the counter electrode is fabricated based on the spray-deposition technique. Spray deposition is a practical process that can deposit thin films of a variety of nanomaterials on a range of substrates with a large area. The spray-doposition method can control the film thickness and uniformity by manipulating the concentration and injection flow rate of the spraying solution in a short time. The CuS/EC solution (5 mg/mL) was carried by the inert gas traveling through the inkjet nozzle and sprayed onto the substrate to form a thin film. Figure 5 shows SEM images of the plan (a) F



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and cross-sectional view (b) of the CuS/EC film. The photograph of the CuS/EC film deposited on the FTO glass indicates the uniform structure without crack. The plan SEM image reveals that the film is dense and no obvious crack is formed. The thickness of film can be controlled to be about 20 μm, by tuning the concentration of solution, pressure of the carrier gas, and the spraying time. Based on the above experimental results, a possible mechanism for the formation of as-prepared CuS/EC heteroarchical nanocomposites was proposed to a chelationanchoring-nucleation-directional growth strategy. The process is schematically expressed as Figure 6. It is known that metal ions can react with L-cysteine to form complexes due to many functional groups in the cysteine, such as -NH2, -COOH, and -SH, which have a strong tendency to coordinate with inorganic cations.33,34 Similarly, Cu2+ can coordinate with L-cysteine to form a complex in a homogeneous system composed of Lcysteine and CuCl2 solution. In the reaction system, one part of 2+ L-cysteine is oxidized into cystine by Cu ions and the other part binds to the as-produced Cu(I) to form CuI−cysteine complex.35 Subsequently, the hydroxyl O− of carboxyl and amine groups of Cu−cysteine complexes could react with the surface hydroxyl groups and carboxyl of electrospun carbon nanofibers, which were demonstrated by the IR spectrum (see Figure S2 in the Supporting Information). Under hydrothermal condition, the C−S and Cu−S bonds of the Cu−cysteine complexes ruptured, and then CuI was oxidized to CuII by a small amount of oxygen in the solution. Hence, CuS nuclei were formed on active growth sites provided by carbon nanofibers templates. After that, CuS seeds would aggregate to CuS nanocrystals at the expense of Cu−cysteine complexes on primary carbon nanofibers. During the process of copper sulfide growth, Cu−cysteine complexes could release lead and sulfur donor gradually so as to decrease the formation rate of copper sulfide, which lead to the controllable growth of numerous CuS nanocrystals monodispersed on primary electrospun carbon nanofibers substrates. Finally, uniform CuS/EC film was obtained through a spray-deposition process. The above proposed mechanism involved in this hydrothermal reaction for the growth of the CuS/EC can be summarized in the following equations.

Figure 7. Nyquist plots of different electrodes.

plots, it could be seen that the electrospun carbon nanofibers counter electrode demonstrated the largest Rct, which was attributed to its relatively lower catalytic activity.39 At the same time, the Pt counter electrode presented a relatively much higher Rct than that of the CuS nanoparticle electrode, which was indicative of its inferior electron transfer kinetics for Sn2− reduction caused by the corrosion of a Pt surface by various S species.40 Moreover, with the electrospun carbon nanofiber supported as the 3-D mat for the CuS nanoparticle, the CuS/ EC counter electrode showed much lower Rct that the counter electrode with bare CuS nanoparticles, suggesting a much enhanced charge transfer in the CuS/EC counter electrode. In the bare CuS nanoparticle electrode, the relatively poor connection between the CuS nanoparticles would slow the electrons flowing from the external circuit to the CuS/ electrolyte interface and, hence, retard the reduction process of the oxidized polysulfide. In this case, the catalytic sites on the CuS nanoparticles may not be fully utilized, resulting into a lower catalytic activity and solar cell performance. As for the CuS/EC composite electrode, all the CuS nanoparticles was grown on the electrospun nanofibers and well connected to each other with the latter as the 3-D framework. The 3-D connected electrospun carbon nanofibers serves as the excellent tunnel for the electron transport. This would reduce the internal resistance of the counter electrode film and ensure the full utilization of the CuS nanoparticles with highly catalytic activity. Hence, the CuS/EC counter electrode solar cell demonstrated a much higher energy conversion efficiency that these of the cells with Pt and bare CuS nanoparticles as the counter electrode. The as-obtained CuS/EC nanocomposite was employed as the counter electrode materials in QDSCs constructed by TiO2/CdS/CdSe as the photoanode and polysulfide as the electrolyte (1 M Na2S and 1 M S in aqueous medium). For comparison, the bare CuS nanoparticles electrode, bare electrospun carbon nanofibers electrode, as well as the traditional Pt electrode were also studied as the counter electrodes. The I−V characteristics for QDSCs with different counter electrodes are presented in Figure 8 and the details are summarized in Table 1. It could be seen that the CuS electrode is a much better candidate than Pt in the QDSCs when polysulfide was used as the electrolyte, as demonstrated by the I−V characteristics. With a short-circuit photocurrent (Jsc) of 12.73 mA cm−2, a open-circuit photovoltage (Voc) of 0.510 V, and a fill factor (FF) of 46.2%, the CuS electrode showed a

Cu 2 + + HOOCCH(NH 2)CH 2SH → Cu+ + HOOCH(NH 2)CH 2S ‐SCH 2CH(NH 2)2 COOH

(1)

Cu+ + HOOCCH(NH 2)CH 2SH → HOOCH(NH 2)CH 2S‐Cu(I)

(2)

HOOCH(NH 2)CH 2S‐Cu(I) + O2 + H 2O → CuS + byproducts

(3)

To investigate the catalytic properties of the various counter electrodes, EIS was performed to study the charge transfer process between the different counter electrodes with polysulfide electrolyte. Symmetrical cells with the sandwich structure of counter electrodes and electrolyte were fabricated for the EIS measurement. Figure 7 shows the Nyquist plots of the different counter electrode and the charge transfer resistance (Rct) can be estimated from the impedance component of the high frequency semicircles.36−38 From the G



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secondary structure of CuS nanoparticles together. This provided the shortcut for the electrons transporting across the 3-D mat to the sulfide active CuS catalyst sites where the oxidized polysulfide was reduced with the electrons,41 as demonstrated in Scheme 1. The facilitated charge transport Scheme 1. Depiction of the Superiority of the QDSC with CuS/EC as the Counter Electrode

Figure 8. I−V characteristics for QDSCs with different materials as the counter electrode.

Table 1. Details of the I−V Parameters of the QDSCs with Different Materials as the Counter Electrode electrodes

Voc (mV)

Jsc (mA cm−2)

FF (%)

η (%)

EC Pt CuS CuS/EC

472 481 510 521

8.10 9.92 12.73 14.60

42.5 44.1 46.2 50.7

1.62 2.10 3.00 3.86

process provided by the CuS/EC heteroarchitecture would reduce the charge transfer resistance (Rct), retard the interfacial recombination, and then enhance the overall efficiency of the solar cells.39

much higher efficiency of 3% than that of 2.1% of Pt electrode. This is due to the fact that the sulfides (S2−, Sx2−) adsorb onto Pt surface and impair the electrocatalytic activity of the latter, which makes the CuS a better counter electrode material than Pt.15,17 Moreover, with the CuS/EC composite as the counter electrode, the efficiency of the QDSC was further enhanced to the best energy conversion efficiency of 3.86% under AM1.5 illumination (100 mW cm−2), with a short-circuit photocurrent (Jsc) of 12.73 mA cm−2, a open-circuit photovoltage (Voc) of 0.521 V, and a fill factor (FF) of 50.7%. Furthermore, we have preliminarily evaluated the stability of the QDSC constructed by CuS/EC counter electrode, which was tested in dark under room temperature. It is found that the QDSC did not show significant degradation and retains 90% of its initial value after 10 days, indicating high stability (see Figure S3 in the Supporting Information). Further stability tests, such as light soaking and relatively high temperature test, will be clarified in the future. It should be noted that the QDSC with bare electrospun carbon nanofibers as the counter electrode showed relatively low efficiency of 1.62%, which was mainly due to its relatively lower catalytic activity.39 This demonstrates that the high performance of the CuS/EC nanocomposite is the result of the combination of the two materials. First, the CuS nanoparticles served as the catalyst to reduce the oxidized polysulfide electrolyte. With the numerous nucleation sites provided by the 3-D nanostructured carbon nanofibers, the CuS nanoparticles were uniformly grown and monodispersed on the surface of the former without any agglomeration. This would increase the contact sites of the CuS nanoparticles with the polysulfide electrolyte and make good use of the as-grown CuS nanoparticles. In other words, the existence of the electrospun carbon nanofibers with 3-D nanostructure promoted high numbers of the CuS reactive sites spread across the surface of the 3-D nanofiber mat. Furthermore, the carbon nanofiber functioned as the primary framework structure bridging all the

4. CONCLUSION In conclusion, the CuS nanoparticles were successfully controlled grown on the 3-D connected electrospun carbon nanofibers. The as-obtained CuS−carbon nanofiber composite was utilized as the efficient counter electrode material in the quantum dot-sensitized solar cells. With the good catalytic activity of the CuS nanoparticles as well as the good charge transport provided by the 3-D nanofiber framework, the composite demonstrated a superior performance of 3.86%, which is much higher the traditional Pt counter electrode and the bare CuS nanoparticle electrode. The presented experimental procedure for the novel CuS/EC heterostructures is quite simple, environmentally benign, and cost-effective, permitting it to be potential application in the future QDSCs.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image of the sample obtained by using TAA as the sulfur source; FT-IR spectra of CuS/EC composite and stability measurement of the QDSC constructed by the CuS/EC counter electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected] Author Contributions †

These authors contributed equally (L.L. and P.Z.).

Notes

The authors declare no competing financial interest. H



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ACKNOWLEDGMENTS The authors would like to thank for the financial support of Singapore-Berkeley Research Initiative for Sustainable Energy (Grant No.: R-279-000-393-592).

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