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cobalt sulfide core shell nanosheets as counter electrodes


Solar Energy Materials & Solar Cells 95 (2011) 2867–2873

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Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat

Preparation of highly electroactive cobalt sul?de core–shell nanosheets as counter electrodes for CdZnSSe nanostructure-sensitized solar cells
Zusing Yang, Chia-Ying Chen, Huan-Tsung Chang n
Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan

a r t i c l e i n f o
Article history: Received 31 December 2010 Received in revised form 30 May 2011 Accepted 4 June 2011 Available online 28 June 2011 Keywords: Ostwald ripening Counter electrode Solar cells Nanomaterials Electrocatalytic activity

abstract
A facile method for the synthesis of Co3S4 core–shell hexagonal nanosheets (NSs) from Co(NO3)2 and thioacetamide under alkaline conditions in the presence of poly(vinylpyrrolidone) has been demonstrated. At the molar ratios of thioacetamide/Co2 ? of 0.26, 0.52, and 2.6, we prepared hollow-, semihollow, and non-hollow Co3S4 core–shell hexagonal NSs. We have found that Ostwald ripening occurring at the core/shell interface accounts for the formation of the hollow Co3S4 core–shell NSs, each consisting of a core of 80 7 30 nm in diameter and a shell of 257 5 nm in thickness. Three CdZnSSe nanostructure-sensitized solar cells incorporating Co3S4 NSs provide an average power conversion ef?ciency of 3.7 7 0.1%, showing high electrocatalytic activity of the Co3S4 NSs toward polysul?de electrolyte. & 2011 Elsevier B.V. All rights reserved.

1. Introduction Preparation of highly electroactive nanomaterials is important in the fabrication of energy storage and conversion devices, such as fuel cells, lithium ions batteries, solar cells, and so on [1–4]. Having large surface area and extremely high catalytic ef?ciency, metallic and semiconductive nanomaterials have become two of the most promising materials for fabrication of ef?cient electrodes. Among them, Pt [5], Ru [6], Pd [7], cobalt oxide [8], and cobalt sul?de [9] are more popular, mainly because of their high electroactivity, ease in preparation, high chemical stability, and large surface-to-volume ratios. The electroactivity of nanomaterials can be enhanced by carefully controlling their structures and compositions. Special nanostructures such as nanosheets (NSs), nanosponges, nanodendrites, and nano?owers are fantastic because of their great electroactivity. Co3O4 NSs relative to their corresponding nanobelts and nanocubes possess a superior activity on the catalytic combustion of methane. Relative to Pt spherical nanoparticles (NPs), Pt nanodendrites, nanosponges, and nanonetworks, and Pt hollow NPs all have higher electrocatalytic activity for the oxidization of methanol [10]. Cobalt sul?de, an important magnetic semiconductor, has been employed as catalysts for oxygen reduction reaction [9], hydrosulfurization [11], used in lithium ions batteries [12], and electrochemical capacitors [13]. Carbon-supported Co3S4 NPs that

were prepared through microwave heating had an excellent electrocatalytic activity for the oxygen reduction reaction [9]. Hollow Co3S4 NPs and nanotubes were prepared by taking advantages of different diffusion rates between cobalt source and hydrogen sul?de [14]. High speci?c capacitance of low-cost CoS nanowires were synthesized through a biomolecule-assisted hydrothermal route [15]. Their use as electroactive electrodes in nanostructure-sensitized solar cells (NSSCs) has not been widely recognized. Very recently, we prepared irregular CoS microstructures (8–80 mm in size) and used them as counter electrodes in CdS/CdSe quantum dot-sensitized solar cells (QDSSCs) that provided power conversion ef?ciency (Z) of 3.4% [16]. In this study, we prepared hollow Co3S4 core–shell hexagonal NSs from Co(OH)2 and thioacetamide using poly(vinylpyrrolidone) (PVP) as stabilizers under alkaline conditions through a hydrothermal process. The as-prepared hollow Co3S4 core–shell NSs were used to fabricate stable and ef?cient counter electrodes in CdZnSSe NSSCs. Three NSSCs each featuring a hollow Co3S4 core–shell electrode provided an average Z of 3.7 70.1%. 2. Material and methods 2.1. Chemicals Commercially available cadmium nitrate, cobalt nitrate, ethylene glycol, hydrogen hexachloroplatinate(IV), thioacetamide, methanol, methyl cellulose, P-25 TiO2 NPs (Degussa), poly(vinylpyrrolidone) (PVP, Mw 55,000), polyethylene glycol (Mw 5000),

n

Corresponding author. Tel./fax: ? 11 886 2 33661171. E-mail address: changht@ntu.edu.tw (H.-T. Chang).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.06.002

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potassium chloride, selenium powder, sodium sul?de, sodium hydroxide, sulfur, and zinc sulfate were purchased from SigmaAldrich (Milwaukee, WI, USA). 2.2. Preparation of Co3S4 core–shell NSs Co(NO3)2 (0.58 g), thioacetamide (0.15 g), and PVP (0.07 g) were added to ultrapure H2O (?nal volume 50 mL) in a glass bottle. After addition of 0.5N NaOH (12 mL) to the mixture, Co(OH)2 immediately formed. The mixture was then heated at 100 1C for 60 min, in which thioacetamide was decomposed to form S2 ? that interacted with Co(OH)2. The instant color changed from aquamarine to black, indicating the formation of Co3S4 NSs. We also conducted similar syntheses using different amounts of Co(NO3)2 and thioacetamide. The resulting Co3S4 NSs (0.25 mL) were isolated from the solution by conducting two centrifugation-wash cycles (10,000 rpm for 10 min; 1.25-mL ethanol for washing). The Co3S4 NSs were then dispersed in ethanol solution (1.0 mL) prior to characterization and use for fabrication of electroactive electrodes. 2.3. Characterization of Co3S4 NSs A double-beam ultraviolet–visible (UV–vis) spectrophotometer (Cintra 10e) from GBC Scienti?c Equipment (Dandenong, Victoria, Australia) was used to measure the absorption spectra of the as-prepared electrodes. Transmission electron microscopes (TEM) of JEOL JSM-1230 (Hitachi, Tokyo, Japan) and high-resolution transmission electron microscope (HRTEM) of FEI Tecnai-G2F20 (GCEMarket, NJ, USA) were used to measure the sizes and shapes of the as-prepared NSs. An energy-dispersive X-ray (EDX) system of Inca Energy 200 (Oxford, Oxfordshire, UK) was used to determine the composition of the prepared NSs. For X-ray diffraction (XRD) measurements, an X’Pert PRO diffractometer (PANalytical, Almelo, Netherlands) and Cu Ka radiation (l ?0.15418 nm) were used; the samples were prepared on glass substrates. 2.4. Preparation of TiO2 ?lms Porous TiO2 ?lms were fabricated from P-25 TiO2 NPs (ca. 20 nm in diameter). First, HNO3 was adsorbed on the TiO2 surface ?TiO2 =NO? ? under heating (80 1C for 8 h) [17,18]. A coating phase 3 was prepared by mixing ?TiO2 =NO? ? (0.6 g), polyethylene glycol 3 (0.18 g), and cellulosic polymer (0.06 g) in 3 mL of ultrapure H2O. A TiO2 ?lm was cast by spreading the TiO2 paste onto transparent ?uorine-doped tin oxide (FTO) glass substrate having a resistance 14 O sq ? 1. The electrode with air dried TiO2 ?lm was then annealed at 450 1C for 30 min. The thickness of TiO2 ?lm was ca. 20 mm [16,18]. 2.5. Preparation of Co3S4 electrodes Three types of the as-prepared core–shell Co3S4 NSs that were dispersed in ethanol solution and FTO glass substrates were used to prepare Co3S4 electrodes as counter electrodes. A representative Co3S4 electrode was prepared by adding one drop (ca. 0.1 mL) of one of the as-prepared core–shell Co3S4 NSs onto the surface of the FTO glass substrate. The solution was then dried at 100 1C for 10 min to obtain a Co3S4 NS counter electrode. 2.6. Preparation of CdZnSSe photoelectrodes. Chemical bath deposition (CBD) was applied to assemble CdZnSSe QDs into the TiO2 coated FTO substrates, based on our previous approach [19]. Each of the substrates was dipped into a

0.5 M Cd(NO3)2 aqueous solution for 5 min, rinsed with ultrapure H2O, dried with an air gun. The treated substrate was dipped for another 5 min into a 0.5 M Na2S aqueous solution, rinsed with ultrapure H2O, and dried with an air gun. The two-step dipping procedure is denoted as one CBD cycle. The process was repeated up to 3 cycles. The as-prepared electrodes are represented as TiO2–(CdS)3 electrodes. The TiO2–(CdS)3 electrodes were then dipped into a solution of Cd2 ? (0.5 M) and Zn2 ? (0.75 M) for 5 min and then immersed into the 0.08 M Na2SeSO3 aqueous solution for 1 h at 50 1C. The process was repeated up to 2 cycles. The as-prepared electrodes are represented as TiO2–CdZnSSe electrodes. The TiO2–CdZnSSe electrodes were immersed into a solution of Zn(NO3)2 (0.5 M) for 5 min, rinsed with ultrapure H2O, and dried with air gun. They were then dipped for 5 min into a 0.5 M Na2S aqueous solution, and followed by another rinsing with ultrapure H2O and drying with an air gun to obtain TiO2–CdZnSSe–ZnS electrodes.

2.7. Assembly of CdZnSSe NSSCs The CdZnSSe NSSCs (active area of 1 cm2) were assembled according to the following procedure. A CdZnSSe photoelectrode and a Co3S4 NS counter electrode were sandwiched between a 20-mm-thick hot-melt ionomer ?lm (Surlyn) under heating (100 1C for 30 min). The electrolyte consisting of 2.0 M Na2S, 0.5 M S, and 0.2 M KCl in 7:3 methanol/ultrapure H2O (in volume ratio) was used in the as-prepared CdZnSSe NSSCs.

2.8. Measurements The irradiation source for photocurrent density–voltage characteristics (J–V) was from a 450 W xenon arc lamp from Oriel (Stratford, CT, USA) through an AM 1.5 ?lter. A Keithley 2400 digital source meter from Test Equipment Connection (Lake Mary, FL, USA) was operated to record the J–V curves of the as-prepared CdZnSSe NSSCs. A metal mask with an area of 0.126 cm2 was placed in front of the NSSCs to con?ne their effective illumination area. A commercial available silicon based reference cell (Oriel, Stratford, CT) was employed to con?rm the light intensity (100 mA/cm2). Triplicate measurements were conducted in this study. All of the J–V measurements were recorded for at least 10 reproducible cycles. The incident-photon-to-current ef?ciency (IPCE) spectra were recorded using a PEC-S20 instrument (Peccell Technologies, Kanagawa, Japan). Potentiostatic current–potential (I–V) curves were recorded using a CHI 660A electrochemical analyzer (CH Instruments, Austin, TX). We note that I–V and J–V used in the electrochemical measurements without and with being under sun light illumination, respectively. I–V measurements were performed using a three-electrode system: a Co3S4 NS electrode as a working electrode, a Pt counter electrode, and an Ag/AgCl reference electrode. The electrolyte is methanol/ultrapure H2O (v/v, 7/3) solution containing 2 M Na2S, 0.5 M S, and 0.2 M KCl. A CHI 614D electrochemical impedance spectrometer (CH Instruments, Austin, TX) was employed to determine the charge transfer resistance (Rct) between the electrode and the electrolyte at zero bias potential over the frequency range 0.1–100 kHz. By ?tting the plots with a Labview software, the values of Rct (the radius of the semicircle in the frequency range 0.1–100 KHz) were estimated [16]. Symmetric cells of Pt/Pt and of Co3S4 NS/Co3S4 NS incorporating the polysul?de redox electrolyte were fabricated separately for the impedance measurements. Each cell had an effective area of 1 cm2.

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3. Results and discussion 3.1. Synthesis and characterization of Co3S4 core–shell NSs To prepare Co3S4 core–shell NSs from Co(NO3)2 and thioacetamide, it is extremely important to form a precursor of Co(OH)2 (solubility product (Ksp) ?2 ? 10 ? 14) [20]. b-Co(OH)2 with a uniform hexagonal shape is a common precursor for preparation of two-dimensional (2D) Co3O4 nanostructures including NSs, nanowalls, and nanoplates [21,22]. To prepare stable b-Co(OH)2, we added NaOH to Co2 ? aqueous solution (96.8 mM, ?nal pH value $ 12.3) in the presence of PVP. The as-prepared b-Co(OH)2 precursors then reacted with S2 ? ions that was produced from thermal decomposition of thioacetamide (32.3 mM) at 100 1C to form hollow core–shell NSs as shown in Fig. 1A. There were no products or precipitates formed if NaOH was not added, revealing the important role of Co(OH)2 precursors in the formation of the hollow core–shell NSs. The average edge length of the hollow NSs estimated from 500 counts to be ca. 80 720 nm. The diameter of the core estimated from 500 counts to be ca. 80 730 nm and the shell thickness was 25 75 nm. In each NS, there is a space with a distance of o5 nm between the core and shell. The EDX spectroscopic analysis reveals the existence of cobalt and sulfur (Fig. 1B) in the as-prepared NSs. The diffraction peaks of XRD patterns further con?rm the formation of Co3S4 NSs (Fig. 1C). The highmagnitude TEM image of an individual Co3S4 NSs depicted in the inset to Fig. 1A clearly reveal that the core of each Co3S4 NS consists of many small spherical NPs (average 5 nm in diameter) that grew in the same direction (Fig. 1D). Similarly, Fig. 1E displays that the shell of each Co3S4 NS consists of many small NPs (average 5 nm in diameter). Detailed structure analyses from d-spacing and Fourier transform (FFT) pattern present the set of (2 2 0) planes with a lattice space of 0.33 nm and the set of (3 1 1) planes each with a lattice space of 0.28 nm in the shell of the hollow Co3S4 NSs [21,22]. Fig. 2 displays time evolution of the formation of the hollow Co3S4 core–shell hexagonal NSs. At 5 min

of reaction time, core–shell hexagonal structures of the Co3S4 NSs already became apparent (Fig. 2A). The growth directions in the core and shell of each Co3S4 NS were the same and all of the NPs exhibited lattice spacing of 0.28 nm that belongs to (3 1 1) planes of Co3S4 (Fig. 2B). The Co3S4 core–shell hexagonal NSs in this stage were assembly from the irregular NPs through the heterogeneous

Fig. 2. (A) and (C) TEM images and (B) and (D) HRTEM images of Co3S4 hexagonal core–shell NSs prepared at the reaction times of 5 and 30 min, respectively. Insets of (A) and (C) are their high-magnitude TEM for individual NS. Insets of (B) and (D) are their corresponding FFT patterns.

Fig. 1. (A) TEM image, (B) EDX spectrum, and (C) XRD pattern of Co3S4 hexagonal core–shell NSs. Inset to (A): high-magnitude TEM image of an individual hollow Co3S4 hexagonal core–shell NS. (D) and (E) HRTEM image of the core (square) and shell (circle) of the NS and their corresponding FFT patterns.

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nucleation and interparticle coursing process [23,24]. The formation of core–shell structure is because of their difference in crystal packing, with the same growth direction. As the reaction time continued to 25 min, Co3S4 core–shell hexagonal NSs were still dominant. After a reaction course of 30 min, hollow structures started to form (Fig. 2C). Ostwald ripening started to occur at the core/shell interface, leading to the formation of the hollow structure. This inward hollowing phenomenon was attributed to the existence of intrinsic crystal packing balance at the core/shell interface. Apparently, the shell crystallites were loosely packed with smaller Co3S4 NPs (o5 nm) that served as nucleation seeds for the progressive shell growth though recrystallization process. Fig. 2D displays that the shell presented the set of (2 2 0) planes with a lattice space of 0.33 nm and the set of (3 1 1) planes with a lattice space of 0.28 nm, and the set of (1 1 1) planes with a lattice space of 0.54 nm, implying that the shell underwent a continued surface growth through the aggregation of smaller NPs in

different orientations. The shell continued to grow upon increasing reaction time; 7–15 and 20–30 nm in thickness at 30 and 60 min, respectively. In contrary, the edge length of the Co3S4 NSs slightly decreased upon increasing reaction time; 90 720 and 80720 nm at 30 and 60 min, respectively. The results revealed the occurrence of Ostwald ripening at the core/shell interface. By controlling the inward hollowing process, we prepared non-hollow and semi-hollow Co3S4 core–shell hexagonal NSs. Non-hollow Co3S4 hexagonal NSs each with a shell width of 7–10 nm (Fig. 3A) were prepared in solutions containing Co(NO3)2 (16.1 mM) and a higher concentration (80.5 mM) of thioacetamide. The average edge length of the non-hollow hexagonal NSs from 500 counts was estimated to be ca. 50710 nm. The observation of two lattice spaces of 0.28 and 0.46 nm (Fig. 3B) reveals that different crystallite orientations suppressed the occurrence of inward hollowing process. By decreasing the concentration of Co(NO3)2 (from 32.2 to 8.1 mM) at a constant thioacetamide concentration (32.2 mM), recrystallization occurring at the core/shell interface decreased. Recrystallization of the shell contributed to a continued surface growth that led to the formation of voids. As a result, semi-hollow Co3S4 core–shell hexagonal NSs formed (Fig. 3C). The shell was 1674 nm in thickness that was thinner than that of the hollow Co3S4 core– shell hexagonal NSs (Fig. 1A), providing the evidence for decreased recrystallization. The average edge length of the semi-hollow hexagonal NSs from 500 counts was estimated to be ca. 110730 nm. The fact that two sets of (3 1 1) planes with a lattice space of 0.28 nm shows that the hollow and semi-hollow Co3S4 core–shell hexagonal NSs had the same orientation (Fig. 3D). To summarize the effect of the concentrations of Co(NO3)2 and thioacetamide on the formation of non-hollow, semi-hollow, and hollow Co3S4 NSs, Scheme 1 is depicted. The non-hollow, semi-hollow, and hollow Co3S4 NSs were preferably formed at the molar ratios of thioacetamide/Co2 ? of 2.6, 0.52, and 0.26, respectively.

3.2. Electrochemical characteristics of Co3S4 NS electrodes To further show the capability of Co3S4 NS electrodes, we determined their adhesion on the surface of each FTO glass substrate by conducting a sonication test [25]. If the catalysts cannot stick to the counter electrode, there will have a great obstacle for the electron transport at electrolyte/counter electrode interfaces [26]. We observed that the hollow Co3S4 core–shell hexagonal NSs had high adhesion (mechanical stability)

Fig. 3. (A) and (B) TEM image of non-hollow and semi-hollow Co3S4 hexagonal NSs, and (C) and (D) their corresponding HRTEM images. Insets of (A) and (B) are their high-magnitude TEM for individual NS. Insets of (C) and (D) are their corresponding FFT patterns.

Scheme 1. Preparations of hollow, semi-hollow, and non-hollow Co3S4 hexagonal NSs.

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Fig. 4. (A) SEM image of the hollow Co3S4 core–shell NSs deposited onto an FTO glass substrate. Inset: the corresponding cross-sectional SEM image. (B) UV–vis spectra of the hollow Co3S4 core–shell NSs deposited onto an FTO glass substrate (a) before and (b) after sonication for 5 min. (C) Chemical stability of the hollow Co3S4 hexagonal core–shell NS electrocatalysts over 100 cycles in a polysul?de redox system. (D) Linear sweep voltammetry measurements in a three-electrode electrochemical cell system using the as-prepared Co3S4 electrode as a working electrode, an Ag/AgCl electrode as a reference, and a Pt wire as an auxiliary electrode. The electrolyte is methanol/ ultrapure H2O (v/v, 7/3) solution containing 2 M Na2S, 0.5 M S, and 0.2 M KCl.

on the FTO glass through a simple treatment (Fig. 4A). PVP that were existent in the hollow Co3S4 core–shell hexagonal NSs are cations under the preparation condition, thus the adhesion of Co3S4 onto FTO glass substrate was mainly through electrostatic, hydrogen, and hydrophobic forces. The thickness of Co3S4 ?lm was o200 nm from cross-sectional SEM measurements (Fig. 4A, inset). The absorption values of the hollow Co3S4 core–shell hexagonal NSs on the FTO substrate slightly decreased after 5-min sonication (Fig. 4B), revealing that desorption of the outmost layer of the NSs was not signi?cant. We then evaluated the electrocatalytic durability and electrocatalytic ability of the hollow Co3S4 core–shell hexagonal NSs in the electrochemical cells using polysul?de as electrolyte [26,27]. The redox reactions occurred at the photoelectrode–electrolyte interface as the following equations [27]: S2? ? 2h -S
?

?1? ?x ? 225? ?2?

S ?S2? -S2? x?1 x

During regeneration in the electrolyte, the oxidized species, S2? , was converted back to S2 ? on the counter electrode x S2? ? 2e? -S2? ?S2? x x?1 ?3?

The decrease in the current density of the electrode was only about 11.3% after 100 cycles (Fig. 4C), revealing that the electrode was considerably stable in the polysul?de solution and thus suitable for long-term use. Cyclic voltammetry was carried out to investigate the electrocatalytic activity of the Co3S4 electrode for the reduction of S2? ions to S2 ? ions in the polysul?de x

Fig. 5. SEM images of (A) TiO2 and (B) CdZnSSe photoelectrodes. (C) Crosssectional SEM image of CdZnSSe photoelectrodes and (D) EDX spectrum of CdZnSSe photoelectrodes.

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electrolyte (Fig. 4D). The greater the current density, the higher the electrocatalytic activity of the electrode [16,26,27]. The electrocatalytic activities of the hollow, semi-hollow, and nonhollow Co3S4 NSs were close ( ? 1.4 $ ? 1.6 mA cm ? 2 at ? 0.8 V), which were higher than those ( ? 12 $ ? 6 mA cm ? 2) of reported Au NPs electrodes [28]. The result shows that the hollow Co3S4 core–shell hexagonal NSs had high electrocatalytic activity for polysul?de reduction. 3.3. Nanostructure-sensitized solar cells The energy-dispersive X-ray (EDX) spectroscopic data depicted in Fig. 5 reveal the formation of CdZnSSe on the TiO2 substrate. The scanning electron microscopy (SEM) image (Fig. 5C) displays that the length and width of the CdZnSSe nanorods were about
Table 1 Comparison of CdZnSSe NSSCs featuring different counter electrodes under one sun illumination. Counter electrode Hollow Co3S4 NSs Semi-hollow Co3S4 NSs Non-hollow Co3S4 NSs Pt NPs VOC (V) 0.520 0.505 0.515 0.530 JSC (mA cm ? 2) 13.15 13.39 12.24 12.72 FF (%) 54.4 54.8 58.4 47.7

Z (%)
3.72 3.70 3.67 3.22

Fig. 6. Nyquist (impedance) plots of (A) hollow (B) semi-hollow, and (C) nonhollow Co3S4 NSs-catalyzed; (D) Pt-catalyzed FTO symmetric cells containing polysul?de redox electrolyte. Device area: 1 cm2. Z0 and Z00 are the virtual and real impedances, respectively.

8007300 and 90 710 nm, respectively. The thickness of CdZnSSe on the TiO2 substrate is ca. 2–3 mm. The CdZnSSe photoelectrodes and the three types of Co3S4 NSs were further employed to fabricate CdZnSSe NSSCs. The detailed device structure of as-fabricated NSSCs is depicted in Fig. S1. Table 1 summarized the photovoltaic performances of as-prepared NSSCs. The CdZnSSe NSSCs provided the values of Z of 3.72, 3.70, 3.67, and 3.22%, respectively; open-circuit voltage (VOC) of 0.520, 0.505, 0.515, 0.515, and 0.530 V, respectively; short-circuit current density (JSC) of 13.15, 13.39, 12.24, and 12.72 mA cm ? 2, respectively; and ?ll factor (FF) of 54.4%, 54.8%, 58.4%, and 47.7%, respectively. We note that the Pt electrodes were unstable mainly because their surface activity and conductivity were suppressed as a result of adsorption of the sulfur atoms [29], leading to smaller FF valve ( o50%) that is in an agreement with most literatures [16,27,29]. The CdZnSSe NSSC featuring three different Co3S4 hexagonal NSs as counter electrodes all provided higher Z than that of featuring a Pt electrode, mainly because of their higher FF (54–58% vs. 47.7%). Notably, the NSSC having a higher FF provides higher electrocatalytic activity. Three QDSSCs each featuring a hollow Co3S4 core–shell hexagonal NS electrode provided an average Z of 3.770.1%. After 6 h, the ef?ciency of each of NSCCs remained almost constant (only slightly decreased to 3.0 70.2%) (Fig. S2), demonstrating high stability of NSSCs toward polysul?de electrolyte. CdZnSSe NSSCs featuring irregular microsized CoS electrodes only provided a Z value of 2.1%, with a low value of FF (38.7%) (Fig. S3). NSSCs featuring with a FTO substrate did not work due to poor catalytic activity of FTO substrate toward electrolyte. In order to con?rm that the electrocatalytic activities of Co3S4 hexagonal NSs indeed had great in?uence on the performance of the QDSSCs, Rct values at the counter electrode/electrolyte interfaces were measured (Fig. 6). Because symmetric cells of Pt/Pt and of Co3S4 NS/Co3S4 NS incorporating the polysul?de redox electrolyte were employed for the impedance measurements, values of 2Rct were estimated from the half of the real component of the impedance [30,31]. According to the literatures, an electrode having a high Rct value provides a low FF value [4]. The hollow, semi-hollow, and nonhollow Co3S4 hexagonal NSs possessed lower Rct values (26.0, 81.8, and 23.5 O cm2) than the Pt electrodes (2465 O cm2) did [32]. As a result, they all provided higher FF values than the Pt electrodes did [4]. The photovoltaic characteristics of a representative CdZnSSe QDSSC featuring a hollow Co3S4 hexagonal NSs electrode were investigated. The detailed J–V curves of photovoltaic performances of as-prepared NSSCs are given in Fig. 7A. The IPCE spectrum (Fig. 7B) displays broad photoresponse over the wavelength range 400–700 nm, which is consistent with the absorption spectrum

Fig. 7. (A) J–V curves of CdZnSSe QDSSCs featuring (a) hollow (b) semi-hollow, and (c) non-hollow Co3S4 NSs and (d) Pt electrodes under one sun illumination or under dark conditions. (B) The IPCE spectrum of a representative CdZnSSe NSSC featuring a hollow Co3S4 NSs electrode. Inset in (B) is the corresponding absorption spectrum.

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(Fig. 7B, inset). A maximum IPCE value of 69% was obtained at 500 nm. A calculated JSC value (12.38 mA cm ? 2) from the IPCE spectrum was in excellent agreement with that obtained from the J–V measurement (13.15 mA cm ? 2) [33].

4. Conclusions Hollow, semi-hollow, and non-hollow Co3S4 core–shell NSs were prepared from Co(NO3)2 and thioacetamide at different thioacetamide/Co2 ? molar ratios—0.26, 0.52, and 2.6, respectively–in the presence of PVP. The Co3S4 core–shell NSs formed through assembly of small spherical Co3S4 NPs. Each of the hollow Co3S4 core–shell NSs had a hollow space between core and shell due to Ostwald ripening at the core/shell interface. The as-prepared Co3S4 core–shell NSs were employed to fabricate counter electrodes for CdZnSSe NSSCs. The CdZnSSe NSSCs featuring the three different Co3S4 NSs all provided the Z values higher than 3.6%. Because of their low cost, ease in preparation, stability, and high electrocatalytic activity, the Co3S4 NSs hold great potential for fabrication of many electroactive devices such as photovoltaic cells and QDSSCs.

Acknowledgments We thank the National Science Council, Taiwan, for ?nancial support (NSC 99-2627-M-002-016 and 99-2627-M-002-017). Z.Y. thanks the National Taiwan University for the award of a postdoctoral fellowship in the Department of Chemistry, National Taiwan University.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2011.06.002.

References
 [1] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366–377. [2] S. Liao, K.-A. Holmes, H. Tsaprailis, V.I. Birss, High performance PtRuIr catalysts supported on carbon nanotubes for the anodic oxidation of methanol, J. Am. Chem. Soc. 128 (2006) 3504–3505. [3] X. Gao, H. Zhu, G. Pan, S. Ye, Y. Lan, F. Wu, D. Song, Preparation and electrochemical characterization of anatase nanorods for lithium-inserting electrode material, J. Phys. Chem. B 108 (2004) 2868–2872. [4] X. Fang, T. Ma, G. Guan, M. Akiyama, E. Abe, Performances characteristics of dye-sensitized solar cells based on counter electrodes with Pt ?lms of different thickness, J. Photochem. Photobiol. A 164 (2004) 179–182. [5] M. Zhao, R.M. Crooks, Homogeneous hydrogenation catalysis with monodisperse, dendrimer-encapsulated Pd and Pt nanoparticles, Angew. Chem. Int. Ed. 38 (1999) 364–366. [6] Z. Song, T. Cai, J.C. Hanson, J.A. Rodriguez, J. Hrbek, Structure and reactivity of Ru nanoparticles supported on modi?ed graphite surface: a study of the model catalysts for ammonia synthesis, J. Am. Chem. Soc. 126 (2004) 8576–8584. [7] J.C. Garcia-Martinez, R. Lezutekong, R.M. Crooks, Dendrimer-encapsulated Pd nanoparticles as aqueous, room-temperature catalysts for the Stille reaction, J. Am. Chem. Soc. 127 (2005) 5097–5103.

[8] L. Hu, Q. Peng, Y. Li, Selective synthesis of Co3O4 nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion, J. Am. Chem. Soc. 130 (2008) 16136–16137. [9] Y. Feng, T. He, N. Alonso-Vante, In situ free-surfactant synthesis and ORRelectrochemistry of carbon-supported Co3S4 and CoSe2 nanoparticles, Chem. Mater. 20 (2008) 26–28. [10] Z.-H. Lin, M.-H. Lin, H.-T. Chang, Facile synthesis of catalytically active platinum nanosponges, nanonetworks, and nanodendrites, Chem. Eur. J. 15 (2009) 4656–4662. [11] T.A. Pecoraro, R.R. Chianelli, Hydrodesulfurization catalysis by transition metal sul?des, J. Catal. 67 (1981) 430–445. [12] J. Wang, S.H. Ng, G.X. Wang, J. Chen, L. Zhao, Y. Chen, H.K. Liu, Synthesis and characterization of nanosize cobalt sul?de for rechargeable lithium batteries, J. Power. Sources 159 (2006) 287–290. [13] F. Tao, Y.-Q. Zhao, G.-Q. Zhang, H.-L. Li, Electrochemical characterization on cobalt sul?de for electrochemical supercapacitors, Electrochem. Commun. 9 (2007) 1282–1287. [14] Y. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Formation of hollow nanocrystals through the nanoscale Kirkendall effect, Science 304 (2004) 711–714. [15] S.-J. Bao, C.M. Li, C.-X. Guo, Y. Qiao, Biomolecule-assisted synthesis of cobalt sul?de nanowires for application in supercapacitors, J. Power. Sources 180 (2008) 676–681. [16] Z. Yang, C.-Y. Chen, C.-W. Liu, H.-T. Chang, Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells, Chem. Commun. 46 (2010) 5485–5487. [17] S. Ito, T. Kitamura, Y. Wada, S. Yanagida, Facile fabrication of mesoporous TiO2 electrodes for dye solar cells: chemical modi?cation and repetitive coating, Sol. Energy Mater. Sol. Cells 76 (2003) 3–13. [18] Z. Yang, H.-T. Chang, CdHgTe and CdTe quantum dot-cosensitized solar cells displaying an energy conversion ef?ciency exceeding 2%, Sol. Energy Mater. Sol. Cells 94 (2010) 2046–2051. [19] Z. Yang, C.-Y. Chen, P. Roy, H.-T. Chang, Quantum dot-sensitized solar cells incorporating nanomaterials, Chem. Commun., in press, doi:10.1039/ c1cc11317h. [20] Z. Yang, C.-Y. Chen, H.-T. Chang, Supercapacitors incorporating hollow cobalt sul?de hexagonal nanosheets, J. Power Source 196 (2011) 7874–7877. [21] S.-J. Bao, Y. Li, C.M. Li, Q. Bao, Q. Lu, J. Guo, Shape evolution and magnetic properties of cobalt sul?de, Cryst. Growth Des. 8 (2008) 3745–3749. [22] Y. Yin, C.K. Erdonmez, A. Cabot, S. Hughes, A.P. Alivisatos, Colloidal synthesis of hollow cobalt sul?de nanocrystals, Adv. Funct. Mater. 16 (2006) 1389–1399. [23] B. Liu, H.C. Zeng, Symmetric and asymmetric ostwald ripening in the fabrication of homogeneous core–shell semiconductors, Small 1 (2005) 566–571. [24] T. He, D. Chen, X. Jiao, Y. Wang, Co3O4 nanoboxes: surfactant-templated fabrication and microstructure characterization, Adv. Mater. 18 (2006) 1078–1082. [25] F. Pichot, J.R. Pitts, B.A. Gregg, Low-temperature sintering of TiO2 colloids: application to ?exible dye-sensitized solar cells, Langmuir 16 (2000) 5626–5630. [26] G. Hodes, J. Manassen, D. Cahen, Electrocatalytic electrodes for the polysul?de redox system, J. Electrochem. Soc. 127 (1980) 544–549. [27] Z. Yang, C.-Y. Chen, C.-W. Liu, C.-L. Li, H.-T. Chang, Quantum dot-sensitized solar cells featuring CuS/CoS electrodes provide 4.1% ef?ciency, Adv. Energy Mater. 1 (2011) 259–264. [28] T. Kiyonaga, T. Akita, H. Tada, Au nanoparticle electrocatalysis in a photoelectrochemical solar cell using CdS quantum dot-sensitized TiO2 photoelectrodes, Chem. Commun. 15 (2009) 2011–2013. [29] Y.-L. Lee, Y.-S. Lo, Highly ef?cient quantum-dot-sensitized solar cell based on co-sensitization of CdS/CdSe, Adv. Funct. Mater. 19 (2009) 604–609. ¨ [30] N. Papageorgiou, W.F. Maier, M. Gratzel, An iodine/triiodide reduction electrocatalyst for aqueous and organic media, J. Electrochem. Soc. 144 (1997) 876–884. [31] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells, Electrochim. Acta 46 (2001) 3457–3466. [32] E. Ramasamy, W.J. Lee, D.Y. Lee, J.S. Song, Spray coated multi-wall carbon nanotube counter electrode for tri-iodide ?I? ? reduction in dye-sensitized 3 solar cells, Electrochem. Commun. 10 (2008) 1087–1089. [33] W. Kubo, A. Sakamoto, T. Kitamura, Y. Wada, S. Yanagida, Dye-sensitized solar cells: improvement of spectral response by tandem structure, J. Photochem. Photobiol. A 164 (2004) 33–39.


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