- Nano Express
- Open Access
Low-Temperature Thermally Reduced Molybdenum Disulfide as a Pt-Free Counter Electrode for Dye-Sensitized Solar Cells
© Lin et al. 2015
- Received: 27 August 2015
- Accepted: 11 November 2015
- Published: 17 November 2015
A two-dimensional nanostructure of molybdenum disulfide (MoS2) thin film exposed layered nanosheet was prepared by a low-temperature thermally reduced (TR) method on a fluorine-doped tin oxide (FTO) glass substrate as a platinum (Pt)-free and highly electrocatalytic counter electrode (CE) for dye-sensitized solar cells (DSSCs). Thermogravimetric analysis (TGA) results show that the MoS2 sulfidization temperature was approximately 300 °C. X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) indicate that the stoichiometry and crystallization of MoS2 were more complete at higher temperatures; however, these temperatures reduce the number of edge-plane active sites in the short-range-order nanostructure. Accordingly, the DSSCs with 300 °C annealed TR-MoS2 CE exhibited an excellent photovoltaic conversion efficiency (PCE) of 6.351 %, up to 91.7 % of which is obtained using the conventional TD-Pt CE (PCE = 6.929 %). The temperature of thermal reaction and the molar ratio of reaction precursors were found to significantly influence the resulting stoichiometry and crystallization of MoS2 nanosheets, thus affecting DSSCs’ performance.
- Molybdenum disulfide
- Thermal reduction
- Counter electrode
- Dye-sensitized solar cells
Since the first demonstration of dye-sensitized solar cells (DSSCs) by O’Regan and Grätzel , much attention has been paid to these third-generation solar cells due to their low cost, easy fabrication, high photo conversion efficiency, and environmental friendliness [2–5]. A DSSC typically comprises of a wide-band semiconductor (usually TiO2) coated with dye molecules on a transparent conductive glass as a working electrode (WE), an electrolyte-containing iodide/triiodide (I−/I3 −) redox couple, and a counter electrode (CE), usually deposited platinum (Pt) on the transparent conductive glass. Pt is conventionally used as the CE catalyst in DSSCs to regenerate the electrolyte redox couple and collect electrons to complete the circuit. However, because the high cost and scarcity of Pt greatly restrict the commercial production of DSSCs, the development of low-cost, good electrical conductivity, and high-electrocatalyst CE materials is highly desired to provide an economic solution for high-performance DSSCs.
Stimulated by the outstanding electrochemical activity of graphene, two-dimensional (2D) nanomaterials have attracted great attention in recent years [5–8]. Transition metal dichalcogenides (TMDCs), MX2, (M = Nb, Ta, Mo, W; X = S, Se, Te), have received much interest due to their 2D layered nanostructures, which are analogous to the graphene structure [9–11]. As a typical TMDC, the layer-dependent properties of molybdenum disulfide (MoS2) have recently attracted considerable attention due to their great potential in the electrochemical fields of catalysis [9, 12], lithium-ion batteries [13–15], hydrogen evolution [9, 16, 17], and DSSCs [18, 19]. MoS2 is composed of three stacked atomic layers (a Mo layer sandwiched between two S layers, S–Mo–S) and held together through van der Walls interactions .
However, MoS2 tends to form zero-dimensional fullerene-like nanoparticles or one-dimensional nanotubes during the synthetic process [21, 22]. Therefore, an efficient way to synthesize 2D layer-nanostructured MoS2 is to use graphene or carbon nanotubes (CNTs) as a template substrate [10, 16, 18, 23]. Although significant efforts have been devoted to the preparation of 2D layer-nanostructured MoS2, including scotch tape-based micromechanical exfoliation , liquid exfoliation [25–28], hydrothermal synthesis [14, 29], physical vapor deposition [30, 31], and chemical vapor deposition [32, 33], the easy synthesis of 2D layer-nanostructured MoS2 at low temperatures by template-free approaches under mild conditions still remains a challenge [34, 35]. Additionally, the electrochemical activities of MoS2 were correlated with the number of catalytically active edge sites [9, 12, 17, 36], for the reason that controlling the nanostructures with more edge sites at the atomic scale is an effective strategy to gain an effective MoS2 catalyst. In this study, we produce an easy, thermally reduced (TR) MoS2 nanosheet thin film on fluorine-doped tin oxide (FTO) glass at low temperature that provides the number of edge-plane active sites in the short-range-order nanostructure of MoS2 nanosheets, and demonstrates good catalytic performance compared with conventional Pt CE DSSCs.
Preparations of the Molybdenum Disulfide Counter Electrodes
A FTO transparent glass (TEC-7, 2.2 mm, Hartford) substrate was ultrasonically cleaned sequentially in detergent, acetone (overnight), distilled water (DI water, 1 h), and ethanol (1 h). Ammonium tetrathiomolybdate ((NH4)2MoS4) powder (ProChem Inc., 99.99 % purity; 0.8 g) was added to 20 mL of N,N-dimethylformide (DMF) to form a 4 wt% solution. The solution was then sonicated for 1 day before use . Furthermore, the dispersed solution was coated on FTO glass by spin coating at 1600 rpm to control the thickness and flatness of the film. The substrate was then dried in air for 1 h and annealed in an H2/Ar = 1:9 gas mixture at various temperatures for 45 min to obtain thermally reduced molybdenum sulfide (TR-MoSx) samples. The annealing temperatures for the MoS2 phase transformation in this study are 250, 300, and 350 °C. The thermally deposited platinum (TD-Pt) CE was prepared as a reference electrode by thermal reduction, which was carried out by dropping a H2PtCl6 isopropanol solution on an FTO glass substrate annealed at 450 °C for 20 min .
Fabrication of the TiO2 Working Electrode
The working electrode utilized the same FTO glass coated with nanocrystalline TiO2 using the print-screen method; the area and thickness of the TiO2 film were about 0.28 cm2 and 10 μm, respectively. The TiO2 WE was then gradually sintered to 550 °C in ambient air for 30 min before being slowly cooled at room temperature (RT). After calcination, the TiO2 WE was then immersed in a N719 (Solaronix) solution (3 × 10−3 M in a 1:1 volumetric mixture of acetonitrile and tert-Butylalcohol) at RT for 24 h. Following the dye adsorption process, the dye-adsorbed TiO2 WE was washed with acetonitrile to remove the remaining dye and dried at RT for a few seconds.
Fabrication of DSSC Devices
The efficiency of the TR-MoSx CEs as well as the standard TD-Pt CE DSSC devices were quantitatively compared. The dye-adsorbed TiO2 WE was future assembled with a CE into a sandwiched configuration and sealed with a 60-μm hot-melt surlyn (SX1170-60, Solaronix) by heating at 100 °C for a few seconds. The DSSC device was fabricated by drilling two holes on the CE and injecting an iodide-based electrolyte (AN-50, Solaronix) in the space between the electrodes after the assembling process. Finally, the holes on the CE were sealed after the electrolyte injection. DSSC devices were then illuminated by a class A quality solar simulator with a light intensity of 100 mW cm−2 (AM 1.5), which was calibrated with a standard silicon cell.
Characterizations of Molybdenum Sulfide Counter Electrodes
In order to investigate the phase transformation and chemical states of the low-temperature thermally reduced MoS2, thermogravimetric analysis (TGA) was conducted using a thermogravimetric analyzer (TGA Q50 V20.10 Build 36, USA) with a heating rate of 5 °C min−1 in ambient Nitrogen. X-ray photoelectron spectroscopy (XPS) was conducted using a PHI Quantera SXM/AES 650 (ULVAC-PHI INC., Japan.) equipment with a hemispherical electron analyzer and a scanning monochromated Al Kα (hv = 1486.6 eV) X-ray source to study the chemical states of Mo and S of the prepared MoSx annealing samples. The XPS curve-fitting program, XPSPEAK 4.1, was used for peak de-convolution and assignment of binding energies, which was referenced to the adventitious C1s peak at 284.6 eV. For spectrum analysis, the background signal was subtracted by Shirley’s method, and the curve fitting was performed by using a Gaussian–Lorentzian peak after Shirley background correction. Raman spectra were collected with a confocal micro-Raman spectroscopy (LABRAM HR 800 UV, Japan) using a 514-nm Ar+ laser source with a spot size of approximately 1 μm. The surface morphology of the prepared MoSx annealing samples was examined by using the field emission scanning electron microscope (FESEM, JEOL, JSM-6330F, Japan). The nanostructures of MoS2 were examined by using the transmission electron microscope (TEM, JEOL-2100F, Japan) equipped with EDS to determine the elements contained in the samples. X-ray diffraction (XRD, PANalytical-X‘Pert PRO MPD) with a CuKα radiation of 0.1541 nm was used to determine the crystallinities of the films.
According to our previous studies [3, 4], cyclic voltammetry (CV) measurements and the electrochemical impedance spectroscopy (EIS) were carried out to examine electrochemical properties. CV measurements were used to measure electrochemical redox ability using a potentiostat/galvanostat (PGSTAT 302N, Autolab, Eco Chemie, Netherlands) in a three-electrode configuration. Platinum wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The solution used for CV measurements contained 1 mM I2, 10 mM LiI, and 0.1 M LiClO4 in acetonitrile. EIS spectra were obtained using the aforementioned potentiostat/galvanostat equipped with a frequency response analysis (FRA) module; EIS results were analyzed using an equivalent circuit model with Autolab FRA software (v4.9, EcoChemie B.V.). The frequencies used in the scan ranged from 106 to 10−2 Hz, and an applied voltage of 10 mV was used.
In addition, Tafel polarization measurements were carried out using the potentiostat/galvanostat equipped with a linear polarization module to examine the electrocatalytic activity at the electrolyte–electrode interface. Both EIS and Tafel polarization measurements were obtained using symmetrical devices in the dark.
The photocurrent–voltage characteristics of DSSC devices were measured under simulated solar illumination (AM 1.5, 100 mW cm−2, Oriel 91160, Newport Corporation, USA), equipped with an AM 1.5G filter (Oriel 81088A, Newport Corporation, USA) and a 300-W xenon lamp (Oriel 6258, Newport Corporation, USA). The simulated incident light intensity was calibrated using a reference Si cell (calibrated at NREL, PVM-81).
Composition and Morphologies
Thermal analysis data for the thiomolybdates decomposition in N2
Temperature range (°C)
Experimental weight loss (%)
(NH4)2MoS4 + DMF
The (NH4)2MoS4 precursor dispersed in DMF was also analyzed by TGA (curve (b) in Fig. 1); the most weight loss occurs in the temperature range from 220 °C to 450 °C, which also indicates the MoS2 phase transformation. Commercial MoS2 powder was also used as a reference (curve (c) in Fig. 1) that shows a broad temperature region (RT to 900 °C) and great thermal stability . For future study, we carried out the three annealing conditions in our homemade furnace thermal CVD in H2 mixed gas (H2/Ar = 1:9) at 350, 300, and 250 °C.
The Mo 3d and S 2p peak positions, atomic percentages, and x values of the TR-MoSx samples annealed at 250, 300, and 350 °C
Peak and identity
Fitting of Mo 3d and S 2p peak binding energy (eV) (atomic percentage (%))
S2 2− 2p3/2
S2 2− 2p1/2
S/Mo ratio (x values)
The TR-MoSx annealed at 350 °C exhibits two main peaks of Mo 3d spectra at 229.22 and 232.36 eV that correspond to Mo4+ 3d5/2 and Mo4+ 3d3/2 orbitals, revealing that the Mo4+ state is dominant in the 350 °C annealed sample and indicating the formation of MoS2 . Additional peaks are observed at 162.05 and 163.24 eV, which correspond to the known S 2p2− 3/2 and S 2p2− 1/2 MoS2 doublet peaks, respectively . The stoichiometric ratio (S/Mo) quantified by relative sensitivity factors (RSF) from the respective integrated peak area of XPS spectra is close to 2.03, also indicating that the structure of the 350 °C annealed sample is MoS2. Whereas the annealing temperature is lowered to 250 °C, in addition to that the XPS peaks of the MoS2 structure, other deconvoluted peaks need the concern. The peaks at 230.18 eV (Mo5+ 3d5/2) and 233.32 eV (Mo5+ 3d3/2), 231.40 eV (Mo6+ 3d5/2) and 234.54 eV (Mo6+ 3d3/2), 232.50 eV (Mo6+ 3d5/2) and 235.64 eV (Mo6+ 3d3/2), representing the Mo 3d5/2 and Mo 3d3/2 of the three valence states can be assigned to Mo2S5, MoS3, and MoO3, respectively [12, 40, 41]. Meanwhile, the binding energy at 163.20 eV (S2 2− 2p3/2) and 164.39 eV (S2 2− 2p1/2) might represent to the intermediate product of Mo2S5 and the MoS3 with a formula of [Mo4+ (S2)2−S2−] [42, 43]. It is worthwhile to mention that the MoS2 fraction decrement is nearly linear with annealing temperature down to 250 °C, and it becomes gradual at lower temperatures. These results suggest an incomplete MoS2 phase transformation at lower annealing temperature, which is consistent with our TGA results. The stoichiometric ratio (S/Mo) estimated from the 250 and 300 °C annealing sample were 2.26 and 2.19, respectively. In addition, compared with the 250 °C annealing sample, note that the line width of MoS2 peaks becomes progressively stronger and narrower for annealing temperatures above 300 °C.
The anodic peak current (I pa) and the cathodic peak current (I pc) corresponded to the oxidation of I− ions and the reduction of I3 − ions, respectively. The magnitude of I pc corresponded to the catalytic activation of a C.E. for I3 − reduction in a DSSC . In addition, the peak to peak voltage separation between the anodic and the cathodic peaks (E pp) can be considered to the redox barrier of I3 −/I− redox couples. Therefore, the higher I pc and lower E pp values means the better electrocatalytic activity of CEs in DSSC . From Fig. 7 (a), it can be observed that the TD-Pt and TR-MoSx CEs exhibit two redox pairs, whereas no significant peak is observed when the annealing temperature is below 300 °C. This indicates that our TR-MoSx annealed at 300 °C exhibits similar electrocatalytic activity to the TD-Pt CE, while the noncrystalline TR-MoSx CE annealed at 250 °C provides poor electrocatalytic activity. Furthermore, the current density of the redox peaks for the TR-MoSx CE annealed at 300 °C is higher than those annealed at 250 and 350 °C. In other words, compared to the other conditions, the 300 °C annealed sample can exhibits the highest I pc and lowest E pp that toward the best I3 − ion electrochemical reduction performance.
Photovoltaic parameters of the DSSCs based on various CEs and electrochemical parameters from EIS and Tafel measurements
R s/Ω cm2
R ct/Ω cm2
J 0/mA cm−2
J sc/mA cm−2
17.056 ± 0.075
0.724 ± 0.003
0.557 ± 0.007
6.929 ± 0.063
250 °C annealed sample
15.442 ± 0.118
0.709 ± 0.004
0.175 ± 0.002
1.917 ± 0.026
300 °C annealed sample
16.905 ± 0.013
0.727 ± 0.003
0.517 ± 0.005
6.351 ± 0.045
350 °C annealed sample
16.063 ± 0.251
0.725 ± 0.005
0.299 ± 0.005
3.479 ± 0.101
Tafel polarization measurements were used to examine the exchange current density (J 0) at the electrolyte–catalyst interface (shown in Fig. 7c). The tangential slope of the Tafel curve provides information about the exchange current density (linear segments extrapolate to an intercept of log J0), which is closely associated with the R ct value (Eq. (5)) . As we can see in Fig. 7c, the 300 °C annealed TR-MoSx electrode has a large exchange current density (J 0) compared with the 250 and the 350 °C annealed samples and was comparable with the TD-Pt electrode (summarized in Table 3), which means higher electrocatalytic activity and lower charge–transfer resistance at the electrolyte–electrode interface.
Photovoltaic Performance of DSSCs
As shown in Fig. 7d, the photovoltaic performance of DSSCs is characterized using the short-circuit current density (J sc), open-circuit voltage (V oc), fill factor (FF), and photoconversion efficiency (ɳ); these parameters are summarized in Table 3. The J sc, V oc, and FF of the DSSC with a reference TD-Pt film CE were 17.056 mA cm−2, 0.724 V, and 0.557, respectively, yielding a photoconversion efficiency of 6.929 %. The DSSC with the TR-MoSx CE annealed at 300 °C exhibits a higher conversion efficiency (6.351 %) compared with those prepared at other annealing temperatures; the corresponding J sc, V oc, and FF were 16.905 mA cm−2, 0.727 V, and 0.517, respectively, which agreed with the CV and EIS measurements. It is worth noting that the values of J sc for the TR-MoS2 CEs annealed at 250 and 350 °C were 15.442 and 16.063 mA cm−2, respectively.
where R is the gas constant, D is the diffusion coefficient of the triiodide, l is the spacer thickness, and F and n have their normal meanings. In other words, EIS and Tafel results explain the good photovoltaic performance of DSSCs based on the TR-MoS2 CE annealed at 300 °C.
The values of J sc and FF can be considered indicative of the number of edge-plane active sites for redox reactions. Although the crystallization of MoS2 was more complete at higher temperature, the J sc and FF of TR-MoSx annealed at 350 °C (J sc = 16.063, FF = 0.299) are smaller than those of the TR-MoSx annealed at 300 °C (J sc = 16.905, FF = 0.517). Here, we suggest that the long-range-order nanostructure of the 350 °C annealed MoS2 reduces the active sites of the edge-planes.
In summary, the independent MoS2 nanosheets annealed at 300 °C provide the best electrocatalytic activity toward I3 − reduction. CV, EIS, and Tafel measurements suggest that the 300 °C annealing temperature should generate a larger active area. This results an excellent photovoltaic conversion efficiency (PCE) of 6.351 % under AM 1.5 illumination of 100 mW cm−2, up to 91.7 % of which is obtained using the conventional TD-Pt CE (PCE = 6.929 %). These results demonstrate that the 300 °C annealed TR-MoS2 CE has a great potential as a low-cost alternative to Pt in DSSCs.
In this work, a two-dimensional nanostructure of MoS2 has been successfully synthesized by a low temperature TR method on FTO glass substrates. This material was also incorporated into a Pt-free DSSC for application. In the TGA results, it was found that the MoS2 sulfidization temperature was approximately 300 °C which provided the effective MoS2 phase transformation process. Additionally, XPS, TEM, and XRD indicate that the stoichiometry and crystallization of MoS2 were more complete at higher temperatures; however, these temperatures reduce the number of edge-plane active sites in the short-range-order nanostructure. The electrochemical analysis also showed that the 300 °C annealed TR-MoS2 CE provided an independent nanosheet nanostructure with numerous active sites that demonstrated Pt-like electrocatalytic activity for I3 − reduction. These short-range-order nanostructure of MoS2 nanosheets provided a great of edge-plane active sites to enhance the catalytic performance to increase the J sc and FF, and an outstanding PCE can be obtained. Accordingly, the DSSC assembled with the 300 °C annealed TR-MoS2 structure exhibited an excellent PCE of 6.351 %; up to 91.7 % of which is obtained using the conventional TD-Pt CE (PCE = 6.929 %). This leads us to the conclusion that the low temperature TR-MoS2 CE is a promising candidate for application as a highly efficient and low-cost CE material in Pt-free DSSCs.
The financial support provided by the Ministry of Science and Technology of Taiwan through Project: MOST 103-2221-E-131 -029 is greatly appreciated.
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