Enhanced Efficiency of Dye-Sensitized Solar Counter Electrodes Consisting of Two-Dimensional Nanostructural Molybdenum Disulfide Nanosheets Supported Pt Nanoparticles

This paper reports architecturally designed nanocomposites synthesized by hybridizing the two-dimensional (2D) nanostructure of molybdenum disulfide (MoS2) nanosheet (NS)-supported Pt nanoparticles (PtNPs) as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). MoS2 NSs were prepared using the hydrothermal method; PtNPs were subsequently reduced on the MoS2 NSs via the water–ethylene method to form PtNPs/MoS2 NSs hybrids. The nanostructures and chemical states of the PtNPs/MoS2 NSs hybrids were characterized by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Detailed electrochemical characterizations by electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel-polarization measurement demonstrated that the PtNPs/MoS2 NSs exhibited excellent electrocatalytic activities, afforded a higher charge transfer rate, a decreased charge transfer resistance, and an improved exchange current density. The PtNPs/MoS2 NSs hybrids not only provided the exposed layers of 2D MoS2 NSs with a great deal of catalytically active sites, but also offered PtNPs anchored on the MoS2 NSs enhanced I3 reduction. Accordingly, the DSSCs that incorporated PtNPs/MoS2 NSs CE exhibited an outstanding photovoltaic conversion efficiency (PCE) of 7.52%, which was 8.7% higher than that of a device with conventional thermally-deposited platinum CE (PCE = 6.92%).


Introduction
A typical dye-sensitized solar cell (DSSC) is usually fabricated by dye-loaded TiO 2 nanoparticles coated on transparent conductive glass to act as the working electrode (WE), a Pt film deposited on the transparent conductive glass as the counter electrode (CE), and an electrolyte containing an iodide/triiodide (I − /I 3 − ) redox couple between the WE and CE [1].Recently, considerable efforts have focused on improving the energy conversion efficiency and long-term stability of the dye, electrolyte, and working electrode [2][3][4][5][6][7].However, a high catalytic material for CE is extremely important for promoting the charge transfer rate for I 3 − reduction.Therefore, a CE with high electrochemical activity and low internal resistance is crucial to yielding high photovoltaic conversion efficiency (PCE).
Coatings 2017, 7, 167 Recently, inspired by the discovery of graphene-which opened up the new research field of two-dimensional (2D) nanomaterials [8,9]-studies on 2D nanomaterials have attracted great attention.As a typical transition metal dichalcogenide, 2D layered nanostructural molybdenum disulfide (MoS 2 ) is similar to the graphene structure, and is composed of three stacked atomic layers (a Mo layer sandwiched between two S layers, S-Mo-S) [10].Layered nanostructural 2D MoS 2 has been extensively investigated as a promising catalyst in electrochemical applications in recent years such as H 2 evolution [11], Li-ion batteries [12], and DSSCs [13].For instance, Jaramillo et al. reported electrochemical H 2 production with MoS 2 nanocatalysts based on their catalytically active sites for H 2 evolution [11].Very recently, Mohammad et al. reported that the ultrathin MoS 2 nanostructured films possessed the outstanding catalytic performance of the CE for DSSCs, with rich catalytic active sites provided by the MoS 2 nanosheet crystal structure [13].Furthermore, due to the high specific surface area of 2D nanomaterials, zero-dimensional (0D) catalytic nanoparticles anchored on the 2D nanomaterials to form 0D/2D functional hybrids bring novel properties that are different from those of their individual intrinsic properties.For example, graphene-supported Pt nanoparticles (PtNPs) such as PtNPs/graphene hybrids, have been investigated as advanced electrocatalysts for achieving superior activities of CEs in producing high-performing DSSCs [14][15][16][17].Yanyan et al. reported that graphene acted as an ideal support for uniformly disperse PtNPs, which was intrinsically important for an effective CE in DSSCs [14].Min-Hsin et al. reported that PtNPs/graphene CE displayed larger realistic electroactive surface areas and a constant higher intrinsic heterogeneous rate to improve the electrocatalytic abilities for the reduction of I 3 − , and therefore improved the PCE of DSSCs [15].
In terms of the above considerations, in this study, we report on our investigation of the performance of DSSCs that used the PtNPs/MoS 2 NSs hybrid nanoarchitecture as the CE.This hybrid CE, constituted by 2D layered MoS 2 NS-supported 0D PtNPs, showed a number of advantages.The MoS 2 NSs not only provided exposed-layer active sites for I 3 − reduction, but also provided large surface areas for PtNP anchoring on the MoS 2 NSs to enhance electrochemical activities.The DSSCs fabricated with the PtNPs/MoS 2 NSs CE had an increased exchange current density and reduced charge-transfer resistance, resulting in a superior PCE of 7.52%, 8.7% higher than that of a conventional thermally-deposited Pt CE (6.92%).

Preparation of MoS 2 NSs and PtNPs/MoS 2 NSs
Two steps were used to synthesize PtNPs/MoS 2 NSs CE.In Step 1, the hydrothermal method was applied to synthesize the MoS 2 NSs.Ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 ) powder (99.99% purity, ProChem Inc., Rockford, IL, USA) weighing 0.5 g was added to an aqueous solution with 5 mL of HCl in 100 mL deionized water.Subsequently, the aforementioned solution was transferred into a Teflon-lined autoclave and heated to 250 • C for 12 h.The suspension was then washed and centrifuged with deionized water and ethanol several times, respectively.The resulting powder was finally dried in vacuum at 60 • C to obtain MoS 2 NSs.In Step 2, PtNPs/MoS 2 NSs hybrids were prepared using a water-ethylene method [17].MoS 2 NSs (50 mg) were ultrasonically dispersed in a mixture containing 30 mL of deionized water, 100 mL of ethylene glycol (EG), and 1.5 mL of 0.05 M H 2 PtCl 6 aqueous solution.This mixture was then heated at 120 • C with stirring for 6 h.Subsequently, the composite was washed and centrifuged with deionized water and ethanol, respectively, for a total of six times.Finally, the resulting powder was dried in vacuum at 60 • C to obtain the PtNPs/MoS 2 NSs hybrids.(DI water, 1 h), and ethanol (1 h).The CEs with MoS 2 NSs and PtNPs/MoS 2 NSs hybrids were fabricated as follows: 5 mg MoS 2 NSs powder and PtNPs/MoS 2 NSs powder were added to 5 mL N,N-dimethylformamide (DMF) for dispersion and then sonicated for 1 h, respectively.Subsequently, the dispersed solutions were coated on FTO glasses by spin coating technology to control the flatness and thickness of the films.Finally, the prepared samples were dried in vacuum at 60 • C for 1 h to obtain the MoS 2 NSs CE and the PtNPs/MoS 2 NSs CE.In addition, a thermal deposition Pt (TD-Pt) CE was also prepared as a reference electrode by dropping a H 2 PtCl 6 isopropanol solution on a FTO glass annealed at 450 • C for 20 min [18].

Fabrication of Various CEs and Assembly of DSSCs
A screen-printing method was carried out to prepare the WEs: 10 µm TiO 2 nanoparticle films were coated onto the FTO glasses, and were then placed in the furnace for calcination at 550 • C in ambient air for 30 min.After slowly cooled to room temperature (RT), the TiO 2 WEs were removed from the furnace and 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.
The DSSCs were assembled as follows.After the dye adsorption process, the dye-adsorbed TiO 2 WE was assembled with various CEs as the sandwich-type cell, and sealed with a 60 µm hot-melt surlyn (SX1170-60, Solaronix, Aubonne, Switzerland) between WE and CE.Then, the commercial iodide-based electrolyte (TDP-LE-M, Jintex Corporation Ltd., Taipei, Taiwan) was injected into the space between the two electrodes after the assembling process.A Class A quality solar simulator with a light intensity of 100 mW•cm −2 (AM 1.5) was used as the light source to illuminate the DSSCs devices to measure the photocurrent-voltage characteristics.

Characterizations
Transmission electron microscopy (TEM, JEM-2100F, JOEL, Tokyo, Japan) was used to examine the nanostructures of the prepared PtNPs/MoS 2 NSs hybrids.The sample for TEM was prepared by dropping the sample solution on a copper grid coated with a carbon film.X-ray photoelectron spectroscopy (XPS) was carried out using a PHI Quantera SXM/AES 650 (ULVAC-PHI Inc., Kanagawa, Japan) system with a hemispherical electron analyzer and a scanning monochromated Al Kα (hv = 1486.6eV) X-ray source to investigate the chemical states of Mo, S, and Pt.To study the chemical states of the PtNPs/MoS 2 NSs hybrids, XPSPEAK 4.1 was used for fitting the obtained curves, peak de-convolution and assignment of binding energies, and referenced to the adventitious C 1s peak at 284.6 eV.For spectrum analysis, the background signal was subtracted by Shirley's method, and curve fitting was performed by using a Gaussian-Lorentzian peak after Shirley background correction.
The catalytic abilities of the CEs were examined by cyclic voltammetry (CV) measurements, equipped with a three-electrode configuration using potentiostat/galvanostat (PGSTAT 302N, Metrohm Autolab, Eco Chemie, Utrecht, The Netherlands) in an acetonitrile-based solution consisting of 10 mM LiI, 1.0 mM I 2 , and 0.1 M LiClO 4 .The Pt wire and an Ag/AgNO 3 electrode were employed as the counter and reference electrodes, respectively.Electrochemical impedance spectroscopy (EIS) was carried out to study the electrochemical properties of the CEs.The aforementioned potentiostat/galvanostat-equipped with a frequency response analysis (FRA) module-was used for the EIS analyses in a frequency range between 10 6 Hz and 10 −2 Hz.Tafel polarization curves were also measured using the potentiostat/galvanostat equipped with a linear polarization module to further investigate the catalytic activities at the electrolyte-electrode interface of various CEs.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, Irvine, CA, USA), equipped with an AM 1.5 G filter (Oriel 81088A, Newport Corporation, Irvine, CA, USA) and a 300 W xenon lamp (Oriel 6258, Newport Corporation, Irvine, CA, USA).The simulated incident light intensity was calibrated using a reference Si cell (calibrated at NREL, PVM-81).

Nanostructural Features and Composition
The morphologies and nanostructures were studied using TEM and HRTEM (JEM-2100F, JOEL, Tokyo, Japan).Figure 1a shows a typical TEM image of the PtNPs/MoS 2 NSs.As we can see from Figure 1a, the PtNPs were uniformly dispersed on the MoS 2 NSs without aggregation.The HRTEM image in Figure 1b shows the plentifully exposed plans with an interlayer distance of the MoS 2 of about 0.64 nm, corresponding to the spacing between (002) planes of MoS 2 , similar to previous studies in Reference [12].In addition, the HRTEM image clearly shows PtNPs with a uniform size on the MoS 2 NSs, and particle sizes of the PtNPs in the range of 3-5 nm.The inset in Figure 1b shows a Pt nanoparticle with the lattice distance of 0.22 nm corresponding to the (111) plane of crystalline Pt.

Nanostructural Features and Composition
The morphologies and nanostructures were studied using TEM and HRTEM (JEM-2100F, JOEL, Tokyo, Japan).Figure 1a shows a typical TEM image of the PtNPs/MoS2 NSs.As we can see from Figure 1a, the PtNPs were uniformly dispersed on the MoS2 NSs without aggregation.The HRTEM image in Figure 1b shows the plentifully exposed plans with an interlayer distance of the MoS2 of about 0.64 nm, corresponding to the spacing between (002) planes of MoS2, similar to previous studies in Reference [12].In addition, the HRTEM image clearly shows PtNPs with a uniform size on the MoS2 NSs, and particle sizes of the PtNPs in the range of 3-5 nm.The inset in Figure 1b shows a Pt nanoparticle with the lattice distance of 0.22 nm corresponding to the (111) plane of crystalline Pt. Figure 2 and Table 1 show the XPS fitting results of PtNPs/MoS2 NSs. Figure 2a shows the XPS spectrum of the wide spectral region of the MoS2 NSs and PtNPs/MoS2 NSs, respectively.The relatively characteristic peaks of the elements are also illustrated in Figure 2a.As seen in Figure 2a, when compared with MoS2 NSs, the Pt 4f peak of PtNPs/MoS2 NSs can be clearly seen.Figures 2b-d show the chemical states of Pt 4f, Mo 3d, and the S 2p orbitals of the PtNPs/MoS2 NSs, respectively.Figure 2b shows the high-resolution Pt 4f spectra, where the main peaks at 71.2 and 74.5 eV correspond to Pt 0 4f7/2 and Pt 0 4f5/2 of the metallic Pt.The smaller peaks at the higher binding energies of Pt 2+ 4f7/2 at 72.0 eV, Pt 2+ 4f5/2 at 75.3 eV, Pt 4+ 4f7/2 at 73.6 eV, and Pt 4+ 4f5/2 at 76.9 eV, correspond to PtO and PtO2, respectively [19]. Figure 2c shows the high-resolution Mo 3d spectra, where the two main peaks of Mo 3d spectra are at 229.2 and 232.3 eV, which correspond to the Mo 4+ 3d5/2 and Mo 4+ 3d3/2 orbitals, and the revealed Mo 4+ states indicated that the major formation was MoS2 [20].The other peaks of Mo 5+ 3d5/2 at 230.3 eV, Mo 5+ 3d3/2 at 233.4 eV, Mo 6+ 3d5/2 at 231.4 eV, Mo 6+ 3d3/2 at 234.5 eV, Mo 6+ 3d5/2 at 232.5 eV, and Mo 6+ 3d3/2 at 235.6 eV, corresponded to the minority products of Mo2S5, MoS3, and MoO3, respectively [20,21].The relatively weak peaks of MoO3 may come from the oxidation of Mo atoms at the edges, or defects on the crystal planes of the MoS2 NSs during the chemical reaction [22].Figure 2d shows the high-resolution S 2p spectra, the S 2− 2p3/2 and S 2− 2p1/2 peaks at 161.9 and 163.Pt is predominantly present as metallic Pt along with surface oxides and hydroxide, as is normally observed in the case of Pt NPs [19].Therefore, as above-mentioned, the oxidation groups on the crystal planes of the MoS2 NSs probably act as nucleation sites to reduce the precursor Pt 4+ to Pt 2+ and Pt 0 in the mixture solution (H2PtCl6-EG-water) for the subsequent formation of PtNPs by EG reduction.Figure 2 and Table 1 show the XPS fitting results of PtNPs/MoS 2 NSs. Figure 2a shows the XPS spectrum of the wide spectral region of the MoS 2 NSs and PtNPs/MoS 2 NSs, respectively.The relatively characteristic peaks of the elements are also illustrated in Figure 2a.As seen in Figure 2a, when compared with MoS 2 NSs, the Pt 4f peak of PtNPs/MoS 2 NSs can be clearly seen.Figure 2b-d show the chemical states of Pt 4f, Mo 3d, and the S 2p orbitals of the PtNPs/MoS 2 NSs, respectively.Figure 2b shows the high-resolution Pt 4f spectra, where the main peaks at 71.2 and 74.5 eV correspond to Pt 0 4f 7/2 and Pt 0 4f 5/2 of the metallic Pt.The smaller peaks at the higher binding energies of Pt 2+ 4f 7/2 at 72.0 eV, Pt 2+ 4f 5/2 at 75.3 eV, Pt 4+ 4f 7/2 at 73.6 eV, and Pt 4+ 4f 5/2 at 76.9 eV, correspond to PtO and PtO 2 , respectively [19]. Figure 2c shows the high-resolution Mo 3d spectra, where the two main peaks of Mo 3d spectra are at 229.2 and 232.3 eV, which correspond to the Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 orbitals, and the revealed Mo 4+ states indicated that the major formation was MoS 2 [20].The other peaks of Mo 5+ 3d 5/2 at 230.3 eV, Mo 5+ 3d 3/2 at 233.4 eV, Mo 6+ 3d 5/2 at 231.4 eV, Mo 6+ 3d 3/2 at 234.5 eV, Mo 6+ 3d 5/2 at 232.5 eV, and Mo 6+ 3d 3/2 at 235.6 eV, corresponded to the minority products of Mo 2 S 5 , MoS 3 , and MoO 3 , respectively [20,21].The relatively weak peaks of MoO 3 may come from the oxidation of Mo atoms at the edges, or defects on the crystal planes of the MoS 2 NSs during the chemical reaction [22].Figure 2d shows the high-resolution S 2p spectra, the S 2− 2p 3/2 and S 2− 2p 1/2 peaks at 161.9 and 163.Pt is predominantly present as metallic Pt along with surface oxides and hydroxide, as is normally observed in the case of Pt NPs [19].Therefore, as above-mentioned, the oxidation groups on the crystal planes of the MoS 2 NSs probably act as nucleation sites to reduce the precursor Pt 4+ to Pt 2+ and Pt 0 in the mixture solution (H 2 PtCl 6 -EG-water) for the subsequent formation of PtNPs by EG reduction.

Electrochemical Properties
EIS measurement was performed in a symmetrical cell comprised of two identical CEs to analyze the correlation between the electrocatalytic activities of the various CEs.Nyquist plots in Figure 3a display the impedance characteristics based on the PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs, and the corresponding electrochemical parameters obtained from the Nyquist plot were fitted with the Autolab FRA software (v4.9, EcoChemie B.V.) and are summarized in Table 2. Based on the equivalent circuit (the inset of Figure 3a), the Rs value was estimated from the intercept on the real axis in the left region, where Rs corresponded to the series resistance of the electrolyte and electrodes.The left semicircle in the high-frequency region corresponded to the charge-transfer resistance (Rct) and the phase angle element (CPE) at the electrolyte-electrode interface, and the right semicircle in the low-frequency region corresponded to the Nernst diffusion impedance (ZN) in the electrolyte.It is well known that a smaller Rs represents a higher conductivity and a smaller Rct brings a faster charge-transfer rate from the CE to the electrolyte to enhance the electrocatalytic activities.The Rs value corresponds to the series resistance and includes the sheet resistance of the FTO substrate and the resistance of the contacts.The Rs of PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs were 27.8 Ω•cm 2 , 27.9 Ω•cm 2 , and 27.8 Ω•cm 2 , respectively.Based on the Rs results, all CEs had similar conductivities.The Rct of PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs were 0.75 Ω•cm 2 , 12.15 Ω•cm 2 and 3.81 Ω•cm 2 ,

Electrochemical Properties
EIS measurement was performed in a symmetrical cell comprised of two identical CEs to analyze the correlation between the electrocatalytic activities of the various CEs.Nyquist plots in Figure 3a display the impedance characteristics based on the PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs, and the corresponding electrochemical parameters obtained from the Nyquist plot were fitted with the Autolab FRA software (v4.9, EcoChemie B.V.) and are summarized in Table 2. Based on the equivalent circuit (the inset of Figure 3a), the Rs value was estimated from the intercept on the real axis in the left region, where Rs corresponded to the series resistance of the electrolyte and electrodes.The left semicircle in the high-frequency region corresponded to the charge-transfer resistance (Rct) and the phase angle element (CPE) at the electrolyte-electrode interface, and the right semicircle in the low-frequency region corresponded to the Nernst diffusion impedance (ZN) in the electrolyte.It is well known that a smaller Rs represents a higher conductivity and a smaller Rct brings a faster charge-transfer rate from the CE to the electrolyte to enhance the electrocatalytic activities.The Rs value corresponds to the series resistance and includes the sheet resistance of the FTO substrate and the resistance of the contacts.The Rs of PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs were 27.8 Ω•cm 2 , 27.9 Ω•cm 2 , and 27.8 Ω•cm 2 , respectively.Based on the Rs results, all CEs had similar conductivities.The Rct of PtNPs/MoS2 NSs, MoS2 NSs and TD-Pt CEs were 0.75 Ω•cm 2 , 12.15 Ω•cm 2 and 3.81 Ω•cm 2 ,

Electrochemical Properties
EIS measurement was performed in a symmetrical cell comprised of two identical CEs to analyze the correlation between the electrocatalytic activities of the various CEs.Nyquist plots in Figure 3a display the impedance characteristics based on the PtNPs/MoS 2 NSs, MoS 2 NSs and TD-Pt CEs, and the corresponding electrochemical parameters obtained from the Nyquist plot were fitted with the Autolab FRA software (v4.9, EcoChemie B.V.) and are summarized in Table 2. Based on the equivalent circuit (the inset of Figure 3a), the R s value was estimated from the intercept on the real axis in the left region, where R s corresponded to the series resistance of the electrolyte and electrodes.The left semicircle in the high-frequency region corresponded to the charge-transfer resistance (R ct ) and the phase angle element (CPE) at the electrolyte-electrode interface, and the right semicircle in the low-frequency region corresponded to the Nernst diffusion impedance (Z N ) in the electrolyte.It is well known that a smaller R s represents a higher conductivity and a smaller R ct brings a faster charge-transfer rate from the CE to the electrolyte to enhance the electrocatalytic activities.The R s value corresponds to the series resistance and includes the sheet resistance of the FTO substrate and the resistance of the contacts.The R s of PtNPs/MoS 2 NSs, MoS 2 NSs and TD-Pt CEs were 27.8 Ω•cm 2 , 27.9 Ω•cm 2 , and 27.8 Ω•cm 2 , respectively.Based on the R s results, all CEs had similar conductivities.The R ct of PtNPs/MoS 2 NSs, MoS 2 NSs and TD-Pt CEs were 0.75 Ω•cm 2 , 12.15 Ω•cm 2 and 3.81 Ω•cm 2 , respectively.The results demonstrated that PtNPs/MoS 2 NSs CE had outstanding charge transfer ability, that the R ct value of PtNPs/MoS 2 NSs CE was 16 times better than that of MoS 2 NSs CE, and five times better than that of TD-Pt CE.
In addition, Figure 3b shows the Bode plots used to investigate the charge-transfer kinetics of the various CEs where the high-frequency peak (f ct ) and the low-frequency peak (f N ) corresponded to the charge-transfer behavior of the catalytic material and the Nernst-diffusion behavior of the electrolyte, respectively.A higher charge-transfer frequency indicated a lower electron lifetime and led to a lower R ct value for the charge transfer rate [25].The charge-transfer frequency of PtNPs/MoS 2 NSs CE was 4.5 kHz, which was higher than that of MoS 2 NSs (0.8 kHz) and TD-Pt (4.3 kHz).The highest frequency of the PtNPs/MoS 2 NSs CE obtained from the Bode plot coincided with the lowest R ct value obtained from the Nyquist plot (Figure 3a).
Figure 3c shows the cyclic voltammetry (CV) curves of various CEs, which was measured using a three-electrode system with the Pt sheet as the CE, Ag/AgNO 3 as the reference electrode, and various CEs as the working electrode.The relative peaks in Figure 3c  was evaluated in terms of its reduction peak current density (I red ) and the voltage separation (E pp ) of the I red peak to the I ox peak, respectively.The magnitude of I red corresponded to the catalytic activity of a CE for an I 3 − reduction in a DSSC [26], and the value of E pp was negatively correlated with the standard electrochemical rate constant of a redox reaction [27].As we can see from Figure 3c and Table 2, the I red value of the PtNPs/MoS 2 NSs (−2.17 mA•cm −2 ) was higher than those of the MoS 2 NSs (−1.66 mA•cm −2 ) and TD-Pt (−1.87 mA•cm −2 ).The E pp of the PtNPs/MoS 2 NSs CE showed a relatively lower value of 0.23 V when compared to that of MoS 2 NSs (0.27 V) and TD-Pt (0.24 V) CEs.Therefore, the higher I red and lower E pp values meant better electrocatalytic activity of PtNPs/MoS 2 NSs CE in DSSC.The results obtained from CV indicated that PtNPs/MoS 2 NSs CE had a better charge transport rate when compared with MoS 2 NSs and TD-Pt CEs, which was consistent with the EIS results and Bode plots (Figure 3a,b).Tafel-polarization curves were used to further investigate the catalytic activities of the exchange current densities (J 0 ) and the limiting current densities (J lim ) at the electrolyte-catalyst interface of various CEs.As shown in Figure 3d, the J 0 was approximately calculated by the Tafel linear extrapolation method, which was closely associated with the R ct value (J 0 is inversely proportional to R ct ) and the J lim is dependent on the intersection of the cathodic branch and the vertical axis (Equations ( 1) and ( 2)) [13].
where R is the gas constant; T is the temperature; F is the Faraday constant; l is the spacer thickness; C is the concentration of I 3 − species; and n represents the number of electrons transferred in the reduction reaction.As shown in Figure 3d and Table 2, the PtNPs/MoS 2 NSs CE showed the highest values for J 0 (5.2 mA•cm −2 ) and J lim (12.1 mA•cm −2 ), which were higher than those of MoS 2 NSs CE (J 0 = 2.7 mA•cm −2 , J lim = 10.2 mA•cm −2 ) and TD-Pt CE (J 0 = 4.4 mA•cm −2 , J lim = 11.9 mA•cm −2 ).
According to the results obtained from the Tafel-polarization curves, the highest J 0 and J lim values of the PtNPs/MoS 2 NSs CE indicated the lowest R ct value at the electrolyte-electrode interface, which was consistent with the EIS measurements.Furthermore, the highest J 0 and J lim values of the PtNPs/MoS 2 NSs CE also coincided with the highest i pc and lowest E pp values obtained from the CV curves, which strongly agreed with the promotion of the I 3 − reduction rate, thus enhancing the catalytic activity.

Photovoltaic Performance of DSSCs
Figure 4 shows the photocurrent-voltage curves of DSSCs assembled with PtNPs/MoS2 NSs, MoS2 NSs, and TD-Pt CEs.The short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE (η) used to characterize the photovoltaic performances of the DSSCs are summarized in Table 3.The highest Jsc, Voc, and FF values of the DSSC for the PtNPs/MoS2 NSs CE were 17.23 mA•cm −2 , 0.71 V, and 0.61, respectively, yielding the highest η value of 7.52% out of all the CEs in this work.The Jsc, Voc, and FF of the DSSC with the reference TD-Pt CE were 17.13 mA•cm −2 , 0.7 V, and 0.57, respectively, yielding an η of 6.92%.The DSSC with the MoS2 NSs CE exhibited an η of 6.76%, and the corresponding Jsc, Voc, and FF were 16.12 mA•cm −2 , 0.7 V, and 0.59, respectively.It is worth noting that the DSSC based on the MoS2 NSs CE displayed a comparable performance with that obtained using the conventional TD-Pt CE, which can be considered indicative of a surface exposed nanosheet possessing rich catalytic active sites [13].

Photovoltaic Performance of DSSCs
Figure 4 shows the photocurrent-voltage curves of DSSCs assembled with PtNPs/MoS 2 NSs, MoS 2 NSs, and TD-Pt CEs.The short-circuit current density (J sc ), open-circuit voltage (V oc ), fill factor (FF), and PCE (η) used to characterize the photovoltaic performances of the DSSCs are summarized in Table 3.The highest J sc , V oc , and FF values of the DSSC for the PtNPs/MoS 2 NSs CE were 17.23 mA•cm −2 , 0.71 V, and 0.61, respectively, yielding the highest η value of 7.52% out of all the CEs in this work.The J sc , V oc , and FF of the DSSC with the reference TD-Pt CE were 17.13 mA•cm −2 , 0.7 V, and 0.57, respectively, yielding an η of 6.92%.The DSSC with the MoS 2 NSs CE exhibited an η of 6.76%, and the corresponding J sc , V oc , and FF were 16.12 mA•cm −2 , 0.7 V, and 0.59, respectively.It is worth noting that the DSSC based on the MoS 2 NSs CE displayed a comparable performance with that obtained using the conventional TD-Pt CE, which can be considered indicative of a surface exposed nanosheet possessing rich catalytic active sites [13].

Conclusions
In summary, the PtNPs/MoS2 NSs hybrids exhibited excellent catalytic activities, acting as an outstanding CE for the DSSCs.The two-dimensional nanostructural MoS2 NSs played an important role in this work, as MoS2 NSs not only provided the rich catalytic active sites for the redox reactions, but also displayed a high specific surface area for supporting plentiful Pt NPs.The combination of the two outstanding catalysts showed superior catalytic behavior and led to the superb redox reaction rate for the I3 − /I − couples.The EIS, Bode plots, CV, and Tafel results all explained the good photovoltaic performance of the DSSCs based on the PtNPs/MoS2 NSs CE, where the lowest Rct value reduced loss during charge transportation, and the highest redox frequency enhanced the charge transfer efficiency, thereby displaying the highest ipc, J0, and Jlim for promoting the charge collection.The superior catalytic properties described above yielded an excellent PCE of 7.52% under AM 1.5 illumination of 100 mW•cm −2 , which was 8.7% higher than that of the conventional TD-Pt CE (6.92%).

Conclusions
In summary, the PtNPs/MoS 2 NSs hybrids exhibited excellent catalytic activities, acting as an outstanding CE for the DSSCs.The two-dimensional nanostructural MoS 2 NSs played an important role in this work, as MoS 2 NSs not only provided the rich catalytic active sites for the redox reactions, but also displayed a high specific surface area for supporting plentiful Pt NPs.The combination of the two outstanding catalysts showed superior catalytic behavior and led to the superb redox reaction rate for the I 3 − /I − couples.The EIS, Bode plots, CV, and Tafel results all explained the good photovoltaic performance of the DSSCs based on the PtNPs/MoS 2 NSs CE, where the lowest R ct value reduced loss during charge transportation, and the highest redox frequency enhanced the charge transfer efficiency, thereby displaying the highest i pc , J 0 , and J lim for promoting the charge collection.The superior catalytic properties described above yielded an excellent PCE of 7.52% under AM 1.5 illumination of 100 mW•cm −2 , which was 8.7% higher than that of the conventional TD-Pt CE (6.92%).
Fluorine-doped tin oxide (FTO) transparent glasses (TEC-7, 2.2 mm, Hartford glass, Hartford, IN, USA) were used as the substrates for CEs and WEs.Prior to the fabrications of CEs and WEs, FTO glasses were ultrasonically cleaned sequentially in detergent, acetone (overnight), distilled water Coatings 2017, 7, 167 3 of 10

Figure 1 .
Figure 1.(a) TEM image of PtNPs distributed over MoS2 NSs; (b) HRTEM of PtNPs/MoS2 NSs, the inset showed the lattice distance of 0.22 nm corresponding to the (111) plane of Pt.
Figure2and Table1show the XPS fitting results of PtNPs/MoS2 NSs.Figure2ashows the XPS spectrum of the wide spectral region of the MoS2 NSs and PtNPs/MoS2 NSs, respectively.The relatively characteristic peaks of the elements are also illustrated in Figure2a.As seen in Figure2a, when compared with MoS2 NSs, the Pt 4f peak of PtNPs/MoS2 NSs can be clearly seen.Figures2b-dshow the chemical states of Pt 4f, Mo 3d, and the S 2p orbitals of the PtNPs/MoS2 NSs, respectively.Figure2bshows the high-resolution Pt 4f spectra, where the main peaks at 71.2 and 74.5 eV correspond to Pt 0 4f7/2 and Pt 0 4f5/2 of the metallic Pt.The smaller peaks at the higher binding energies of Pt 2+ 4f7/2 at 72.0 eV, Pt 2+ 4f5/2 at 75.3 eV, Pt 4+ 4f7/2 at 73.6 eV, and Pt 4+ 4f5/2 at 76.9 eV, correspond to PtO and PtO2, respectively[19].Figure2cshows the high-resolution Mo 3d spectra, where the two main peaks of Mo 3d spectra are at 229.2 and 232.3 eV, which correspond to the Mo 4+ 3d5/2 and Mo 4+ 3d3/2 orbitals, and the revealed Mo 4+ states indicated that the major formation was MoS2[20].The other peaks of Mo 5+ 3d5/2 at 230.3 eV, Mo 5+ 3d3/2 at 233.4 eV, Mo 6+ 3d5/2 at 231.4 eV, Mo 6+ 3d3/2 at 234.5 eV, Mo 6+ 3d5/2 at 232.5 eV, and Mo 6+ 3d3/2 at 235.6 eV, corresponded to the minority products of Mo2S5, MoS3, and MoO3, respectively[20,21].The relatively weak peaks of MoO3 may come from the oxidation of Mo atoms at the edges, or defects on the crystal planes of the MoS2 NSs during the chemical reaction[22].Figure2dshows the high-resolution S 2p spectra, the S 2− 2p3/2 and S 2− 2p1/2 peaks at 161.9 and 163.1 eV, which corresponds to MoS2.The S2 2− 2p3/2 and S2 2− 2p1/2 of binding energy at 163.2 eV and 164.4 eV might represent the intermediate products of Mo2S5 and the MoS3 with a formula of [Mo 4+ (S2) 2− S 2− ] [23,24].Pt is predominantly present as metallic Pt along with surface oxides and hydroxide, as is normally observed in the case of Pt NPs[19].Therefore, as above-mentioned, the oxidation groups on the crystal planes of the MoS2 NSs probably act as nucleation sites to reduce the precursor Pt 4+ to Pt 2+ and Pt 0 in the mixture solution (H2PtCl6-EG-water) for the subsequent formation of PtNPs by EG reduction.

Figure 1 .
Figure 1.(a) TEM image of PtNPs distributed over MoS 2 NSs; (b) HRTEM of PtNPs/MoS 2 NSs, the inset showed the lattice distance of 0.22 nm corresponding to the (111) plane of Pt.
Figure2and Table1show the XPS fitting results of PtNPs/MoS 2 NSs.Figure2ashows the XPS spectrum of the wide spectral region of the MoS 2 NSs and PtNPs/MoS 2 NSs, respectively.The relatively characteristic peaks of the elements are also illustrated in Figure2a.As seen in Figure2a, when compared with MoS 2 NSs, the Pt 4f peak of PtNPs/MoS 2 NSs can be clearly seen.Figure2b-dshow the chemical states of Pt 4f, Mo 3d, and the S 2p orbitals of the PtNPs/MoS 2 NSs, respectively.Figure2bshows the high-resolution Pt 4f spectra, where the main peaks at 71.2 and 74.5 eV correspond to Pt 0 4f 7/2 and Pt 0 4f 5/2 of the metallic Pt.The smaller peaks at the higher binding energies of Pt 2+ 4f 7/2 at 72.0 eV, Pt 2+ 4f 5/2 at 75.3 eV, Pt 4+ 4f 7/2 at 73.6 eV, and Pt 4+ 4f 5/2 at 76.9 eV, correspond to PtO and PtO 2 , respectively[19].Figure2cshows the high-resolution Mo 3d spectra, where the two main peaks of Mo 3d spectra are at 229.2 and 232.3 eV, which correspond to the Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 orbitals, and the revealed Mo 4+ states indicated that the major formation was MoS 2[20].The other peaks of Mo 5+ 3d 5/2 at 230.3 eV, Mo 5+ 3d 3/2 at 233.4 eV, Mo 6+ 3d 5/2 at 231.4 eV, Mo 6+ 3d 3/2 at 234.5 eV, Mo 6+ 3d 5/2 at 232.5 eV, and Mo 6+ 3d 3/2 at 235.6 eV, corresponded to the minority products of Mo 2 S 5 , MoS 3 , and MoO 3 , respectively[20,21].The relatively weak peaks of MoO 3 may come from the oxidation of Mo atoms at the edges, or defects on the crystal planes of the MoS 2 NSs during the chemical reaction[22].Figure2dshows the high-resolution S 2p spectra, the S 2− 2p 3/2 and S 2− 2p 1/2 peaks at 161.9 and 163.1 eV, which corresponds to MoS 2 .The S 2 2− 2p 3/2 and S 2 2− 2p 1/2 of binding energy at 163.2 eV and 164.4 eV might represent the intermediate products of Mo 2 S 5 and the MoS 3 with a formula of [Mo 4+ (S 2 ) 2− S 2− ] [23,24].Pt is predominantly present as metallic Pt along with surface oxides and hydroxide, as is normally observed in the case of Pt NPs[19].Therefore, as above-mentioned, the oxidation groups on the crystal planes of the MoS 2 NSs probably act as nucleation sites to reduce the precursor Pt 4+ to Pt 2+ and Pt 0 in the mixture solution (H 2 PtCl 6 -EG-water) for the subsequent formation of PtNPs by EG reduction.