Next Article in Journal
Shape-Controlled Synthesis of Au Nanostructures Using EDTA Tetrasodium Salt and Their Photothermal Therapy Applications
Next Article in Special Issue
High-Efficiency Visible Transmitting Polarizations Devices Based on the GaN Metasurface
Previous Article in Journal
Thermal Calcination-Based Production of SnO2 Nanopowder: An Analysis of SnO2 Nanoparticle Characteristics and Antibacterial Activities
Previous Article in Special Issue
Study on the Coupling Mechanism of the Orthogonal Dipoles with Surface Plasmon in Green LED by Cathodoluminescence
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Ti Porous Film-Supported NiCo2S4 Nanotubes Counter Electrode for Quantum-Dot-Sensitized Solar Cells

1
Shaanxi Key Laboratory of Industrial Automation, School of Physics and Telecommunication Engineering, Shaanxi University of Technology, Hanzhong 723001, China
2
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
3
Henan Key Laboratory of Photovoltaic Materials, College of Physics and Materials Science, Henan Normal University, Xinxiang 453000, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2018, 8(4), 251; https://doi.org/10.3390/nano8040251
Submission received: 22 March 2018 / Revised: 13 April 2018 / Accepted: 13 April 2018 / Published: 17 April 2018
(This article belongs to the Special Issue Optoelectronic Nanodevices)

Abstract

:
In this paper, a novel Ti porous film-supported NiCo2S4 nanotube was fabricated by the acid etching and two-step hydrothermal method and then used as a counter electrode in a CdS/CdSe quantum-dot-sensitized solar cell. Measurements of the cyclic voltammetry, Tafel polarization curves, and electrochemical impedance spectroscopy of the symmetric cells revealed that compared with the conventional FTO (fluorine doped tin oxide)/Pt counter electrode, Ti porous film-supported NiCo2S4 nanotubes counter electrode exhibited greater electrocatalytic activity toward polysulfide electrolyte and lower charge-transfer resistance at the interface between electrolyte and counter electrode, which remarkably improved the fill factor, short-circuit current density, and power conversion efficiency of the quantum-dot-sensitized solar cell. Under illumination of one sun (100 mW/cm2), the quantum-dot-sensitized solar cell based on Ti porous film-supported NiCo2S4 nanotubes counter electrode achieved a power conversion efficiency of 3.14%, which is superior to the cell based on FTO/Pt counter electrode (1.3%).

1. Introduction

In recent years, the quantum-dot-sensitized solar cell (QDSSC) has aroused a widespread attention due to the large absorption coefficient, multiple exciton generation, and the tunable absorption spectrum based on quantum confinement effect. QDSSC is composed of three parts: QD-sensitized TiO2 or ZnO photoanode, electrolyte (Sn2−/S2−), and counter electrode (CE) [1]. As one of the most important parts in QDSSC, CE is used as a catalyst to reduce Sn2−/S2− after the electron injection from external circuit so that QD can be regenerated. For achieving this function, CE materials should provide superior catalytic activity and high chemical stability against the corrosive polysulfide electrolyte.
As is well known, Pt is a poor electrocatalyst for reducing Sn2−/S2− due to its strong chemisorption with S2− ions, resulting in the serious corrosion and much higher overpotentials for electrolyte regeneration. Therefore, the QDSSC with Pt CE show a low fill factor (FF) and power conversion efficiency (PCE) [2], and the high cost is another disadvantage. Recently, some Pt-free CE materials with low cost, such as carbon materials [3,4,5], conductive polymers [6], and inorganic compound [7], have been widely developed and demonstrated to have attractive performances. Of these Pt-free CE materials, the transition-metal sulfides, such as CuS, FeS, CoS, NiS, and NiCo2S4 [8,9,10,11,12], have attracted tremendous interest. Especially, NiCo2S4 contains higher electrochemical characteristics compared with binary NiS and CoS. NiCo2S4 has been regarded as one of the most potential electrode materials for a super-capacitor for are several main reasons, as follows: (1) NiCo2S4 has a high electric conductivity, which is approximately 100 times higher than that of NiCo2O4 and higher than that of NiS and CoS [13]; (2) NiCo2S4 has good mechanical and thermal stability and two different metal cations (Co and Ni) supplying richer redox reactions, leading to better electrochemical performance [14,15,16,17]. Up until now, NiCo2S4 as an efficient CE has also been widely used in dye-sensitized solar cell (DSSC). Shi et al. reported that NiCo2S4 nanosheet films were used as a CE of DSSC, the photocurrent density is increased by 3 mA/cm2 [18]. Huo et al. fabricated the flower-like NiCo2S4/NiS micro-spheres, then the NiCo2S4/NiS was coated on FTO (fluorine doped tin oxide) conductive glass as a CE for DSSC and the PCE of DSSC increased by 8.24% compared with that of the DSSC based on Pt CE [19]. A compact NiCo2S4 film with a thickness of 40 nm and a cross-linked network of NiCo2S4 nanosheet film coated FTO conductive glass were used as CEs for DSSC [20,21], and the DSSC with NiCo2S4 CE exhibited higher PCE compared with that of Pt CE-based DSSC. In addition, one-dimensional (1D) nanomaterials (e.g., nanorod, nanowire, nanotube) with direct electrical pathways show excellent application prospects in nanoscale electronic devices. One-dimensional NiCo2S4 nanotube arrays were used for supercapacitors [22] and 1D Co9S8 hollow nanoneedle arrays were used as CE for QDSSC [11].
The substrate supporting CE materials is also very important for the performance of DSSC; it should have large surface area, excellent conductivity, and good corrosion resistance to the electrolyte. Owing to the poor conductivity of metal sulfides, improving the catalytic activity of CE by increasing the thickness of CE is limited. In order to solve this problem, the porous microstructure with a large surface is used to load CE materials. For example, porous SnO2, ZnO, TiO2, and carbon and nickel foam were used as catalyst support [7,9,23,24]. The porous Cu2S and FeS CEs were directly prepared on Cu and Fe substrates by in situ corrosion method [25], but they can easily peeled off from the substrates because of the reaction of Cu and Fe substrates with S2− in electrolyte. In addition, it is important that the electrons from the external circuit quickly transfer to CE materials and reduce the electrolyte. So far, various substrates have been used, such as FTO and ITO (indium doped tin oxide) conductive glass [3,4,5,6], C fiber cloth and C paper [16], Ti mesh [26], and abovementioned Fe and Cu. Generally, two methods are available to prepare CE: one is the in situ growth method, the other is the ex situ coating method. For the former, there is a good adhesion between the substrate and CE materials, but the load of CE materials is limited. For the latter, although CE materials can be increased by increasing the coating several times, the adhesion is poor. In order to solve the adhesion problem, the adhesive was added into the CE materials [27], but it increased the electron transfer resistance.
In this paper, we have designed NiCo2S4 nanotubes supported on Ti porous film (Ti-PF) as CE for QDSSC. Firstly, Ti-PF was prepared by acid etching, then NiCo2S4 nanotubes were synthesized on Ti-PF by two-step hydrothermal method. NiCo2S4 nanotubes not only provide the effective path for electron transport but also have more electroactive sites for reducing polysulfide electrolyte. In addition, Ti-PF/NiCo2S4 CE exhibits lower charge-transfer resistance compared with FTO/Pt CE owing to the high conductivity of Ti and the porous structure increases the load of CE materials and improves the stability via the pore-wall. As a result, the PCE (3.14%) of QDSSC based on Ti-PF/NiCo2S4 CE is higher than that (1.3%) of QDSSC based on Pt CE.

2. Experimental Section

In this work, NiCo2S4 nanotubes supported on Ti-PF were prepared by the following three steps (Figure 1): (1) Ti-PF was prepared by acid etching as the substrates (Step 1); (2) Ni−Co precursor ((Ni,Co)2(CO3)(OH)2) nanorods were hydrothermally grown (Step 2); and (3) (Ni,Co)2(CO3)(OH)2 nanorods were converted into NiCo2S4 nanotubes in Na2S solution via an anion-exchange reaction (Step 3).

2.1. Preparation of Ti-PF

Firstly, Ti sheets with high purity (TA1, 99.9%) and 0.2 mm thickness were washed in the acetone and ethanol using an ultrasonic bath for 30 min, respectively, and rinsed with deionized water. Then, the cleaned Ti sheets were immersed in 90 mL of HCl solution (25 wt·%) for 24–72 h at room temperature. Next, Ti-PF sheets were washed thoroughly with deionized water until the pH was close to 7 and further dried in air.

2.2. Fabrication of NiCo2S4 Nanotubes

NiCo2S4 nanotubes were prepared by two-step hydrothermal method according to the literature [14,28]. All the reagents were of analytical grade in this experiment and purchased from Sinopharm (Beijing, China). Firstly, 4 mmol CoCl2·6H2O, 2 mmol NiCl2·6H2O, and 12 mmol urea were dissolved in 35 mL deionized water and stirred to form a pink homogeneous solution. Subsequently, the mixed-solution and Ti-PF sheets were transferred into 50 mL Teflon-lined stainless-steel autoclave and then heated at 120 °C for 10 h. After being cooled to room temperature, Ti-PF sheets with pink product were washed with deionized water and ethanol and then dried at 60 °C in air for 10 h. The (Ni,Co)2(CO3)(OH)2 nanorods were obtained. In the next step, the (Ni,Co)2(CO3)(OH)2 was transformed into NiCo2S4 by a hydrothermal process in 0.1 M Na2S·9H2O solution at 120 °C for 10 h. After being cooled to room temperature, Ti-PF sheets with black products were washed with deionized water and ethanol and then dried in air at 60 °C. The NiCo2S4 nanotube CE was obtained.

2.3. Assembly of QDSSCs

The QDSSC was fabricated using a screen printing technique with the home-made TiO2/ZnO paste [29]. Firstly, the TiO2 compact layer was prepared via a spin-coating method and followed by calcination at 400 °C for 0.5 h. Subsequently, the mesoscopic photoanodes were prepared through four circulars screen printing of TiO2/ZnO paste on FTO conductive glass with TiO2 compact layer and sintered at 450 °C for 0.5 h. The active area of the film is 0.25 cm2. Next, the growth of ZnO nanowires was performed in a procedure similar to that in our previous paper. Lastly, CdS/CdSe/ZnS QDs were deposited by successive ionic layer adsorption and reaction (SILAR) method [1,29]. A polysulfide electrolyte used in the QDSSCs and the symmetric cells was prepared by dissolving 1 M Na2S, 1 M S and 0.2 M KCl in a methanol/water solution (7:3, v/v).

2.4. Characterization and Measurements

Field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan) and transmission electron microscope (TEM, Tecnai G2F20, FEI, Columbus, OH, USA) were carried out to investigate the morphology and composition. The X-ray diffraction (XRD) patterns were obtained by D/max-2400 X-ray diffraction spectrometer (Rigaku, Akishima-Shi, Japan) with Cu Ka radiation at 40 kV and 100 mA. The current-voltage (I–V) characterization was performed under AM 1.5 G simulated sunlight (100 mW/cm2) and recorded by a Keithley 2400 Source Meter (Keithley Instruments, Inc., Cleveland, OH, USA). The cyclic voltammetry (CV), Tafel polarization curves, and electrochemical impedance spectroscopy (EIS) were performed in the symmetric cells on the workstation (CHI660E, CH Instruments Ins., Shanghai, China). These tests were used to investigate the electrocatalytic ability of CE towards the reduction of Sn2−/S2− electrolyte and the electronic transport properties of CE. In CV tests, the scanning potential range is from −1.2 V to 1 V with a scan rate of 100 mV/s. EIS curves were recorded at bias voltage of 0 V over a frequency range of 0.1 Hz to 1 MHz with AC amplitude of 10 mV, all EIS spectra were analyzed by ZsimpWin software. Polarization Tafel curves were recorded from −0.6V to 0.6 V at the scan rate of 10 mV/s.

3. Results and Discussion

3.1. Morphology of Ti-PF

Figure 2 shows the SEM images for Ti-PF with different etching time in HCl. Generally, metal Ti is stable in low concentration of HCl at room temperature. However, Ti is slowly etched when HCl concentration is greater than 20%. In our experiment, HCl solution with concentration of 25% is used to etch Ti sheets at room temperature and the morphology of Ti-PF is controlled by adjusting the etching time. Figure 2a–f show the morphologies of Ti-PF with etching 24 h, 48 h, and 72 h, respectively. It can be seen clearly that with the increase of etching time, the porous structure has changed significantly. When the etching time is 24 h, the holes with the size range of about 5–10 μm exist only in very few places (Figure 2a,b). When the etching time increases to 48 h, the holes with the size range of 10–20 μm are uniformly formed on the surface of Ti sheet (Figure 2c,d). With an increase in etching time (up to 72 h), the holes disappeared completely (Figure 2e) and the shallow pits with the size of below 5 μm were formed (Figure 2f). In this experiment, Ti-PF plays three roles to improve the catalytic activity of CE to polysulfide electrolyte: (1) metal Ti provides a fast electronic transmission channel, (2) the porous structure gives a large surface area and thus increases the load of CE materials, and (3) the deeper holes in which NiCo2S4 nanotubes are limited by the wall increase the stability of CE. Therefore, the Ti-PF etched for 48 h is most suitable for using as the CE substrate.

3.2. Morphology of NiCo2S4 Nanotubes

The conversion of (Ni,Co)2(CO3)(OH)2 nanorods into NiCo2S4 nanotubes can be explained by the anion-exchange reaction mechanism [22,30,31]. Firstly, S2- in the Na2S solution exchanges with CO32− and OH on the surface of (Ni, Co)2(CO3)(OH)2 nanorods to form NiCo2S4, CO32−, and OH- react with H+ in the solution to produce CO2 and H2O. At the same time, the internal (Ni,Co)2(CO3)(OH)2 diffuse spontaneously to the surface of the nanorod, where it provides a source of (Ni,Co)2(CO3)(OH)2 for further anion exchange. The continuous outward diffusion results in the generation of void space inside the nanorod. When (Ni,Co)2(CO3)(OH)2 has been completely converted into NiCo2S4, nanorods become nanotubes.
To illustrate the morphology of as-synthesized samples, SEM measurements were performed. Figure 3a,b,e show the representative low-magnification and high-magnification SEM images of (Ni,Co)2(CO3)(OH)2 nanorods, respectively. Figure 3c,b,f correspond to the low-magnification and high-magnification SEM images of NiCo2S4 nanotubes, respectively. In Figure 3a–d, (Ni,Co)2(CO3)(OH)2 nanorods and NiCo2S4 nanotubes were homogeneously deposited on Ti-PF, suggesting that the two-hydrothermal method is favorable in forming this structure. Moreover, the NiCo2S4 nanotubes were well preserved during sulfurization process. It can be seen that the (Ni,Co)2(CO3)(OH)2 array film is composed by many multi-directional nanorods due to the nucleation sites from the wall of holes and there is considerable inter-nanorod space, it will help electrolyte full contact with CE materials at the bottom, improving the utilization rate of the CE materials. A rough comparison between Figure 3b,d the diameters of NiCo2S4 nanotubes are larger than that of (Ni,Co)2(CO3)(OH)2 nanorods, owing to the Ni and Co ions diffusion from inside to outside of the nanorod during an anion-exchange process. As shown in Figure 3e,f (Ni,Co)2(CO3)(OH)2 nanorods with a smooth surface and solid structure and NiCo2S4 nanotubes with a rough surface are clearly seen from the damaged film.
The detailed structure of NiCo2S4 nanotube scraped from Ti-PF sheet was further confirmed by TEM, as shown in Figure 4. From Figure 4a, the nanotube structure and the porous wall can be evidently seen, indicating the successful preparation of NiCo2S4 nanotubes on Ti-PF. By a closer examination of the wall in Figure 4b, it is found that NiCo2S4 nanotube is composed of many nanoparticles with a size of about 25 nm (marked with red line) and numerous pores locate at the nanotube. The NiCo2S4 nanotube with a rough surface (Figure 4a–c) and a thin wall of about 25 nm (Figure 4c) effectively increases the electroactive sites. The nanotube structure can greatly enhance the electrolyte penetration and improve the performance of cells. In Figure 4c, the corresponding selected area electron diffraction (SAED) pattern indicates the polycrystalline nature of NiCo2S4 nanotubes and the diffraction rings can be readily indexed to the (111), (220), (311), (400), (511), and (440) planes of NiCo2S4 phase. In addition, Figure 4d reveals that the lattice spacings are about 0.51 nm, 0.284 nm, and 0.234 nm, which can be assigned to the (111), (311), and (400) crystal planes of the cubic NiCo2S4 phase, respectively, indicating the successful formation of crystalline NiCo2S4.
The as-synthesized NiCo2S4 nanotubes were further confirmed by the XRD and electron energy loss spectroscopy (EELS) elemental mapping. In order to investigate the effect of annealing treatment on the catalytic activity of NiCo2S4 CE and the performance of cells, NiCo2S4 CE was annealed at 400 °C for 30 min in the nitrogen atmosphere (NiCo2S4-an). Figure 5a shows XRD images for Ti-PF, NiCo2S4, and NiCo2S4-an on Ti-PF. The diffraction peaks located at 31.6°, 38.3°, 50.5°, and 55.3° can be indexed to the (311), (400), (511), and (440) planes of the cubic phase NiCo2S4 (JCPDS 20-0782), which is consistent with SAED analysis. There are also two strong peaks at 29.84° and 52.0°, which may correspond to the (311) and (440) planes of the cubic phase Co9S8 (JCPDS no.19-0364). The existence of Co9S8 phase is because that (Ni,Co)2(CO3)(OH)2 nanorods were incompletely sulfurized in Na2S solution, which was verified by many reports [14,32]. It can also be seen that the intensity of (311) and (440) diffraction peaks of NiCo2S4 increased after annealing, indicating an increase in the crystallinity. Moreover, the TEM and EELS mapping images (Figure 5b) indicate that the elements (Ni, Co, and S) are uniformly distributed in the NiCo2S4 nanotube.

3.3. Electrochemical Properties of CEs

To investigate the electrochemical properties of Pt, NiCo2S4, and NiCo2S4-an CEs, CV test of a symmetrical cell was carried out, as shown in Figure 6a. The peaks explain the catalytic reaction at the interface between CE and electrolyte as follows Sn2− + e → nS2−. From the CV curves, NiCo2S4 and NiCo2S4-an CEs present a similar shape with two typical pairs of redox peaks [16,21]. As a matter of fact, the reduction peak of the left pair is assigned to the reaction Sn2 + e → nS2 and the right one is assigned as S + 2ne → Sn2− [19,20,33]. The role of the CE in a QDSSC is to catalyze the reduction of Sn2− to S2− ions in the polysulfide electrolyte, so the left pair of redox peaks is directly related to the catalytic activity of CE, the positive and negative peaks correspond to the oxidation of S2− and the reduction of Sn2−, respectively [34,35]. However, because of high over-potential, Pt CE has only one pair of redox peaks, which correspond to the oxidation of S2− and the reduction of Sn2− [36]. The higher current density of left cathodic peak indicates that the CE has an excellent electrocatalytic activity for the reduction of Sn2− to nS2− [17]. It can be seen that NiCo2S4 and NiCo2S4-an CEs show higher current than Pt CE and the reduction current density of NiCo2S4-an CE is the biggest. This results indicate that the NiCo2S4 CE is expected to enhance QD regeneration and photoelectron generation, thus beneficial for improving QDSSC’s photocurrent, and the annealing treatment enhanced the crystallinity of NiCo2S4 CE, which increases the reduction current.
Tafel polarization technique is an important method to evaluate the catalytic activity of CEs. Theoretically, the Tafel curve includes the diffusion, Tafel and polarization zones at the high-, middle-, and low-potential areas, respectively. In Tafel analysis, the exchange current density (J0) (Tafel zone) and the limiting diffusion current density (Jlim) (diffusion zone) are two key parameters to evaluate the electrocatalytic activity of CEs. Tafel polarization curves of Pt, NiCo2S4, and NiCo2S4-an CEs are shown in Figure 6b. It can be seen that, in the Tafel zone, the slopes of the anodic or cathodic branches are in the order of NiCo2S4-an > NiCo2S4 > Pt. A larger slope indicates a higher J0. According to the following equation:
J0 = RT/nFRct,
where R, T, F and n are the gas constant, the temperature, Faraday’s constant, and the electron number involved in Sn2−/S2− redox couple, respectively [35]; the charge-transfer resistance (Rct) can be calculated by J0 values. The change trends of the Rct is NiCo2S4-an < NiCo2S4 < Pt, which is consistent with the EIS results. In addition, Jlim derived from the horizontal part of the curve at high potential is also closely related to the catalytic activity of CEs, which is given by equation:
D = L Jlim/2nFC,
where D, L, F, C, and n are the diffusion coefficient of the polysulfide, the electrolyte thickness, the Faraday constant, the polysulfide concentration, and the number of electrons involved in the reduction of disulphide at the counter electrode, respectively [37]. It can be noticed that the change trend of Jlim is consistent with CV results, suggesting the D of redox couple in the electrolyte increases with enhanced electrocatalytic activity of CEs.
To further understand the reason for the good performance of the as-prepared CEs, EIS was carried out using the symmetrical cells, as shown in Figure 6c, and the corresponding parameters are shown in Table 1. Figure 6c shows Nyquist plots of Pt, NiCo2S4, and NiCo2S44-an CEs and the insets of Figure 6c are the equivalent circuit and the magnified Nyquist impedance. In the equivalent circuit, Rs represents the series resistance including the sheet resistance of the substrates (FTO and Ti sheet) and the contact resistance of the symmetrical cell, which is mainly correlated to electron transfer rates to the interface of CE/electrolyte and it can be estimated from the intercept on the real axis at the high frequency. The intercept of the middle frequency semicircle on the real axis represents Rct at the interface between CE and electrolyte. The Rct, which is closely related to the electrocatalytic activity and the reaction kinetics of the CE, is an important parameter to determine the FF of cell [36,38,39]. Generally, because of the symmetrical structure, Rct1 and Rct2 at the two CE/electrolyte interfaces are equal (Rct1 = Rct2), so every Nyquist plot has one semicircle [35,40]. The obtained impedance spectra are fitted by Z-View software, as shown in Table 1. The Rs values of Pt, NiCo2S4 and NiCo2S4-an CEs are 8.639 Ω, 3.139 Ωand 3.01 Ω, respectively. Among them, the Rs values of NiCo2S4, and NiCo2S4-an CEs are close, which may be ascribed to the same Ti-PF substrates. The Rs of Pt CE is much higher than that of NiCo2S4 and NiCo2S4-an CEs, which is attributed to the strong chemisorption of S2− ions on Pt. Rct directly reflects the electrochemical reaction at CE/electrolyte interface, Rct values of Pt, NiCo2S4, and NiCo2S4-an CEs are 6860 Ω, 67.47 Ω, and 33.31 Ω, respectively; this means that it is easier for charges transfer through the NiCo2S4-an/electrolyte interface than Pt/electrolyte and NiCo2S4/electrolyte interfaces. Thus, it is anticipated that the QDSSC with Ti-PF-supported NiCo2S4 nanotube CE will show better photovoltaic performance. Furthermore, the proper annealing treatment reduced the Rct of NiCo2S4/electrolyte interface and improved the short-circuit photocurrent density (Jsc) and PCE of QDSSC.
Complete photovoltaic cells based on NiCo2S4 and NiCo2S4–an CEs were fabricated and the cell based on Pt CE is used as the reference. In this study, Pt, NiCo2S4, and NiCo2S4-a CEs were soaked in Sn2−/S2− electrolyte for 24 h and then were used in the complete photovoltaic cells. The complete photovoltaic cells and symmetric cells were fixed by clip and spacer with 90 µm thickness. The photovoltaic curves and the photovoltaic parameters (open-circuit voltage (Voc), Jsc, FF, and PCE) are shown in Figure 6d and Table 1, respectively. From Figure 6d, obviously, the performance of QDSSCs based on NiCo2S4 and NiCo2S4-an CEs are better than that of QDSSC with Pt CE.
The champion QDSSC based on Pt CE has an Voc of 0.489V, a Jsc of 11.76 mA/cm2, a FF of 22.56%, and a PCE of 1.3%. The champion QDSSC with NiCo2S4 CE has a Voc of 0.456V, a Jsc of 13.72 mA/cm2, a FF of 40.6%, and a PCE of 2.54%. The champion QDSSC employing NiCo2S4-an CE has a Voc of 0.489V, a Jsc of 16.68 mA/cm2, a FF of 38.52%, and a PCE of 3.14%. Notably, the PCE increased from 1.3% to 3.14% when Pt CE was replaced with NiCo2S4-an CE. In addition, the average values obtained of the three best cells (up to nine) based on an optimal photoanode and three CEs are given in brackets, as shown in Table 1, and the change trends of the average values of Voc, Jsc, FF, and PCE are consistent with the champion QDSSCs. This improvement in the cell performance originates from the significant increases in both Jsc and FF, which closely related to the higher electrocatalytic ability of CE. Furthermore, the QDSSC with NiCo2S4-an CE shows a higher Jsc and Voc than that with NiCo2S4 CE and thus obtains a higher PCE. Voc of QDSSC depends upon the difference between the quasi Fermi level of the photoanode and the redox potential of the electrolyte. The annealing treatment improved the crystallinity and the conductivity of NiCo2S4, so the fast charge transfer at CE/electrolyte interface can cause a change in the concentration gradient in the electrolyte solution, which influences the recombination rates at the photoanode/electrolyte interface and consequently the conduction band position of the photoanode. Meanwhile, the high conductivity of NiCo2S4-an CE also increased the photocurrent of cell [41]. J–V parameters are in line with the electrocatalytic ability of CEs discussed in the CV, Tafel polarization, and EIS.

4. Conclusions

In summary, we have prepared Ti-PF by the acid etching technique and Ti-PF supporting NiCo2S4 nanotubes via two-step hydrothermal method; furthermore, Ti-PF supporting NiCo2S4 nanotubes are used as CE in QDSSCs. The morphology of Ti-PF is affected with the etching time. When etching time is 48 h in hydrochloric acid with a weight concentration of 25% at room temperature, the holes are uniformly formed on the surface of Ti sheet, which is most suitable for use as the substrate to support CE materials. SEM, TEM, and XRD results show that the as-synthesized NiCo2S4 nanotube with porous surface is the cubic phase. Using a polysulfide electrolyte in the symmetric cells, Ti-PF/NiCo2S4 CE provided greater electrocatalytic activity (a higher reduction current density, a higher J0 and Jlim) and lower internal resistance (Rs and Rct). Also, Ti-PF/NiCo2S4 was used to fabricate QDSSC, it has a higher performance (Jsc = 16.68 mA/cm2, Voc = 0.489 V, FF = 38.52%, and PCE = 3.14%) than that based on FTO/Pt CE (Jsc = 11.76 mA/cm2, Voc = 0.489 V, FF = 22.56%, and PCE = 1.3%).

Acknowledgments

Scientific Research Project of Education Department of Shaanxi Provincial Government (Grant No. 16JS016), Scientific Research Project of Science and Technology Department of Shaanxi Provincial Government (Grant No. 2014SZS16-K02), Natural Science Foundation of Shaanxi Province (Grant No. 2017JM6090). The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 61176056 and 51572216), the industrial science and technology research project in Shaanxi province (2015GY005), the Key Scientific and Technological Project of Henan Province, China (No. 172102210344), and the Key Program of the Higher Education Institutions of Henan Province in China (grant No. 17A140008).

Author Contributions

Jianping Deng and Minqiang Wang designed the experiments. Jianping Deng and Zhaolin Yuan performed the experiments. Xiaohui Song and Zhi Yang analyzed the data and wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Deng, J.; Wang, M.; Yang, Z.; Yang, Y.; Zhang, P. Preparation of TiO2 Nanoparticles Two-Dimensional Photonic-Crystals: A Novel Scattering Layer of Quantum Dot-Sensitized Solar Cells. Mater. Lett. 2016, 183, 307–310. [Google Scholar] [CrossRef]
  2. Radich, J.G.; Dwyer, R.; Kamat, P.V. Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2/Sn2 at the Counter Electrode. J. Phys. Chem. Lett. 2011, 2, 2453–2460. [Google Scholar] [CrossRef]
  3. Wang, H.; Hu, Y.H. Graphene as a Counter Electrode Material for Dye-Sensitized Solar Cells. Energy Environ. Sci. 2012, 5, 8182–8188. [Google Scholar] [CrossRef]
  4. Batmunkh, M.; Biggs, M.J.; Shapter, J.G. Carbon Nanotubes for Dye-Sensitized Solar Cells. Small 2015, 11, 2963–2989. [Google Scholar] [CrossRef] [PubMed]
  5. Hwang, S.; Batmunkh, M.; Nine, M.J.; Chung, H.; Jeong, H. ChemPhysChem Dye-Sensitized Solar Cell Counter Electrodes Based on Carbon Nanotubes. ChemPhysChem 2015, 16, 53–65. [Google Scholar] [CrossRef] [PubMed]
  6. Jafari, F.; Behjat, A.; Khoshro, A.R.; Ghoshani, M. A Dye-Sensitized Solar Cell Based on Natural Photosensitizers and a PEDOT:PSS/TiO2 Film as a Counter Electrode. Eur. Phys. J. Appl. Phys. 2015, 69, 20502. [Google Scholar] [CrossRef]
  7. Song, X.; Wang, M.; Deng, J.; Ju, Y.; Xing, T.; Ding, J.; Yang, Z.; Shao, J. ZnO/PbS Core/Shell Nanorod Arrays as Efficient Counter Electrode for Quantum Dot-Sensitized Solar Cells. J. Power Sources 2014, 269, 661–670. [Google Scholar] [CrossRef]
  8. Yang, Z.; Chen, C.-Y.; Liu, C.-W.; Li, C.-L.; Chang, H.-T. Quantum Dot–Sensitized Solar Cells Featuring CuS/CoS Electrodes Provide 4.1% Efficiency. Adv. Energy Mater. 2011, 1, 259–264. [Google Scholar] [CrossRef]
  9. Geng, H.; Zhu, L.; Li, W.; Liu, H.; Quan, L.; Xi, F.; Su, X. FeS/Nickel Foam as Stable and Efficient Counter Electrode Material for Quantum Dot Sensitized Solar Cells. J. Power Sources 2015, 281, 204–210. [Google Scholar] [CrossRef]
  10. Faber, M.S.; Park, K.; Caban-Acevedo, M.; Santra, P.K.; Jin, S. Earth-Abundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1843–1849. [Google Scholar] [CrossRef] [PubMed]
  11. Chen, C.; Ye, M.; Zhang, N.; Wen, X.; Zheng, D.; Lin, C. Preparation of Hollow Co9S8 Nanoneedle Arrays as Effective Counter Electrodes for Quantum Dot-Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 6311–6314. [Google Scholar] [CrossRef]
  12. Manjceevan, A.; Bandara, J. Optimization of Performance and Stability of Quantum Dot Sensitized Solar Cells by Manipulating the Electrical Properties of Different Metal Sulfide Counter Electrodes. Electrochim. Acta 2017, 235, 390–398. [Google Scholar] [CrossRef]
  13. Xiao, J.; Wan, L.; Yang, S.; Xiao, F.; Wang, S. Design Hierarchical Electrodes with Highly Conductive NiCo2S4 Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors. Nano Lett. 2014, 14, 831–838. [Google Scholar] [CrossRef] [PubMed]
  14. Yan, M.; Yao, Y.; Wen, J.; Long, L.; Kong, M.; Zhang, G.; Liao, X.; Yin, G.; Huang, Z. Construction of a Hierarchical NiCo2S4@PPy Core–Shell Heterostructure Nanotube Array on Ni Foam for a High-Performance Asymmetric Supercapacitor. ACS Appl. Mater. Interfaces 2016, 8, 24525–24535. [Google Scholar] [CrossRef] [PubMed]
  15. Peng, S.; Li, L.; Li, C.; Tan, H.; Cai, R.; Yu, H.; Mhaisalkar, S.; Srinivasan, M.; Ramakrishna, S.; Yan, Q. In Situ Growth of NiCo2S4 Nanosheets on Graphene for High-Performance Supercapacitors. Chem. Commun. 2013, 49, 10178–10180. [Google Scholar] [CrossRef] [PubMed]
  16. Chen, L.; Zhou, Y.; Dai, H.; Yu, T.; Liu, J.; Zou, Z. One-Step Growth of CoNi2S4 Nanoribbons on Carbon Fibers as Platinum-Free Counter Electrodes for Fiber-Shaped Dye-Sensitized Solar Cells with High Performance: Polymorph-Dependent Conversion Efficiency. Nano Energy 2015, 11, 697–703. [Google Scholar] [CrossRef]
  17. Shi, Z.; Deng, K.; Li, L. Pt-Free and Efficient Counter Electrode with Nanostructured CoNi2S4 for Dye-Sensitized Solar Cells. Sci. Rep. 2015, 5, 9317. [Google Scholar] [CrossRef] [PubMed]
  18. Shi, Z.; Lu, H.; Liu, Q.; Cao, F.; Guo, J.; Deng, K.; Li, L. Efficient p-Type Dye-Sensitized Solar Cells with All-Nano-Electrodes:NiCo2S4 Mesoporous Nanosheet Counter Electrodes Directly Converted from NiCo2O4 Photocathodes. Nanoscale Res. Lett. 2014, 9, 608. [Google Scholar] [CrossRef] [PubMed]
  19. Huo, J.; Wu, J.; Zheng, M.; Tu, Y.; Lan, Z. Flower-Like Nickel Cobalt Sulfide Microspheres Modified with Nickel Sulfide as Pt-Free Counter Electrode for Dye-Sensitized Solar Cells. J. Power Sources 2016, 304, 266–272. [Google Scholar] [CrossRef]
  20. Huang, N.; Zhang, S.; Huang, H.; Liu, J.; Sun, Y.; Sun, P.; Bao, C.; Zheng, L.; Sun, X.; Zhao, X. Pt-Sputtering-Like NiCo2S4 Counter Electrode for Efficient Dye-Sensitized Solar Cells. Electrochim. Acta 2016, 192, 521–528. [Google Scholar] [CrossRef]
  21. Khoo, S.Y.; Miao, J.; Yang, H.; He, Z.; Leong, K.C.; Liu, B.; Thatt, T.; Tan, Y. One-Step Hydrothermal Tailoring of NiCo2S4 Nanostructures on Conducting Oxide Substrates as an Efficient Counter Electrode in Dye-Sensitized Solar Cells. Adv. Mater. Interfaces 2015, 2, 1500384. [Google Scholar] [CrossRef]
  22. Chen, H.; Jiang, J.; Zhang, L.; Xia, D.; Zhao, Y.; Guo, D.; Qi, T.; Wan, H. In Situ Growth of NiCo2S4 Nanotube Arrays on Ni Foam for Supercapacitors: Maximizing Utilization Efficiency at High Mass Loading to Achieve Ultrahigh Areal Pseudocapacitance. J. Power Sources 2014, 254, 249–257. [Google Scholar] [CrossRef]
  23. Park, J.T.; Lee, C.S.; Kim, J.H. High Performance Electrocatalyst Consisting of CoS Nanoparticles on an Organized Mesoporous SnO2 Film: Use as Counter Electrodes for Pt-Free, Dye-Sensitized Solar Cells. Nanoscale 2015, 7, 670–678. [Google Scholar] [CrossRef] [PubMed]
  24. Xu, S.; Luo, Y.; Zhong, W.; Xiao, Z.; Luo, Y.; Ou, H. Nanoporous TiO2/SnO2/Poly(3,4-ethylene-dioxythiophene): Polystyrenesulfonate Composites as Efficient Counter Electrode for Dye Sensitized Solar Cells. J. Nanosci. Nanotechnol. 2016, 16, 392–399. [Google Scholar] [CrossRef] [PubMed]
  25. Chen, H.; Zhu, L.; Liu, H.; Li, W. Efficient Iron Sulfide Counter Electrode for Quantum Dots Sensitized Solar Cells. J. Power Sources 2014, 245, 406–410. [Google Scholar] [CrossRef]
  26. Du, J.; Du, Z.; Hu, J.-S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; et al. Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201–4209. [Google Scholar] [CrossRef] [PubMed]
  27. Yang, Y.; Zhu, L.; Sun, H.; Huang, X.; Luo, Y.; Li, D.; Meng, Q. Composite Counter Electrode Based on Nanoparticulate PbS and Carbon Black: Towards Quantum Dot-Sensitized Solar Cells with Both High Efficiency and Stability. ACS Appl. Mater. Interfaces 2012, 4, 6162–6168. [Google Scholar] [CrossRef] [PubMed]
  28. Fu, W.; Zhao, C.; Han, W.; Liu, Y.; Zhao, H.; Ma, Y.; Xie, E. Cobalt Sulfide Nanosheets Coated on NiCo2S4 Nanotube Arrays as Electrode Materials for High-performance Supercapacitors. J. Mater. Chem. A 2015, 3, 10492–10497. [Google Scholar] [CrossRef]
  29. Deng, J.; Wang, M.; Zhang, P.; Ye, W. Preparing ZnO Nanowires in Mesoporous TiO2 Photoanode by an in-Situ Hydrothermal Growth for Enhanced Light-Trapping in Quantum Dots-Sensitized Solar Cells. Electrochim. Acta 2016, 200, 12–20. [Google Scholar] [CrossRef]
  30. Li, R.; Wang, S.; Huang, Z.; Lu, F.; He, T. NiCo2S4@Co(OH)2 Core-Shell Nanotube Arrays in Situ Grown on Ni Foam for High Performances Asymmetric Supercapcitors. J. Power Sources 2016, 312, 156–164. [Google Scholar] [CrossRef]
  31. Park, J.; Zheng, H.; Jun, Y.; Alivisatos, A.P. Hetero-Epitaxial Anion Exchange Yields Single-Crystalline Hollow Nanoparticles. J. Am. Chem. Soc. 2009, 131, 13943–13945. [Google Scholar] [CrossRef] [PubMed]
  32. Lu, F.; Zhou, M.; Li, W.; Weng, Q.; Li, C.; Xue, Y.; Jiang, X.; Zeng, X.; Bando, Y.; Golberg, D. Engineering Sulfur Vacancies and Impurities in NiCo2S4 Nanostructures Toward Optimal Supercapacitive Performance. Nano Energy 2016, 26, 313–323. [Google Scholar] [CrossRef]
  33. Su, A.-L.; Lu, M.-N.; Chang, C.-Y.; Wei, T.-C.; Lin, J.-Y. Scalable Fabrication of Efficient NiCo2S4 Counter Electrodes for Dye-sensitized Solar Cells Using a Facile Solution Approach. Electrochim. Acta 2016, 222, 1410–1416. [Google Scholar] [CrossRef]
  34. Du, F.; Yang, Q.; Qin, T.; Li, G. Morphology-Controlled Growth of NiCo2O4 Ternary Oxides and Their Application in Dye-Sensitized Solar Cells as Counter Electrodes. Sol. Energy 2017, 146, 125–130. [Google Scholar] [CrossRef]
  35. Wang, S.; Dong, W.; Fang, X.; Zhou, S.; Shao, J.; Deng, Z.; Tao, R.; Zhang, Q.; Hu, L.; Zhu, J. Enhanced Electrocatalytic Activity of Vacuum Thermal Evaporated CuxS Counter Electrode for Quantum Dot-Sensitized Solar Cells. Electrochim. Acta 2015, 154, 47–53. [Google Scholar] [CrossRef]
  36. Dennyson Savariraj, A.; Viswanathan, K.K.; Prabakar, K. CuS Nano-flakes and Nano-platelets as Counter Electrode for Quantum Dots Sensitized Solar Cells. Electrochim. Acta 2014, 149, 364–369. [Google Scholar] [CrossRef]
  37. Zakeeruddin, S.M.; Gratzel, M. Solvent-Free Ionic Liquid Electrolytes for Mesoscopic Dye Sensitized Solar Cells. Adv. Funct. Mater. 2009, 19, 2187–2202. [Google Scholar] [CrossRef]
  38. Meng, K.; Surolia, P.K.; Byrne, O.; Thampi, K.R. Efficient CdS Quantum Dot Sensitized Solar Cells Made Using Novel Cu2S Counter Electrode. J. Power Sources 2014, 248, 218–223. [Google Scholar] [CrossRef]
  39. Li, D.-M.; Cheng, L.-Y.; Zhang, Y.-D.; Zhang, Q.-X.; Huang, X.-M.; Luo, Y.-H.; Meng, Q.-B. Development of Cu2S/Carbon Composite Electrode for CdS/CdSe Quantum Dot Sensitized Solar Cell Modules. Sol. Energy Mater. Sol. C 2014, 120, 454–461. [Google Scholar] [CrossRef]
  40. Liberatore, M.; Decker, F.; Burtone, L.; Zardetto, V.; Brown, T.M.; Reale, A.; Di Carlo, A. Using EIS for Diagnosis of Dye-Sensitized Solar Cells Performance. J. Appl. Electrochem. 2009, 39, 2291. [Google Scholar] [CrossRef]
  41. Liu, F.; Zhu, J.; Li, Y.; Wei, J.; Lv, M.; Xu, Y.; Zhou, L.; Hu, L.; Dai, S. Earth-Abundant Cu2SnSe3 Thin Film Counter Electrode for High Efficiency Quantum Dot-Sensitized Solar Cells. J. Power Sources 2015, 292, 7–14. [Google Scholar] [CrossRef]
Figure 1. Schematic diagram to illustrate the preparation process of NiCo2S4 nanotubes on Ti porous film (Ti-PF).
Figure 1. Schematic diagram to illustrate the preparation process of NiCo2S4 nanotubes on Ti porous film (Ti-PF).
Nanomaterials 08 00251 g001
Figure 2. SEM images for Ti-PF from different etching time. (a,b)24 h, (c,d)48 h, and (e,f )72 h.
Figure 2. SEM images for Ti-PF from different etching time. (a,b)24 h, (c,d)48 h, and (e,f )72 h.
Nanomaterials 08 00251 g002aNanomaterials 08 00251 g002b
Figure 3. SEM images for (Ni,Co)2(CO3)(OH)2 nanorods (a,b,e) and NiCo2S4 nanotubes (c,d,f) on Ti-PF.
Figure 3. SEM images for (Ni,Co)2(CO3)(OH)2 nanorods (a,b,e) and NiCo2S4 nanotubes (c,d,f) on Ti-PF.
Nanomaterials 08 00251 g003
Figure 4. (ac) TEM and (inset) SAED and (d) HRTEM images of NiCo2S4 nanotube.
Figure 4. (ac) TEM and (inset) SAED and (d) HRTEM images of NiCo2S4 nanotube.
Nanomaterials 08 00251 g004
Figure 5. (a) XRD images for Ti-PF, NiCo2S4, and NiCo2S4-an on Ti-PF; (b) TEM image and the corresponding EELS elemental mapping images of a single NiCo2S4 nanotube.
Figure 5. (a) XRD images for Ti-PF, NiCo2S4, and NiCo2S4-an on Ti-PF; (b) TEM image and the corresponding EELS elemental mapping images of a single NiCo2S4 nanotube.
Nanomaterials 08 00251 g005
Figure 6. (a) Cyclic voltammetry (CV); (b) Tafel curves; and (c) EIS of the symmetric cells with Pt, NiCo2S4, and NiCo2S4-an CEs and (d) J–V characteristics for QDSSCs based on Pt, NiCo2S4, and NiCo2S4-an CEs, respectively.
Figure 6. (a) Cyclic voltammetry (CV); (b) Tafel curves; and (c) EIS of the symmetric cells with Pt, NiCo2S4, and NiCo2S4-an CEs and (d) J–V characteristics for QDSSCs based on Pt, NiCo2S4, and NiCo2S4-an CEs, respectively.
Nanomaterials 08 00251 g006
Table 1. Photovoltaic parameters obtained from J–V curves of quantum-dot-sensitized solar cells (QDSSCs) and EIS parameters of symmetric cells.
Table 1. Photovoltaic parameters obtained from J–V curves of quantum-dot-sensitized solar cells (QDSSCs) and EIS parameters of symmetric cells.
SamplesVoc (V)Jsc (mA/cm2)FF (%)PCE (%)Rs (Ω)Rct (Ω)
Pt0.489 (0.483)11.76 (10.29)22.56 (25.19)1.30 (1.21)8.6396860
NiCo2S40.456 (0.45)13.72 (13.56)40.60 (40.46)2.54 (2.51)3.13967.47
NiCo2S4-an0.489 (0.478)16.68 (15.32)38.52 (38.61)3.14 (2.82)3.01033.31

Share and Cite

MDPI and ACS Style

Deng, J.; Wang, M.; Song, X.; Yang, Z.; Yuan, Z. Ti Porous Film-Supported NiCo2S4 Nanotubes Counter Electrode for Quantum-Dot-Sensitized Solar Cells. Nanomaterials 2018, 8, 251. https://doi.org/10.3390/nano8040251

AMA Style

Deng J, Wang M, Song X, Yang Z, Yuan Z. Ti Porous Film-Supported NiCo2S4 Nanotubes Counter Electrode for Quantum-Dot-Sensitized Solar Cells. Nanomaterials. 2018; 8(4):251. https://doi.org/10.3390/nano8040251

Chicago/Turabian Style

Deng, Jianping, Minqiang Wang, Xiaohui Song, Zhi Yang, and Zhaolin Yuan. 2018. "Ti Porous Film-Supported NiCo2S4 Nanotubes Counter Electrode for Quantum-Dot-Sensitized Solar Cells" Nanomaterials 8, no. 4: 251. https://doi.org/10.3390/nano8040251

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop