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

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%).


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 TiO 2 or ZnO photoanode, electrolyte (S n 2− /S 2− ), and counter electrode (CE) [1]. As one of the most important parts in QDSSC, CE is used as a catalyst to reduce S n 2− /S 2− 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 S n 2− /S 2− due to its strong chemisorption with S 2− 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

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.

Fabrication of NiCo 2 S 4 Nanotubes
NiCo 2 S 4 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 CoCl 2 ·6H 2 O, 2 mmol NiCl 2 ·6H 2 O, 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 (CO 3 )(OH) 2 nanorods were obtained. In the next step, the (Ni,Co) 2 (CO 3 )(OH) 2 was transformed into NiCo 2 S 4 by a hydrothermal process in 0.1 M Na 2 S·9H 2 O 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 NiCo 2 S 4 nanotube CE was obtained.

Assembly of QDSSCs
The QDSSC was fabricated using a screen printing technique with the home-made TiO 2 /ZnO paste [29]. Firstly, the TiO 2 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 TiO 2 /ZnO paste on FTO conductive glass with TiO 2 compact layer and sintered at 450 • C for 0.5 h. The active area of the film is 0.25 cm 2 . 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 Na 2 S, 1 M S and 0.2 M KCl in a methanol/water solution (7:3, v/v).

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/cm 2 ) 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 S n 2− /S 2− 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. 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 (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 NiCo 2 S 4 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.

Morphology of NiCo 2 S 4 Nanotubes
The conversion of (Ni,Co) 2 (CO 3 )(OH) 2 nanorods into NiCo 2 S 4 nanotubes can be explained by the anion-exchange reaction mechanism [22,30,31]. Firstly, S 2in the Na 2 S solution exchanges with CO 3 2− and OH − on the surface of (Ni, Co) 2 (CO 3 )(OH) 2 nanorods to form NiCo 2 S 4 , CO 3 2− , and OHreact with H + in the solution to produce CO 2 and H 2 O. At the same time, the internal (Ni,Co) 2 (CO 3 )(OH) 2 diffuse spontaneously to the surface of the nanorod, where it provides a source of (Ni,Co) 2 (CO 3 )(OH) 2 for further anion exchange. The continuous outward diffusion results in the generation of void space inside the nanorod. When (Ni,Co) 2 (CO 3 )(OH) 2 has been completely converted into NiCo 2 S 4 , 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 (CO 3 )(OH) 2 nanorods, respectively. Figure 3c,b,f correspond to the low-magnification and high-magnification SEM images of NiCo 2 S 4 nanotubes, respectively.
In Figure 3a-d, (Ni,Co) 2 (CO 3 )(OH) 2 nanorods and NiCo 2 S 4 nanotubes were homogeneously deposited on Ti-PF, suggesting that the two-hydrothermal method is favorable in forming this structure. Moreover, the NiCo 2 S 4 nanotubes were well preserved during sulfurization process. It can be seen that the (Ni,Co) 2 (CO 3 )(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 NiCo 2 S 4 nanotubes are larger than that of (Ni,Co) 2 (CO 3 )(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 (CO 3 )(OH) 2 nanorods with a smooth surface and solid structure and NiCo 2 S 4 nanotubes with a rough surface are clearly seen from the damaged film. The detailed structure of NiCo 2 S 4 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 NiCo 2 S 4 nanotubes on Ti-PF. By a closer examination of the wall in Figure 4b, it is found that NiCo 2 S 4 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 NiCo 2 S 4 nanotube with a rough surface (Figure 4a  The as-synthesized NiCo 2 S 4 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 NiCo 2 S 4 CE and the performance of cells, NiCo 2 S 4 CE was annealed at 400 • C for 30 min in the nitrogen atmosphere (NiCo 2 S 4 -an). Figure 5a shows XRD images for Ti-PF, NiCo 2 S 4 , and NiCo 2 S 4 -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 NiCo 2 S 4 (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 Co 9 S 8 (JCPDS no.19-0364). The existence of Co 9 S 8 phase is because that (Ni,Co) 2 (CO 3 )(OH) 2 nanorods were incompletely sulfurized in Na 2 S solution, which was verified by many reports [14,32]. It can also be seen that the intensity of (311) and (440) diffraction peaks of NiCo 2 S 4 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 NiCo 2 S 4 nanotube.

Electrochemical Properties of CEs
To investigate the electrochemical properties of Pt, NiCo 2 S 4 , and NiCo 2 S 4 -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 S n 2− + e − → nS 2− . From the CV curves, NiCo 2 S 4 and NiCo 2 S 4 -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 S n 2− + e − → nS 2− and the right one is assigned as S + 2ne − → S n 2− [19,20,33]. The role of the CE in a QDSSC is to catalyze the reduction of S n 2− to S 2− 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 S 2− and the reduction of Sn 2− , respectively [34,35]. However, because of high over-potential, Pt CE has only one pair of redox peaks, which correspond to the oxidation of S 2− and the reduction of S n 2− [36]. The higher current density of left cathodic peak indicates that the CE has an excellent electrocatalytic activity for the reduction of S n 2− to nS 2− [17]. It can be seen that NiCo 2 S 4 and NiCo 2 S 4 -an CEs show higher current than Pt CE and the reduction current density of NiCo 2 S 4 -an CE is the biggest. This results indicate that the NiCo 2 S 4 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 NiCo 2 S 4 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 (J 0 ) (Tafel zone) and the limiting diffusion current density (J lim ) (diffusion zone) are two key parameters to evaluate the electrocatalytic activity of CEs. Tafel polarization curves of Pt, NiCo 2 S 4 , and NiCo 2 S 4 -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 NiCo 2 S 4 -an > NiCo 2 S 4 > Pt. A larger slope indicates a higher J 0 . According to the following equation: where R, T, F and n are the gas constant, the temperature, Faraday's constant, and the electron number involved in Sn 2− /S 2− redox couple, respectively [35]; the charge-transfer resistance (R ct ) can be calculated by J 0 values. The change trends of the R ct is NiCo 2 S 4 -an < NiCo 2 S 4 < Pt, which is consistent with the EIS results. In addition, J lim 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: 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 J lim 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, NiCo 2 S 4 , and NiCo 2 S 4 4-an CEs and the insets of Figure 6c are the equivalent circuit and the magnified Nyquist impedance. In the equivalent circuit, R s 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 R ct at the interface between CE and electrolyte. The R ct , 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, R ct1 and R ct2 at the two CE/electrolyte interfaces are equal (R ct1 = R ct2 ), 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 R s values of Pt, NiCo 2 S 4 and NiCo 2 S 4 -an CEs are 8.639 Ω, 3.139 Ωand 3.01 Ω, respectively. Among them, the R s values of NiCo 2 S 4 , and NiCo 2 S 4 -an CEs are close, which may be ascribed to the same Ti-PF substrates. The R s of Pt CE is much higher than that of NiCo 2 S 4 and NiCo 2 S 4 -an CEs, which is attributed to the strong chemisorption of S 2− ions on Pt. R ct directly reflects the electrochemical reaction at CE/electrolyte interface, R ct values of Pt, NiCo 2 S 4 , and NiCo 2 S 4 -an CEs are 6860 Ω, 67.47 Ω, and 33.31 Ω, respectively; this means that it is easier for charges transfer through the NiCo 2 S 4 -an/electrolyte interface than Pt/electrolyte and NiCo 2 S 4 /electrolyte interfaces. Thus, it is anticipated that the QDSSC with Ti-PF-supported NiCo 2 S 4 nanotube CE will show better photovoltaic performance. Furthermore, the proper annealing treatment reduced the R ct of NiCo 2 S 4 /electrolyte interface and improved the short-circuit photocurrent density (J sc ) and PCE of QDSSC. Complete photovoltaic cells based on NiCo 2 S 4 and NiCo 2 S 4 -an CEs were fabricated and the cell based on Pt CE is used as the reference. In this study, Pt, NiCo 2 S 4 , and NiCo 2 S 4 -a CEs were soaked in S n 2− /S 2− 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 (V oc ), J sc , FF, and PCE) are shown in Figure 6d and Table 1, respectively. From Figure 6d, obviously, the performance of QDSSCs based on NiCo 2 S 4 and NiCo 2 S 4 -an CEs are better than that of QDSSC with Pt CE. The champion QDSSC based on Pt CE has an V oc of 0.489V, a J sc of 11.76 mA/cm 2 , a FF of 22.56%, and a PCE of 1.3%. The champion QDSSC with NiCo 2 S 4 CE has a V oc of 0.456V, a J sc of 13.72 mA/cm 2 , a FF of 40.6%, and a PCE of 2.54%. The champion QDSSC employing NiCo 2 S 4 -an CE has a V oc of 0.489V, a J sc of 16.68 mA/cm 2 , 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 NiCo 2 S 4 -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 V oc , J sc , FF, and PCE are consistent with the champion QDSSCs. This improvement in the cell performance originates from the significant increases in both J sc and FF, which closely related to the higher electrocatalytic ability of CE. Furthermore, the QDSSC with NiCo 2 S 4 -an CE shows a higher J sc and V oc than that with NiCo 2 S 4 CE and thus obtains a higher PCE. V oc 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 NiCo 2 S 4 , 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 NiCo 2 S 4 -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.

Conclusions
In summary, we have prepared Ti-PF by the acid etching technique and Ti-PF supporting NiCo 2 S 4 nanotubes via two-step hydrothermal method; furthermore, Ti-PF supporting NiCo 2 S 4 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 NiCo 2 S 4 nanotube with porous surface is the cubic phase. Using a polysulfide electrolyte in the symmetric cells, Ti-PF/NiCo 2 S 4 CE provided greater electrocatalytic activity (a higher reduction current density, a higher J 0 and J lim ) and lower internal resistance (R s and R ct ). Also, Ti-PF/NiCo 2 S 4 was used to fabricate QDSSC, it has a higher performance (J sc = 16.68 mA/cm 2 , V oc = 0.489 V, FF = 38.52%, and PCE = 3.14%) than that based on FTO/Pt CE (J sc = 11.76 mA/cm 2 , V oc = 0.489 V, FF = 22.56%, and PCE = 1.3%).