Effect of TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO2 nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO2 nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl4) treatment, the walls of TiO2 nanotubes were coated with TiO2 nanoparticles. XRD patterns showed that the oxygen-annealed TiO2 nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO2 nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl4 treatment, the efficiency of TiO2 nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of Rk (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO2 nanotubes were treated with TiCl4. These results indicate that TiO2 nanotubes treated with TiCl4 can increase the surface area of TiO2 nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.


Background
Dye-sensitized solar cells (DSSCs) have received considerable attention lately because they are cost-effective and environmentally friendly with efficiencies comparable to those of the traditional silicon-based cells [1]. Generally, granular titanium dioxide powder is commonly used in dye-sensitized solar cell light anode structure. The solgel method is used to produce porous film structure, but small pores form between particles of the transmission path of clutter, resulting in a more dye adsorption capacity and low clutter of electron transfer path. The path is too long and will make the leakage current and the probability of electron recombination, thus affecting the overall conversion efficiency of solar cells. The titanium dioxide nano-tubular structure of high surface area and large aspect ratio can be beneficial to the dye adsorption, and more rules of order can be reduced when the electron and hole in the transmission probability of recombination. TiO 2 nanotubes have been synthesized by various methods including hydrothermal method [2], seeded growth [3], template-assisted deposition [4], and anodization [5]. Especially, anodization is a relatively simple method for synthesizing large-area and self-organized TiO 2 nanotube arrays [6][7][8]. In this paper, we used the electrochemical anodization to prepare TiO 2 nanotubes arrays with different thickness and applied them on the photoelectrode of dye-sensitized solar cells. The TiO 2 nanotubes and solar cells were investigated by fieldemission scanning electron microscopy, X-ray diffraction (XRD), I-V characteristic analyses, and electrochemical impedance spectroscopy (EIS) to study the effect of titanium tetrachloride (TiCl 4 ) treatment on the photoelectrode of TiO 2 nanotubes for dye sensitized solar cells.

Methods
In this study, the growth of nanotubes was anodized on Ti foils (purity of 99.6%, thickness of 0.2 mm) by constant current at 15 mA in the ethylene glycol solution containing 0.3 wt.% NH 4 F and 2 vol.% deionized water kept at 20°C. The anodized TiO 2 nanotubes were annealed in oxygen at 450°C for 60 min. For the treatment of TiCl 4 , TiO 2 nanotubes were immersed in 0.2 M TiCl 4 solution for 1 h and annealed in air at 350°C for 30 min. Pt counter electrodes were prepared by coating with a drop of H 2 PtCl 6 solution and heating at 400°C for 15 min [9]. To adsorb N3 dye, TiO 2 nanotubes were immersed in 3×10 −4 M solution containing N3 dye and ethyl alcohol at 45°C for 8 h in the oven. The working electrodes were then rinsed with ethanol. Electrolyte solution is adopted from Everlight Chemical Industrial Corporation (ESE-20). The electrode was assembled into a sandwich-type open cell using platinum plate as a counter electrode. Both electrodes were spaced by a kind of polymer films. The thickness was 60 μm, and the size of TiO 2 working electrode was 0.25 cm 2 (0.5 ×0.5 cm). The surface morphology of the TiO 2 nanotubes was observed by scanning field emission electron microscopy. Structural analysis was carried out by powder Results and discussion Figure 1 shows the SEM images of the TiO 2 nanotubes before and after TiCl 4 treatment. Clearly, after the samples were treated with TiCl 4 , the walls of TiO 2 nanotubes were coated with TiO 2 nanoparticles, which could increase the surface area of TiO 2 . In order to explore the impact of annealing gas on the properties of TiO 2 nanotubes, the samples were carried out with XRD characterization. XRD patterns of TiO 2 nanotubes are shown in Figure 2. It is found that the as-formed TiO 2 nanotubes are amorphous and are converted to anatase after annealing. The oxygen annealed TiO 2 nanotubes have a better anatase phase than that annealed in air. After the treatment of TiCl 4 , TiO 2 nanotubes also show a good anatase phase. Figure 3 shows the currentvoltage characteristics of DSSCs with the electrodes of different lengths of TiO 2 nanotubes without TiCl 4 treatment. The parameters for the short-circuit current density (J sc ), the open circuit potential (V oc ), the fill factor, and the overall conversion efficiency (η) are listed in Table 1. From the results of Figure 3 and Table 1, it is found that the best conversion efficiency of DSSCs is 4.35%, while the length of TiO 2 nanotubes is 20 μm.
The result of conversion efficiency is quite higher than the previous reports [10][11][12]. This may be due to the length of TiO 2 nanotubes in this study, which is quite longer than those of the previous reports. It is advantage to adsorb N3 dye on the TiO 2 nanotubes. Figure 4 shows the current-voltage characteristics of DSSCs with the electrodes of different lengths of TiO 2 nanotubes after TiCl 4 treatment. The parameters for the J sc , the V oc , the fill factor, and the η are listed in Table 2. From the results of Figure 4 and Table 2, it is found that the best conversion efficiency of DSSCs can be improved up to 6.58%, while the length of TiO 2 nanotubes is 20 μm.
In order to study the effect of TiCl 4 treatment on the transport properties of TiO 2 nanotubes, the analysis of EIS for TiO 2 nanotubes has been investigated. Figure 5 shows the spectra of EIS for the dye-sensitized solar cells with and without TiCl 4 treatment. The simulation of equivalent circuit is referred to the previous reports [13][14][15]. The parameter R k , which is represented by charge transfer resistance related to recombination of electrons, is also listed in Table 3. The value of R k decreases from 26.1 to 17.4 Ω after TiCl 4 treatment. These results indicate that the effect of TiCl 4 treatment     on TiO 2 nanotubes can increase the surface area of TiO 2 and the adsorption of N3 dye, resulting in better transport properties of TiO 2 nanotubes and the improvement of conversion efficiency for DSSCs.

Conclusions
In summary, we prepared TiO 2 nanotube arrays by electrochemical anodization to apply on the electrode of dye-sensitized solar cell. After TiCl 4 treatment, the walls of TiO 2 nanotubes were coated with TiO 2 nanoparticles. It can increase the surface area of TiO 2 and the adsorption of N3 dye, resulting in better transport properties of TiO 2 nanotubes and the improvement of conversion efficiency of DSSCs.