Effects of Surface Modification of TiO₂ Nanotube Arrays on the Performance of CdS Quantum-Dot-Sensitized Solar Cells

2.1. Preparation and Modification of TiO₂ Nanotube Arrays

Ti foil (0.5 mm thickness, 99.4% purity) was first cut into 1.5 cm × 3 cm. The foils were ultrasonically cleaned in acetone, ethanol, and deionized water for 30 min, respectively, and dried in ambient air. The Ti foil was anodized in ethylene glycol solution containing 0.3 wt% NH₄F and 2 wt% H₂O by applying a 60 V DC potential for 4 h. The anodization was performed in a two-electrode configuration with the Ti foil as the working electrode and a stainless steel foil as the counterelectrode. After anodization, the TNAs on Ti foil were ultrasonically treated in ethanol for 5 min to remove the electrolyte and debris on top, followed by thermal treatment at 450°C for 3 h. Then the samples were soaked in TiCl₄ (0.04 M), HNO₃ (pH = 3), and KOH (pH = 12), respectively, for 30 min at 70°C. And the sample soaked in TiCl₄ subsequently re-annealed at 450°C for 30 min.

2.2. Deposition of CdS QDs

The CdS-sensitized TiO₂ photoanodes were achieved by SILAR of CdS QDs on TNAs. The as-prepared photoanodes were firstly immersed into a solution of 0.05 M Cd(NO₃)₂ in ethanol for 2 min, then rinsed with ethanol, and then immersed into 0.05 M Na₂S in methanol/water (1 : 1/v : v) for another 2 min, followed by another rinsing with deionized water and methanol. Such an immersion cycle was repeated 8 times until the desired deposition of CdS QDs was achieved. The sensitized photoanodes were passivated with ZnS by alternately dipping into 0.5 M Zn(NO₃)₂ ethanol solution and 0.5 M Na₂S methanol/water solution three times for 2 min for each time.

2.3. Fabrication of Counterelectrodes and Solar Cells Assembly

To assemble the solar cell, a Pt-coated counterelectrode was prepared by dropping 5 mM H₂PtCl₆ in isopropanol onto FTO glass and then annealed at 385°C for 30 min. The cells were assembled in a sandwiched structure with a thermal adhesive film (Surlyn, 60 μm, DuPont). A methanol/water (7 : 3 by volume) solution containing 0.5 M Na₂S, 2 M S, and 0.2 M KCl was used as the redox electrolyte. The electrolyte was injected by vacuum backfilling, and then the hole was sealed with a Surlyn film. The effective area of the cell is 0.3 cm².

2.4. Characterization

The morphology and microstructure of the photoanodes were characterized by scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, JOEL JEM2100), and the crystal structure was studied by X-ray diffraction measurements (XRD, Bruker AXS, D8 Advance). The absorption spectrum was obtained by a UV-vis spectrophotometer (Persee, TU-1901). The current and voltage measurements were recorded by Keithley 2400 sourcemeter when the cells were irradiated by a solar simulator (Aulight, CEL-S500) under AM 1.5 illumination of 100 mW/cm². All the electrochemical impedance spectroscopy (EIS) measurements of QDSSCs were carried out in dark and under the open circuit potentials. The superimposed signal has the frequency ranging from 0.1 Hz to 100 KHz with AC amplitude of 10 mV (Zahner, Zennium).

3.1. Contact Angle Analysis

The top-view and side-view images of highly ordered TNAs prepared by anodic oxidation are shown in Figure 1(a). The tube length is measured to be 20 μm while the average inner diameter 150 nm. To observe the surface characteristics of the resulting surfaces, modified TNAs were investigated by equilibrium contact-angle measurements. The TNAs without any surface modification were used as a reference, and the corresponding contact angle is displayed in Figure 1(b). The water contact angle is 6.3 ± 2°, and the surface is practically wetting and represents a superhydrophilic behavior. We doubt that the reason for the phenomenon of the surface derives from the spreading water on the entire surface and into the pores [23]. Figures 1(c)–1(e) exhibit the contact angles of TNAs modified by TiCl₄, HNO₃, and KOH, respectively, and the resulting contact angles are 12.2 ± 2°, 1.2 ± 2°, and 9.4 ± 2°, respectively. It can be seen that the contact angles are different and the TNAs treated by HNO₃ represent the best hydrophilicity. Besides the reason that water is spreading into the pores which is similar to the reference sample, the changes of active site and surface property play a key role in the surface wetting behavior of the TNAs modified with HNO₃. Moreover, notice that the contact angles of modified samples are similar to that of the reference, all of which are smaller than 50° and show a superhydrophilic behavior. In order to study the effect of the superhydrophilic/superhydrophobic surface on the QDSSCs performance, a TNAs sample was modified with also methyltrimethoxysilane (MTMS) which is usually used for surface hydrophobic modification. The originally superhydrophilic surface becomes hydrophobic with a contact angle of about 111.2 ± 2° as shown in Figure 1(f).

3.2. Morphological Analysis

Figure 2 shows the top-view and side-view SEM images of CdS-sensitized TiO₂ nanotube arrays. In Figure 2(a), CdS quantum dots are well distributed on the top surface of the TNAs without any modification, while some CdS quantum dots are found to aggregate on the openings of TNAs. The side-view SEM image in Figure 2(a) depicts that the vast majority of QDs are found to aggregate and attach to the external tube wall. The results indicate that because of the effect of surface tension, the CdS precursor solutions could not penetrate deeply into the nanotubes and cause large aggregation of CdS on the top surface and external tube wall [24]. Figure 2(b) represents the great majority of CdS quantum dots aggregated in the openings of TiO₂ nanotubes treated by TiCl₄. The side-view SEM image shows that CdS QDs are located on the inner tube wall of TNAs and distributed uniformly. As depicted in Figure 2(c), CdS QDs treated by HNO₃ are homogeneous and rarely aggregate in the openings. Moreover, the distribution of CdS QDs in the inner spaces is also uniform and few aggregated as illustrated in the inset of Figure 2(c). In Figure 2(d), a great majority of CdS QDs aggregate in the openings of TNAs treated by KOH, while the side-view image shows a similar distribution of the sample treated by TiCl₄. In the case of MTMS-treated TNAs shown in Figure 1(e), large quantity of CdS QDs blocked in the surface openings, and the side-view image in inset shows a uniform distribution in the inner spaces of TNAs. According to the surface characteristics in Figure 1, the results shown in Figure 2 suggest that superhydrophilic surface might have a uniform QDs distribution, while hydrophobic surface might be easy to cause QDs aggregation.

To further clarify whether or not the CdS QDs are introduced into the nanotubes wall under the influence of hydrophilic and hydrophobic modification of TNAs coated with CdS, the samples were investigated by TEM. Figure 3 shows TEM images of the reference sample and CdS/TNAs treated by TiCl₄, HNO₃, KOH, and MTMS, respectively. As shown in Figure 3(a), few CdS QDs are deposited in the nanotube wall of the reference sample without surface treatment. Compared with the reference sample, many CdS QDs have been attached to the nanotube wall of the sample treated by TiCl₄ which is shown in Figure 3(b). And the CdS QDs are found to be aggregated in the tube wall. After being treated by HNO₃, as depicted in Figure 3(c), a great majority of CdS QDs deposited in nanotube wall are better distributed. In addition, the sizes of QDs are more uniform. Similarly, the CdS QDs are also found to be aggregated for the HNO₃-treated sample. As shown in Figure 3(d), CdS QDs apparently aggregate together for the CdS/TiO₂ electrode treated by KOH. Besides, the CdS/TNAs electrode modified by MTMS represents the phenomenon of aggregation and the nonuniform distribution of CdS QDs in Figure 3(e). By combining Figures 2 and 3, we suggest that the CdS QDs are more uniformly distributed on the tube wall of TNAs after the treatment of HNO₃ because of its superhydrophilic characteristic and smaller surface tension. Close observation by high-resolution TEM demonstrates that the size of CdS QDs is about 5–10 nm in diameter, as depicted in Figure 3(f).

3.3. XRD Analysis

The crystal structure of the deposited CdS was examined by X-ray diffraction experiments. For comparison, the X-ray diffraction pattern from the TNAs sample is also given as curve 1 in Figure 4, where diffraction peaks are the anatase phase of TiO₂ (JCPDS card no. 21-1272) and Ti metal phase. Curves 2–6 represent the diffraction peaks of CdS/TNAs treated without and with TiCl₄, HNO₃, KOH, and MTMS, respectively. Compared to the TiO₂ nanotubes arrays, CdS/TNAs electrodes exhibited new peaks corresponding to (111), (220), (331) planes of CdS, indicating the presence of the cubic phase CdS (JCPDS card no. 80-0019). The XRD patterns of CdS/TNAs electrodes with different treatments are quite similar, which suggests that surface modification could not change the crystal structure of the photoanode.

3.4. UV-Vis Absorption Spectra Analysis

3.5. Photovoltaic Performance and Charge Transfer

The J-V curves of QDSSCs treated without and with TiCl₄, HNO₃, KOH, and MTMS are shown in Figure 6. The open circuit potential (V_oc), short circuit current density (J_sc), fill factor (FF), and conversion efficiency (η) of all cells are listed in Table 1. The J_sc and V_oc of the reference electrode are 0.943 mA/cm² and 0.37 V, respectively, resulting in a very low value of conversion efficiency of 0.12%. And the effect of the TiCl₄ treatment TNAs is also evaluated using the same parameters. An increase in the J_sc of TiCl₄-treated solar cell is observed in Table 1, while the efficiency of the cell represents a small decrease due to the reduction of the V_oc and FF. The lower V_oc is possibly ascribe to the decrease of the electrons transported from CdS QDs to TiO₂ conduction band, leading to a decrease in the quasi-Fermi level of TiO₂ [29], while lower FF may be attributed to the low driving force for the electron injection [29]. Moreover, these results clearly reflect that the TiCl₄ treatment does not have a beneficial effect on QDSSCs. In the case of HNO₃-treated solar cell, a systematic beneficial effect is observed. The J_sc,  V_oc, and η are 1.460 mA/cm², 0.36 V, and 0.17%, respectively. The efficiency of 0.17% is 42% higher than that of the reference solar cell. Accordingly, the higher J_sc seems to be mostly owing to the better connectivity between TNAs and CdS QDs. This implies that the strong oxidizing property and high acidity of the HNO₃ to some extent help make better contact between TNAs and CdS QDs [30]. Therefore, well-connected structures are able to promote electron transfer and improve J_sc. For the solar cell treated with KOH, the photovoltaic performance is similar to the TiCl₄-treated solar cell. The J_sc is higher while V_oc and FF are lower than those of the reference cell. In addition, the MTMS-treated solar cell shows lower V_oc and efficiency than that of the reference cell. These results are in good agreement to that of Figures 2 and 3, and the lower efficiencies of the solar cells are due to the blocking QDs in the opening of nanotubes. Moreover, the J_sc of HNO₃-treated solar cell increases to a maximum value of 1.460 mA/cm², which is possibly because of the increase amount of CdS QDs deposited on TiO₂ and the uniform distribution of the QDs.

In order to find out the reasons why J_sc increases in the photoanode after HNO₃ treatment, we further investigated the electron transport and charge recombination processes of the solar cells by applying EIS. Figure 7 shows the Nyquist plots and Bode plots of CdS-sensitized solar cells modified with and without HNO₃, respectively, which are measured under dark conditions and open circuit potentials. EIS is a useful technique to detect kinetic processes in QDSSCs. Two distinct arcs of each plot could be seen from Figure 7(a), representing the electron transport processes at the interface of TNAs/CdS/electrolyte (frequency region 1–1000 Hz) and the feature of polysulfide solution diffusion in the electrolyte (<1 Hz). Generally, the plots of QDSSCs exhibit three arcs, while in our case the interface of the counterelectrode/electrolyte is not so obvious in the frequency region 1–100 kHz which is shown in the inset of Figure 7(a). The electrochemical parameters obtained from equivalent circuit analysis are presented in Table 2. The equivalent circuit of the solar cells is represented in the inset of Figure 7(b). For relatively low resistance, the transfer resistance of counterelectrode/electrolyte interface does not play a key role in the whole resistance of solar cell. Therefore, charge transfer is dominated by the transfer resistance at the interface of TNAs/CdS/electrolyte and the diffusion resistance of electrolyte. From Table 2, it can be seen that the values of R_CT2 and R_E for reference and HNO₃-treated solar cells are 62.45 and 180.4 Ω, 37.04 and 127.1 Ω, respectively. It is inferred that HNO₃-treated solar cell shows a lower charge transfer resistance which is responsible for the observed higher photocurrent density (J_sc = 1.460 mA/cm²) compared to reference sample. And the results are in good agreement with the photovoltaic performance of solar cells shown in Figure 6. This phenomenon may be due to the better hydrophilicity and lower surface tension of the solar cells treated by HNO₃. Moreover, because of the decrease of excessive CdS hindering the access of electrolyte deep into the inner tube, J_sc of the HNO₃ treatment solar cells is larger than other cells. Correspondingly, the characteristic frequency peaks in Bode phase plots are shown in Figure 7(b).