Titanium foils with thickness of 0.25 mm (99.5% purity; Alfa Aesar, Ward Hill, MA, USA) were used for anodic growth of TNA. Titanium foils were first polished by sonication in chemical polishing solvent which contained nitric acid, ammonia fluoride, urea, ethanol, and hydrogen peroxide in 12:5:5:3:12 v/v ratio and rinsed subsequently with deionized (DI) water, acetone, and methanol. The anodization reaction was carried out in a two-electrode electrochemical cell with polished Ti foil (2 × 2.5 cm²) which served as the anode-working electrode and Pt foil (thickness 0.025 mm; Alfa Aesar) as the counter electrode. The separation between Ti electrode and Pt electrode was about 3.5 cm. The anodization electrolyte contains 0.3 wt% NH₄F and 2 vol% H₂O in ethylene glycol solution. The anodization was operated under a constant potential of 60 V at low temperature of 15°C with magnetic stirring. The reaction period controlled the thickness of TiO₂ nano-tube arrays. Typically the TNA samples with tube length of approximately 15 μm were obtained after 2 h of anodization process. It is evident that increasing the TNA length leads to the increase of short-circuit photocurrent density due to the higher surface area available for dye adsorption. The TNA foils were then carefully washed with deionized water to remove the surface residual electrolyte in the nano-tube arrays. Such prepared TNA samples were then annealed at 450°C for 3 h with a heating rate of 1°C/min in order to transform the TNA from amorphous to anatase crystalline phase.

Figure 1 summarizes the procedures for post-treatment of annealed TNA. The TiCl₄-treated TNA (TNA-TiCl₄) was prepared from annealed TNA which was soaked in 0.2 M TiCl₄ solution (in ethanol) at 60°C for 30 min followed by heat treatment at 450°C for 30 min. The TnB-treated TNA (TNA-TnB) was prepared as follows: titanium (IV) n-butoxide (Ti(O-Bu)₄, TnB) (ACROS Organics, New Jersey, USA) was mixed with 2 M CH₃COOH (pH = 2.5) at room temperature under magnetic stirring for approximately 5 days until a homogeneous sol solution was obtained. The TiO₂ sol and the annealed TNA were both transferred to a teflon-lined autoclave to perform the hydrothermal treatment at 200°C for 5 h. The Ti foil was then removed from the autoclave, rinsed with DI water, and heated at 450°C for 30 min to form TNA-TnB.

The surface morphology and crystal phase of TNA, TNA-TiCl₄, and TNA-TnB were investigated by scanning electron microscopy (SEM) (SM6500F, JEOL Ltd., Akishima, Tokyo, Japan) and X-ray diffraction (XRD) (PANalytical X'Pert PRO, Almelo, The Netherlands), respectively. The results were confirmed by high-resolution transmission electron microscopy (Hitachi H-7100, Hitachi Ltd., Chiyoda, Tokyo, Japan).

To fabricate DSSCs devices, three kinds of TNAs including TNA, TNA-TiCl₄, and TNA-TnB, served as photoanodes, were combined with a transparent Pt counter electrode (cathode). The TNAs samples were sensitized by dye molecules (3 × 10^-4 M, N719 in a mixed solvent of acetonitrile and tertbutyl acohol (volume ratio = 1:1)) for 24 h. The amount of dye adsorbed on TNA electrodes was determined by desorbing the N719 from TNAs surfaces into a solution of 0.1 M NaOH. The concentration of the adsorbed N719 was analyzed by UV-visible spectrophotometer (V-630, JASCO Corp., Easton, MD, USA). The Pt cathode was made by a 'two-step dip coating' process developed by Wei et al. 12 . We have first prepared the poly-N-vinyl-2-pyrrolidone (PVP)-capped Pt nano-particles by dissolving PVP (M.W. = 8000) and H₂PtCl₆ (Pt precursor) into deionized water at room temperature and well stirred until a light-yellow solution was obtained. A NaBH₄ solution was then added drop by drop to the H₂PtCl₆-PVP solution, and the solution quickly turned into a black color, indicating the formation of Pt nano-particles (Pt-PVP solution).

FTO glass (8Ω/sq., Solaronix SA, Aubonne VD, Switzerland) was pretreated by 1% ML-371 aqueous solution at room temperature for 1 min in order to increase adhesion between the PVP-capped Pt nano-particles and FTO surface. The ML-371-modified FTO substrate was then dipped into the Pt-PVP solution for 5 min and rinsed with deionized water followed by heat-treatment at 400°C for 5 h to remove completely the organic component and complete the preparation of counter electrode.

To assemble the DSSCs, the liquid electrolyte of 0.1 M lithium iodide, 0.05 M iodine (I₂), 0.5 M 4-tert-butylpyridine, 0.5 M 1,2-Dimethyl-3-propylimidazolium iodide in acetonitrile was applied to the above-prepared Pt electrode which was then placed over the N719-coated TNAs electrodes. The edges of the cells sealed with a hot-melt film (Surlyn, 125 μm) and the electrolyte (I^-/I₂/I₃ ^- redox couple) were injected into the space. The active cell area studied in this work is 0.25 cm² (0.5 cm × 0.5 cm). The photoelectrochemical performance of the resultant solar cells were measured by back illuminated through the Pt counter electrode due to the nonpenetration of light through the photoanode Ti metal substrate.

The current (I)-voltage (V) characteristics were performed using a digital source meter (Keithley model 2400, Keithley Instruments Inc., Cleveland, OH, USA) with the TNA-based DSSCs devices under one-sun AM 1.5 irradiation from a solar simulator (300 W Xe light and filters, Oriel Instruments, Irvine, CA, USA) on a 0.25 cm² sample area.

TiO₂ electrode is one of the major concerns in DSSCs application. Since the TiO₂ phase, morphology, and surface area of TiO₂ films will affect the dye adsorption, electron transport, and electrolyte diffusion in the cell as well as the DSSCs performance. In this work, we have devised two kinds of Ti-precursor solutions to fabricate TiO₂ nano-particles decorated TNA on Ti substrates which can serve as photoanodes for DSSCs device. During the optimization stage of anodization process, the important roles of the anodization condition, including the thickness of Ti-foil (0.05-0.25 mm), temperature (15-30°C), anodization potential (20-60 V), reaction period (10 min-24 h), and various kinds of F-containing electrolytes (HF, KF, and NH₄F) in controlling the thickness (length of TiO₂ nano-tubes), homogeneity, and morphology of TNA were revealed. The data presented below were obtained with NH₄F/H₂O/ethylene glycol electrolyte solution after anodization procedure at 15°C for 2 h. Figure 2 shows a typical current-time plot recorded during the constant potential anodization process. Within the first few seconds, the current dropped drastically to a local minimum indicating the oxidation of Ti-foil to form surface pits acting as nucleation sites for tube formation 13 . Upon increasing the pit density, the current increased to a maximum where the pit density reached saturation. After further anodization, the current gradually decreased due to the continuously lengthen of TiO₂ nano-tubes. Such anodization behavior is commonly observed in the self-organized pore formation process 14 in which the competition between TiO₂ oxide layer formation and dissolution of titanium progressed concurrently. Finally, the formation of vertically oriented TNA was achieved. The initially grown TNA was gray, which turned to yellow color after annealing at 450°C for 3 h.

Figure 3a shows the SEM images of the untreated TNA formed by anodizing a titanium foil with two types of magnification: 10,000× (top view) and 50,000× (side view, inset). The SEM image of lower magnification of TNA (Figure 3a) shows high porosity character of anodic TiO₂ films with some nano-scale cracks. The higher magnification image of TNA (inset of Figure 3a) shows the self-organized TiO₂ nano-tubes aligned densely with hexagonal close-packed arrangement. The inner diameters of these TiO₂ nano-tubes based on SEM images are in the range of 100-120 nm, and the wall thickness is approximately 10 nm. The thickness of TNA corresponding to the length of TiO₂ nano-tubes is about 15 μm obtained from the SEM cross section analysis shown in Figure 4. It is consistent with other research works that high-aspect-ratio TiO₂ nano-tubes can be fabricated with rapid growth rate by anodization 4 9 15 .

Figure 3b, c shows the top-view SEM images of the decorated TNAs after post-treatment by TiCl₄ and TnB solution, respectively, with the corresponding side view images with higher magnification as shown in the inset of Figure 3b, c. The length of TiO₂ nano-tubes remains the same after different post-treatment. For both TiCl₄ and TnB treated TNAs samples (denoted as TNA-TiCl₄ and TNA-TnB), additional materials can be observed on the top of TNAs and inside the TiO₂ nano-tubes. Yet, outside the TnB-treated TNA, the TiO₂ nano-tubes were apparently coated with an extra layer of TiO₂ nano-particles. Transmission electron microscope (TEM) experiments had been performed on TNA-TiCl₄ and TNA-TnB samples detached from the Ti foil and dispersed on a copper grid. The bulk crystallites were observed (Figure 5a) inside TiO₂ nano-tubes in the case of TiCl₄-treated TNA sample. In TnB-treated TNA, bulk crystallites were observed both inside and outside the nano-tubes as shown in Figure 5b. The average inner diameter of TiO₂ nano-tubes in TNA-TiCl₄ and TNA-TnB after different post-treatment was about 85-120 nm based on TEM analysis which is consistent with SEM results.

XRD was used to confirm the crystalline phase of TiO₂ nano-structure. Figure 6 shows the XRD patterns of (a) as-prepared TNA, (b) annealed-TNA, (c) TiCl₄-treated TNA, and (d) TnB-treated TNA. The as-prepared TNA (before annealing) were amorphous (Figure 6a). Upon annealing to 450°C, the sharp anatase diffraction peaks appeared (Figure 6b) with crystal domain of approximately 20 nm. After post-treatment by TiCl₄ and TnB, the XRD patterns remain the same for both TNA-TiCl₄ and TNA-TnB, suggesting that the TiO₂ crystalline phase was not affected by post-treatment. The slight increase of TiO₂ crystal domain to 22 nm (for TNA-TiCl₄) and 29 nm (for TNA-TnB) was due to the two times annealing at 450°C for post-treated TNAs samples.

The above-prepared TiO₂ nano-tube arrays (TNA, TNA-TiCl₄, and TNA-TnB) were used to fabricate the DSSCs for photoelectrochemical performance study. The results obtained from I-V curve measurements for N719-sensitized DSSCs under simulated AM 1.5 illumination is shown in Figure 7. The thickness of TNAs layers on Ti foil was fixed at approximately 15 μm. Table 1 lists the TNAs thickness, the amount of dye adsorbed on TNAs layers (N719_ads), and photoelectric data of the DSSCs in Figure 7 including the open circuit voltage, the short-circuit photocurrent density (J_sc), the fill factor, and the photocurrent conversion efficiency (η). It is apparent that by post-treatment of the TNA, the performance of DSSCs was notably enhanced. The device based on the untreated TNA has shown the lowest J_sc (3.84 mA/cm²) and η (1.38%). This is due to the low surface area of the untreated TNA which uptakes less amount of dye molecules (0.092 μmole/cm²). Higher efficiency and current density of DSSCs device might be attributed to the higher amount of adsorbed N719 and the fast electron transportation on TiO₂ electrodes. As shown in Table 1, TnB treatment assisted the higher dye adsorption amount, raised from 0.092 to 0.116 μmole/cm² (approximately 26% increase of N719_ads), and J_sc also raised from 3.84 to 5.97 mA/cm² (approximately 55% increase of J_sc). Both results lead to the efficiency improvement from 1.38% to 2.40% (approximately 74% increase of η). The significant increase of dye adsorption is due to the increased surface area from the decorated TiO₂ nano-particles on TNA, as clearly seen from the SEM images (Figure 3c). TEM images (Figure 5b) also vindicated the presence of bulk TiO₂ crystallites inside and outside the TNA. The further increase of J_sc as well as η is attributed to the increase of TiO₂ crystallinity of TNA-TnB, as evidenced by XRD. The dye loading for TNA-TiCl₄ sample is lower compared to that of TNA-TnB because the TiO₂ nano-particles only subsist inside the nano-tubes. The slightly higher efficiency of TNA-TiCl₄-based DSSCs compared to the untreated TNA one is due to the better TiO₂ crystallinity after two times annealing process. Variation in the performance of different devices might be due to the variations in the TiO₂ tube length. Nevertheless, same trends were observed within each batch of device study.