The sol-gel method for preparing anode TiO₂ films, sol-gel method is a common method for preparing wet chemical materials [1] [13] - [16] . Sol-gel derived samples high uniformity, high purity of products, easy control of the reaction process, has great advantages in the application of the film, becoming one of the most commonly used method for preparing thin films. TiO₂ film composition prepared using different process methods or parameters, structure, orientation and thickness are the differences.

Tetrabutyl titanate hydrolysis reaction in an acidic solution [17] :

The hydrolyzed reaction mainly (1) and (2) out of the way, the product was incomplete hydrolyzate Ti(OR)₃(OH) and Ti(OH)₄, Ti(OH)₄ lose a water molecules generated TiO₂ colloidal particles. Polycondensation reaction (3) and (4) happened after 10 hours, polycondensation product gel form titanium oxide.

Chemically pure tetrabutyl titanate as raw material in this experiment, using ethanol as solvent, diethanolamine as complexing agent, nitric acid as catalyst. Experiment steps were as follows:

1) Prepare a mixture A with tetrabutyl titanate, ethanol and diethanolamine, fully stirred to give a homogeneous mixture.

2) Prepare a uniformly mixed mixture A with ethanol, deionized water and nitric acid.

3) Under magnetic stirring, the above mixture B was added into the mixture A dropwise to obtain a uniform, light yellow transparent sol, and the hydrolysis polycondensation reaction at room temperature to obtain sol C.

The mole ratio of tetrabutyl titanate, ethanol, diethanolamine and and nitric acid was 12:48:4:1:0.6.

The desired ionic salt was put into B solution. Adding the solution B in solution A dropwise under magnetic stirring to give a homogeneous, light yellow transparent sol. The hydrolysis polycondensation reaction at room temperature to obtain solution C.

The TiO₂ film was prepared by sol-gel method, drying at room temperature and then placing the crucible in a muffle furnace at 20˚C/min at 500˚C heated to a constant temperature for two hours, then cooled in the furnace. After hydrolysis of the intermediate product suitable high-temperature annealing was completely decomposed, residual organic matter can be completely removed, and finally completely dehydrated, only closely integrated with the substrate of titanium dioxide films.

Using DMF solution with Eosin Y as sensitizer, the concentration of 0.001 g/mL. Putting the annealed TiO₂ film into the dye solution and rushing floated color with ethanol after 24 hours to make the anode. Using acetonitrile solution with KI and I₂ as electrolyte. The counter electrode was prepared by electroplating method on FTO conductive glass with pt.

The prepared TiO₂ film colored face up on the table, the pore size of 0.5 cm² of silicone sheet on the anode and the counter electrode on top, with a clip to clip up on both sides of the battery, electrolyte injected with a syringe. Cell production is complete to measure optical properties.

Testing the DSC with Beijing changtuo company CHF-XM-500W xenon lamp as the light source, the incident light intensity was 73.1 mW/cm², the open circuit voltage of V_oc and short circuit current density I_sc. The open circuit voltage can be seen from Figure 1, Nd³⁺ doped TiO₂ thin films was lower, Zn²⁺ doped TiO₂ thin film photovoltaic best performance, cell power was highest, indicating that the effect of doping of Zn²⁺ was the best in these three kinds of ions.

As can be seen from the Figure 1, not only the curve hardness of ion-doped TiO₂ film was significantly better than undoped TiO₂ film, but also the current and voltage was higher than the undoped film. It illustrated the truth of certain mount of doping ions can improve the property of TiO₂ thin films. This was because the oxide melting point have a certain influence on the phase transformation of TiO₂, when the oxide melting point was higher than TiO₂, can inhibit the transformation of anatase to rutile, and when the oxide melting point lower than TiO₂, can promote the transformation, and the lower the melting point effect more obvious.

Using the RIGAKU D/MAX-2200 type PC X-ray diffraction of the phase of the sample were measured for the Cu target radiation. λ = 0.15418 nm, the working voltage of 40 kV, current of 30 mA, the scanning range of 10˚ - 80˚, the scanning speed was 20/min. As can be seen from the Figure 2, doping ions easily lead redox reaction in the titania lattice surface, then produced oxygen vacancy or interstitial titanium by diffusion, thereby inhibiting the interaction between different titanium atoms, transition hinder anatase to rutile phase, to improve the light absorption ability of TiO₂ thin films.

The characteristics of samples were examined through a UV-Vis spectrophotometer type UV-2550 produced by Japanese Shimadzu Company, scan speed was medium, the slit width was 2 nm, wavelength range was 200 nm to 800 nm.

Figure 3 was the UV-Vis spectra of TiO₂ films doped with ions in the 300 - 600 nm. It can be seen from that the spectra in the wavelength range of 500 nm or more, whether the TiO₂ film was doped with ions or not, light absorption was relatively small. Absorb light beginning between 300 - 400 nm, and the wavelength was shifted to shorter wavelength direction as the absorption peak was more obvious. While in the UV range, TiO₂ films have a strong absorption of light.

The spectral curve changed on the form compared with the non-doped TiO₂ films. The absorption peak shifted to longer wavelengths, spectral red shift. Expand the scope of TiO₂ nanoparticles in response to visible light direction, improved the absorption properties in a certain extent. The band gap of TiO₂ thin film doped with Zn²⁺ is 2.77 eV, the band gap of TiO₂ thin film doped with La³⁺ is 2.51 eV, the band gap of TiO₂ thin film doped with Nd³⁺ is 2.71 eV, all smaller than the band gap of undoped TiO₂ films (3.0 eV), demonstrated that the doping ion can improve the photocatalytic activity of TiO₂ thin films.

Many studies have shown that ion doping with an optimal concentration. With increasing the concentration of the doping, the surface space charge layer is narrowed, electrons and holes are generated by light excitation TiO₂ and can be effectively separated, the lifetime of photo-induced carriers prolonged, but when the doping concentration is lower than the optimal concentration, there is not enough traps in the semiconductor to capture the photo-induced carriers, electrons and holes can not reach the most effective separation; When the optimum doping concentration, the space charge layer thickness is exactly equal to the incident light penetration depth, the photo-generated electrons and holes have the optimal separation, the most favorable to the photocatalytic reaction; It would cause an increase of photo-induced carriers recombination in the surface when excess dopant to reduce the photocatalytic efficiency.

The doping of Zn²⁺ as an example to illustrate the effects of different doping concentration on the properties of DSC. The doping concentration in solution was respectively 0.001%, 0.03%, 0.05%, 0.1%, 0.2% (mole fraction). As can be seen from Figure 4, the open circuit voltage and short circuit current increased with the ions concentration increased. However, when reached a certain value, the open circuit voltage and short circuit current decreased, which also proved the existence of an optimum value of ions concentrationt, and the optimum value was 0.05%.

Semiconductor ZnO and TiO₂ belong to different band gap, in both the interface the Ti⁴⁺ into the ZnO lattice instead of Zn²⁺ causing a charge imbalance due to Zn²⁺ ionradius is larger than the radius Ti⁴⁺, in order to bal- ance the charge, more −OH was absorbed to the surface, and the surface adhesion of the −OH to accept the photogenerated holes. This reduces the probability of the photogenerated hole and electron recombination, improves the separation of photogenerated electrons and holes, which exhibits better photocatalytic activity. However, when doped Zn²⁺ too much, ZnO and TiO₂ solid solution has reached saturation, the excess of Zn²⁺ in the form of ZnO deposited on the surface of TiO₂ grains, reducing the effective Specific surface area of TiO₂ films. And because the band gap of ZnO is greater than TiO₂, the UV-visible irradiation does not cause photocatalytic reaction, so the accumulation of ZnO on the surface of TiO₂ hinder the photocatalytic reaction, causing lower photocatalytic activity of TiO₂ films.

The Zn²⁺ optimum doping amount is 0.05%, because then the transfer rate of carriers of the fastest, the separation of electron hole pair efficiency is the highest, the photocatalytic activity is the strongest. When Zn²⁺ doping amount is less than 0.05%, the defect levels and impurity levels is separating center of electron-hole pairs; When Zn²⁺ doping greater than 0.5%, the defect levels and impurity levels is recombination center of electron-hole pairs. carrier recombination rate accelerated, electron-hole pairs separation efficiency decreases.

Figure 5 was the UV-Vis spectra of TiO₂ films doped with Zn²⁺ between 300 nm and 600 nm, including 1, 2, 3, 4, 5 respectively represent the doping concentrations were 0.001%, 0.03%, 0.05%, 0.1%, 0.2% (mole fraction). As we can be seen from the spectra, the shape of the curve changed with increasing the concentration, the absorption peak shifted to longer wavelengths. However, when the concentration continues to increase, the absorption peak moves to shorter wavelength. It also showed the presence of an optimum doping concentration, and the most preferably concentration of 0.05% (mole fraction). This was consistent with previous analyzes. It presented the first band gap decreased after the increase. It also showed that there was an optimal doping concentration. The band gap of the mixture narrowed compared with pure TiO₂, the right concentration of Zn²⁺ doped TiO₂ improved ability to absorb long-wave photons.