High-pressure Synthsis Of CdS/TiO₂、NiO/TiO₂ Heterostructure And Their Photoelectric Performance

Recently, with the rapid develop of world economy and the advancement of global industrialization, human beings face two big problems due to a large amount of consumption of fossil fuel:（a） energy shortage;（b） environment pollution. Followed by acid rain and smog, green house effect, the natural disasters occurred frequently. Therefore, looking for a green environmental protection and renewable new energy has become one of the first problems that all mankind must face. Solar energy attracts a great deal of attention due to its rich resources, green environmental protection, and the advantages of recycle. The solar cell is economical and practical, no noise and no pollution. It is one of the important applications of solar energy and has received the extensive attention of the researchers.The titanium dioxide（TiO₂） nanomaterial is widely used in photovoltaic and photocatalysis field, due to its suitable valence band and conduction band position, stable chemical property, good optical activity, the advantages of low cost and non-toxic harmless. TiO₂ nanowires possess high specific surface area and good surface carrier transmission rate and have attracted a great deal of attention of scientific research workers. But because of its broad band gap, TiO₂ can only absorb the ultraviolet light. However, ultraviolet light holds less than 5% in sunlight. In order to solve this problem, titanium dioxide with modification is carried out, such as ion doping, quantum dot sensitization and so on. The quantum dot sensitization is one of the hot research topics recently. The Cd S, Ni O and so on semiconductor materials have a good energy level matching relationship with TiO₂. They are commonly used as photosensitizer to broaden the scope of TiO₂ light absorption, and improve the photoelectric properties. At present, the researchers used quantum dot sensitization method is the subsequent ion layer adsorption and reaction. Because in the process of chemical synthesis, space steric hindrance will affect the samples and form the finite effective area and heterojunction greatly influenced the improvement of the photoelectric performance.With the continuous development of technology of high temperature and high pressure, the technology exhibits unique property in discovery of new structural materials and synthetic testing, such as the shorter and efficient synthetic time of samples, the improved mass density of the material, the increased cationic coordination number, the gibbs free energy modulation of reactants and products according to the expected direction, preparation of the target product, etc. At the same time, the change of pressure and temperature can effectively adjust the atomic spacing of samples. In turn, its configuration and rule have affected, resulting in the change of the electronic structure of the samples. Therefore, under the dual function of pressure and temperature, the band gap of semiconductor materials can be changed and affect the photoelectric performance. The high temperature and high pressure technology has the advantages in the material synthesis. For the first time this paper adopts the method of high temperature and high pressure to prepare Cd S/TiO₂ and Ni O/TiO₂ heterojunction and the photoelectric properties were investigated. The main contents of the paper are as follows:（1）Through the hydrothermal method, using tetrabutyl titanate ethanol solution and sodium hydroxide aqueous solution as reactants, white precipitate（Na₂Ti₃O₇） was prepared firstly. After repeated centrifugal dilute hydrochloric acid cleaning and annealing at 600 ℃, TiO₂ nanowires is prepared. The average diameter of the nanowires is about 200 nm by SEM images. Subsequently, we adopted sequent ion layer adsorption and reaction method（SILAR） to fabricate the Cd S quantum dots（QDs） on TiO₂. The characteristics of the Cd S/TiO₂ were carried out by FESEM, XRD, TEM and so on.The Cd S was successfully grown on the TiO₂ nanowires. Cd S QSs is around 10 nm. Then, the Cd S/TiO₂ nanowires were by cold-press molding and assembled into the domestic six sides jacking machine. In the high temperature and high pressure experiments, we studied two aspects as follows. Firstly, photoelectric properties of Cd S/TiO₂ nanowires under different pressures have been investigated, maintaining that the temperature was 100 ℃ and the time is 30 min. Different pressure of Cd S/TiO₂ nanowires photoelectric properties shows that with increasing pressure, the Cd S/TiO₂ firstly increases and then decrease. When the pressure was P =3 GPa, high photoelectric conversion efficiency of 2.13% was obtained and the photocurrent density was about 3.14 m A/cm2. Secondly, based on the above experiments, we further investigated the effect of different temperature on the properties of Cd S/TiO₂ nanowires photoelectric effect. With the increase of temperature, the short circuit current density Cd S/TiO₂ is increased firstly and then exists reducing phenomenon. When the pressure was P = 3 GPa, the time was t = 30 min, and the temperature was t = 300 ℃ temperature, the photocurrent density was about 4.91 m A/cm2 and the highest photoelectric conversion rate of 3.51% was obtained.（2） With ethanol solution of nickel nitrate and oxalic acid ethanol solution as reactant, the light blue precipitate was prepared by chemical deposition. After repeated centrifugal process and annealing at 400 ℃, Ni O nanocrystalline was obtained. The preparation of sample is pure（JCPDS No. 47-1049） the Ni O nanocrystals by XRD. Then by the methods mentioned above, the preparation of TiO₂ and Ni O experienced cold pressing and molded into the top six sides compresso. The sample was prepared processing conditions, exploring effect of different pressures under the same T = 100℃. The conditions T=100 ℃, T=30 min and P = 3 GPa were the best parameters. Then different temperature on the Ni O/TiO₂ heterojunction photoelectric properties were investigated maintaining the pressure was P = 3 GPa, the time was T = 30 min. When the temperature was T = 200 ℃, photocurrent density was about 0.425 m A/cm2 and the highest photoelectric conversion efficiency of 0.084% was obtained.