Preparation Of Sorghum Semiconductor Composite Films By Electrodeposition And Their Photocatalytic Properties

Photocatalytic technique has been become one of the most potential methods to solve the energy and environment problems due to their application in water splitting and photodegradation of organic pollutants. The focus of photocatalytic technology is photocatalyst, so the development of high photocatalytic activity photocatalyt has been a hot topic. Most semiconductor photocatalysts, which have been widely studied in past decaades, are only efficient under the ultraviolet (UV) light irradiation, such as TiO2, ZnS, ZnO. However, almost 43% of the solar radiation energy is in visible light area (400-750 nm), and only 4% comes from UV radiation. If the photocatalytic technology would be widely used, the key point is preparation of high activity, good stability and low cost visible-light response photocatalysts. There are four strategies for developing visible-light response photocatalysts: doping on the metal-or nonmetal-site of the UV-light response photocatalysts; dye-sensitized photocatalysts; semiconductor combinations; novel photocatalysts (such as metal chalcogenides, metal phosphide). In this study, we focused on the last two strategies, the composite films (including CdS/CdSe, TiO2/CdSe and SnS/SnSe) and novel ternary metal sulfide (Ag8SnS6) were fabricated by using facile electrodeposition method.This thesis mainly consists of following parts.1. Firstly, CdS thin films were prepared using the chemical bath deposition (CBD) process and annealed in argon atmosphere. The grain size and light absorbance increased after annealing. CdSe layer was overlain on the CdS films using galvanostatically with different deposition times. The resulting CdS/CdSe bilayer structure exhibits significant enhancements in optical absorption, photocurrent density, and photoconversion efficiency. And these parameters increased with the increase of film thickness. In particular, the CdS/CdSe (6 min) sample exhibited a maximum photocurrent density of 8 mA/cm2 and maximum photoconversion efficiency of 4.90%.In the next work, various kinds of CdS and CdS/CdSe thin films were fabricated on ITO conductive glass by chemical bath deposition (CBD) and/or electrodeposition (ED) methods. First, two kinds of CdS thin films were firstly deposited on ITO conductive glass by CBD or ED. It was found that the two kinds of CdS films showed different compositions and crystal structures. The CdS film prepared by CBD is S-rich and has a cubic zincblende structure, while the CdS film prepared by ED is Cd-rich and has a hexagonal wurtzite structure. Furthermore, CdSe was deposited on the as-obtained CdS films by a galvanostatical ED method, resulting in CdS/CdSe bilayer thin films. The achieved CdS/CdSe bilayer thin films showed enhanced optical absorption than the CdS monolayer thin film. For the photoelectrochemical properties, the CdS/CdSe bilayer thin film also exhibited enhanced photocurrent density and photoconversion efficiency in comparison with the CdS monolayer thin film, In particular, The CdS/CdSe bilayer thin film in which the CdS layer was obtained by ED, showed a maximum photocurrent density of about 6 mA/cm2 and maximum photoconversion efficiency of 3.41%.2. TiO2/CdSe composite films were deposited on FTO conductive glass by electrodeposition method. XRD and SEM characterization results proved that the composite films were successfully prepared. The photodegradation properties of obtained thin films were tested by the degradation of methyl ene blue (MB) under a simulate sunlight. Compared with mono-TiO2 film, the degradation rate of TiO2/CdSe composite films had been enhanced. After 3 h irradiation, the degradation rate of MB was 83%.In order to test the stability of TiO2/CdSe composite film, the photocatalytic degradation process was repeated three times by using the same sample. The degradation rate was just a slight decrease. It indicated that the composite film had a good reusability.3. The electrolyte of SnS and SnSe was analyzed by using cyclic voltammetry and linear sweep voltammetry, and the deposition parameters were obtained. SnS and SnSe thin films were electrodeposted with potentiostatic method. The component and crystal structure of SnS and SnSe films were characterized by EDS and XRD. The stoichiometric ratios of Sn/S and Sn/Se were close to 1. By observing the XRD patterns, the experiment results and standard cards matched very well. This proved that the SnS and SnSe films were successfully deposited. SEM was used to analyze the morphology and measure the thickness of films. The grain of SnS films was a rectangular shape, and SnSe films was irregular lamellar. The absorption property was tested by UV-Vis spectrophotometry; these three kinds of films (SnS, SnSe and SnS/SnSe) had a good absorbance in the visible light region. The energy band gaps of SnS and SnSe were calculated, and the calculated results and literatures reported matched very well. Finally, the photoelectrochemical properties of samples were tested, and the electrolyte was a aqueous solution of Eu(NO3)3 (0.2 M). The results indicated that SnS, SnSe and SnS/SnSe films had a good photoresponse under irradiation. Compared with literature reports, these films had greater photocurrent density and excellent stability.4. In this section, electronic, optical properties, surface energies and work functions of Ag8SnS6 was calculated by first-principles method. In order to understand the optical properties of Ag8SnS6, the dielectric function, absorption coefficient and refractive index were performed from the energy range 0 to 11 eV. The absorption spectrum indicates that Ag8SnS6 has a good absorbency in visible light area. Surface energies and work functions of (411), (413), (211), and (112) orientations had been calculated. All of these four orientations had a greater surface energy, which meant organic pollutants could be adsorbed on these surfaces. These results revealed the reason of Ag8SnS6 with an outstanding photocatalytic activity.The parameters of Ag8SnS6 parapration had been explored. A two-step method was used in the study. Firstly, the Ag-Sn alloy film was electrodeposited on FTO conductive glass with different current denisy. The compositon of alloy films was characteried by EDS. When the current denisy was -0.5 mA/cm2, the atomic ratio of Ag/Sn was close to 8. And then the Ag-Sn alloy films were sulfurized with sulphur vapour in a tubular furnace. A ternary sulfide of Ag-Sn-S was obtained. Through the analysis of the XRD, it contains the Ag8SnS6 grain in the Ag-Sn-S film.