Preparation And Characterization Of Semiconductor Materials Of Ta-doped ZnO And Cu₂ZnSnS₄by Wet Chemical Method For Their Applications

The rapid development of modern industry not only brings abundant material wealth to human society, but also causes energy shortage and environmental pollution. The environmentally friendly solar cell and photocatalytic materials with low cost and high efficiency open a feasible way to solve these problems.One hand, the traditional photocatalytic semiconductor materials such as TiO2and ZnO show excellent UV photocatalytic performance and wide application prospects, but the main drawback of low absorption in the visible light leads to the doping and modification of traditional photocatalytic materials. At present, the water shortages in many parts of the world make clean and safe drinking water more and more important, because of many diseases caused by microorganisms. The semiconductor photocatalytic oxidation technology, as an efficient and environmentally friendly water treatment method, can degrade the hazardous industrial waste water and toxic exhaust gas and disinfect water with some advantages such as simple and mild reaction conditions, wide use, and recycling.On the other hand, solar is the most abundant, clean and renewable energy source. The photovoltaic energy keeps growing at a rate of40%per year in the past decade. CIGS thin film solar cells has been applied in commercialization, but the complex processing, scare elements and high cost prevent its large-scale production. Therefore to explore new semiconductor light-absorbing material with simple process, low cost, and high energy conversion efficiency is nowadays an attractive research area. Cu2ZnSnS4(CZTS) as a new type of photovoltaic light-absorbing materials has drawn great attention.In this thesis, semiconductor materials of Ta-doped ZnO nanopowders, CZTS films and nanocrystals were prepared by wet-chemical method and their basic properties were characterized deeply. The antimicrobial effect and antibacterial mechanism of Ta-doped ZnO nanoparticles by a modified Pechini method have been studied on four common bacteria systematically. Meanwhile the processing, phase structure and optical band gap of sol-gel and hydrothermal method-derived CZTS film and nanocrystalline, respectively, have been investigated in depth. The fabrication procedure of CZTS thin film solar energy cells has been explored. Main achievements are summarized as follows:1. The Ta doping concentration effect on the minimum inhibitory concentration (MIC) of four bacteria of Gram-positive B. subtilis, S. aureus and Gram-negative E. coli, P. aeruginosa under dark ambient has been evaluated. It is found that compared to pure ZnO,5mol%Ta-doped ZnO nanopowders show significantly improved antimicrobial properties with MIC of160-200μg/ml for S. aureus, E. coli and B. subtilis. The strong bactericidal efficacy of Ta-doped ZnO on P. aeruginosa, E. coli, and S.aureus is confirmed under visible light illumination. The enhanced antibacterial acitivity of Ta-doped ZnO is attributed to larger photocatalytical acitivity under visible light, more surface defects and hydroxyl groups, and smaller grain size due to the incorporation of Ta5+ions into ZnO. The possible antimicrobial mechanisms in Ta-doped ZnO systems under visible light and dark conditions are also proposed. The photocatalytic generation of H2O2under visible light plays a crucial role during the antimicrobial test, which provides a possibility for applications in environmental and water treatments. In addition, the antimicrobial effect of Nb-doped ZnO nanoparticles from a modified Pechini method was also characterized, which is almost similar to that of Ta-doped ZnO nanoparticles. In comparison with pure ZnO,3mol%Nb-doped ZnO nanopowders show optimal antimicrobial properties.2. Using ethylenediamine as a coupling and stabilizer agent, the impact of annealing atmosphere and temperature on sol-gel derived CZTS phase structure, morphology, and optical band gap have been investigated systematically. The CZTS films shows basically kesterite phase with optical band gap of1.48eV by annealing at 500℃in ambient containing H2S, which is suitable as a light-absorbing material for solar cells. The growth curve, crystal structure, surface morphology and optical properties of CdS films prepared by chemical bath deposition have been examined. Based on the above results, the CZTS thin film solar cells with two different electrode configurations have been attempted. CZTS film cells on Mo electrodes have an open voltage of214mV with very low short current, which seriously affect the energy conversion efficiency. The fabrication procedure of CZTS film cells still need further optimizing and improving.3. CZTS nanocrystals with high purity kesterite phase in the range of3-10.5nm have been synthesized at180℃by hydrothermal method successfully. The effect of hydrothermal treatment time on the grain size and crystalline behavior of CZTS nanoparticles have been studied systematically. With decreasing CZTS nanocrystal size, the broadening Raman spectra appear, moving to the high wavenumber region. The size effect of CZTS nanocrystals on the optical absorption and band gap has been characterized. The optical absorption spectra of CZTS nanocrystals have a blue shift at4.8nm and below. It is found that when CZTS nanocrystal size of3nm is in the same order of magnitude with Bohr radius (2.5-3.4nm), the CZTS quantum dots exhibit obvious quantum confinement effect with bandgap bluwshift of0.47eV. The hydrophilic CZTS quantum dot by hydrothermal method is expected to apply in quantum dots-sensitized solar cells.