Synthesis and characterization of silver phosphate from lyotropic liquid crystalline mesophase template as a photocatalyst

Dağ, Ömer
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Bilkent University
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Increasing energy demands and environmental problems are the driving forces of the current literature. Over the years, many new compounds have been synthesized and also morphological control of the well-known compounds have been the major topics to improve/contribute to the solutions of energy demand and environmental issues. One of these issues is finding an efficient and stable photocatalyst for some of the environmental problems. Ag3PO4 has been a target material for dye degradation and water splitting processes. Silver phosphate has a suitable band gap for photo-oxidation process under visible light irradiation. However, it has stability and reusability problems that needs to be resolved to effectively use as an efficient photo-catalyst. Because of that, many research worked on the synthesis and stability issues of this material. In this thesis, the work focuses on surfactant:Ag(I):H3PO4 lyotropic liquid crystalline mesophase to synthesize mesoporous Ag3PO4. Two different surfactants (small, 10-lauryl ether, C12EO10 and large pluronic, triblock copolymer, P123, HO(CH2CH2O)20-(CH(CH3)CH2O)70-(CH2CH2O)20H), two different silver salts (AgNO3, SN and AgCF3SO3, AgOTf) and two different phosphate precursors (H3PO4 and LiH2PO4) have been used throughout this investigation. Solutions were prepared in water or ethanol by first dissolving surfactant, then adding stoichiometric ratio of AgNO3, and H3PO4. To achieve clear and homogenous solution, a small amount of HNO3 is added to the above solution. Without HNO3, some yellow precipitation occurs that needs to be filtrated out. According to XRD patterns, SEM images, and N2 adsorption-desorption isotherms, the yellow precipitate is bulk Ag3PO4. Decanted solution and normal acidified solution compares well with each other and the results are similar in further steps of the synthesis. Therefore, adding small amount of HNO3 to the solution overcomes the precipitation of bulk Ag3PO4 and used in further steps of the synthesis. Then, the solutions can be spin or drop-cast coated over glass slides to form the mesophases and thin/thick films. The films diffract at small angles, indicating the formation of the mesophase. However, the mesophases are not stable and gradually transform into soft mesocrystals that diffract at small and high angles. Later step is to determine a desired calcination temperature for mesoporosity. Therefore, first a high temperature (over 300˚C) treatments have been applied to burn all surfactant in the films. This ensures mesoporosity, but it also results some bulk formations; silver metal forms at high temperatures. Therefore, the calcination or heat treatment temperature has been gradually reduced down to room temperature (RT). At RT, soft mesocrystal forms that can be heat treated at various low temperatures (70-150˚C) to form Ag3PO4 in many different morphologies; these samples have no silver metal. All Ag3PO4 samples, obtained under different conditions, were tested in Rhodamine-B (Rh-B) dye degradation by visible light irradiation with a good activity. But the catalyst is not stable under catalytic conditions. To solve this problem, some samples were prepared under vacuum to convert surfactants carbons to coat the surface of the catalyst by carbon that stabilized the catalyst. In the last section of the thesis, cation exchange method has been developed to convert pre-formed mesoporous LiMPO4 (M = Mn, Co, and Ni) to Ag3PO4. Mesoporous Ag3PO4 has been obtained from all precursors but the ones obtained from LiCoPO4 performed the best in photo-degradation of dye under visible light and the ones obtained from LiMnPO4 is almost inactive. Therefore, this part needs further studies to understand details of these observations. Introducing carbon and cation exchange methods seem to be effective solutions for the stability problem of this photocatalyst. All synthesis products are tested in the photodegradation experiment and compared with each other. This thesis is partially clarified; how to synthesize mesoporous Ag3PO4, what the behavior of silver in system is, and how to stabilize the catalyst. Furthermore, the cation exchange process opens a new horizon for the Ag3PO4 synthesis.

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Ag3PO4, Lyotropic Liquid Crystalline Mesophase, Soft Mesocrystal, Photocatalyst, Cation Exchange
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