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ที่มาและความสำคัญ(Background and importance) *:
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Carbon dioxide (CO2) emission has become a topical issue due to global policies and strategic goals for a low carbon economy. The policies and goals are the main driving forces for the development of technologies for carbon capture and storage (CCS). Conventional approaches for CO2 management utilize a liquid or solid base for CO2 capture. The capture process is usually followed by a regeneration of the liquid or solid base, in which a significant amount of energy is consumed and the CO2 is released back into the atmosphere. The other approach is CO2 sequestration, involving the injection and storage of CO2 deeply in an underground site. The sequestration promises a dramatic reduction in CO2 emissions but is inhibited by the risk of CO2 leakage and requirements for long-term inspection. One alternative to CCS is CO2 conversion to a high-value product. The approach concerns not only the removal of CO2 but also the generation of commodities, such as methane, carbon monoxide, formaldehyde, methanol, and ethanol.
Different techniques were introduced and demonstrated for CO2 conversion, including CO2 fixation and conversion by microalgae, CO2 hydrogenation by metal oxide catalyst, and CO2 splitting using a metal oxide electrocatalyst. Photoreduction of CO2 to liquid fuels is an attractive alternative that relies on photocatalysts such as zinc oxide (ZnO), cadmium sulfide (CdS), and titanium dioxide (TiO2). TiO2 is a popular photocatalyst that has been used in the decomposition of organic pollutants in wastewater. It has also featured heavily as a potent photocatalyst for CO2 conversion. An issue regarding TiO2 concerns the wide bandgap energy, which limits the number of photo-generated electrons resulting in a fast pairing rate of electrons and holes. TiO2 also responds only to UV light, preventing it from utilizing the full intensity of natural sunlight.
A photocatalytic heterostructure between two 2-dimensional nanostructures, defined as a 2D–2D heterostructure, can be an ideal photocatalytic platform that provides excellent charge mobility and charge separation. The heterostructure contains interfaces between two semiconductors with unequal bandgap values, which induces a local electric field that directs the flow of charge carriers. The team of J. Sun (J. Sun , H. Zhang , L. H. Guo and L. Zhao , ACS Appl. Mater. Interfaces, 2013, 5 , 13035 —13041) synthesized a TiO2 nanosheets/graphene 2D–2D heterostructure by introducing hydrofluoric acid (HF) to a titanate–graphene oxide (GO) mixture in a solvothermal process. The TiO2 nanosheets grew on and were in good contact with the GO. Zhao et al.(B. Zhao , L. Lin and D. He , J. Mater. Chem. A, 2013, 1 , 1659 —1668) and Keerthana et al. (B. Gomathi Thanga Keerthana , T. Solaiyammal , S. Muniyappan and P. Murugakoothan , Mater. Lett., 2018, 220 , 20 —23) demonstrated the use of alkali solutions such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) as a soft template in the formation of sodium dititanate (Na2Ti2O5) nanosheets. The mechanism involved hydrolysis of a titanate precursor, followed by a formation of the dititanate interlayers. The layers were further intercalated by the alkali ions, stabilized, and became nanosheets.
Graphene is a superior choice for one-half of the 2D–2D heterostructure since it has a good charge transfer ability, chemical stability, and outstanding light absorption properties. It can be synthesized following a chemical exfoliation approach, yielding GO, which is a few layers of graphene sheet with carbon–hydrogen–oxygen functional groups. The functional groups serve as defects in the nanostructure and provide sites for the precipitation and immobilization of metal/metal oxide nanostructures. The sodium dititanate nanosheets can be synthesized and immobilized on GO via a hydrothermal process in an alkali solution. The 2D–2D photocatalytic heterostructure can be of great use to the photoreduction of CO2 to liquid fuels.
In this work, we synthesized the 2D–2D photocatalytic heterostructure of copper-doped sodium dititanate nanosheets/GO (CTGN). The heterostructure was synthesized using a one-step hydrothermal process with the addition of a NaOH soft template. Some chemical, physical and crystallographic properties of the solid samples were studied using analytical instruments, including X-ray diffraction (XRD), Raman microscope, Fourier-transform infrared spectroscopy (FTIR), UV-Visible spectroscopy (UV-Vis), high-resolution transmission electron microscope (HR-TEM), field effect scanning electron microscope (FE-SEM) and electron dispersive spectroscopy (EDS). The photocatalytic property was characterized using photoluminescence spectroscopy (PL). Liquid samples from the photoreduction of CO2 were analyzed using gas chromatography (GC) to obtain composition of the liquid fuels.
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