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ที่มาและความสำคัญ(Background and importance) *:
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"Carbon capture, utilization, and storage (CCUS) is a research and industrial approach for managing CO2. CO2 is either captured, converted to high-value products, or stored underground. Researchers worldwide have contributed to the continual improvement of CCUS technologies, but there are several issues to tackle.
Photoreduction of CO2 to liquid fuels is a potential approach to CO2 conversion to high-value products. It involves the coupling of photoelectrons, generated by a photocatalyst, and dissolving CO2 in an aqueous medium. This triggers the catalytic transformation of CO2 into liquid fuels, such as methanol and ethanol. The major key to the approach is assistance from a good photocatalyst, such as zinc oxide (ZnO) or titanium dioxide (TiO2). The photocatalyst utilizes photon energy from light, mainly in the ultraviolet region (UV), to generate photoelectrons that assist in the photoreaction. TiO2 is particularly well known for its performance in water purification. It is commercially available, chemically stable, and relatively nontoxic. However, the role of TiO2 is limited by its large energy band gap (3.2–3.5 eV), narrow light adsorption range, and fast electron–hole recombination rates, which lead to a slow photoreaction and inadequate production yield. Precious metals, such as silver (Ag), palladium (Pd), and copper (Cu), have been shown to enhance the photoactivity of TiO2 by providing a transition state for photoelectrons to separate from holes and interact with other active radicals. Such acts decrease the band gap energy and reduce the electron–hole recombination rate, leading to more photoelectrons becoming involved in the photoreaction and better photoactivity of the photocatalyst. In another approach, immobilization of TiO2 on carbon nanostructures, such as carbon nanotubes and graphene, was found to extend the light adsorption range from UV to the visible region. It can also create interfaces between TiO2 and carbon nanostructures that serve as N–P heterojunctions. The heterojunctions enhance separation between photoinduced electrons and holes, promoting photoactivity of such materials.
Graphene is a 2D carbon nanostructure with outstanding charge transfer abilities, chemical stability, and optical properties. It can be chemically synthesized using a chemical exfoliation approach to yield graphene oxide (GO). This provides a multilayered graphene sheet with functional groups, such as carboxyl, hydroxyl, and epoxy. Interactions between the functional groups and metal precursors lead to the accumulation of metal ions and eventually immobilization of metal nanoparticles on the GO sheet. The TiO2/graphene composite is a high-performance photocatalyst with enhanced light adsorption and high surface activity. Various methods have been demonstrated for the synthesis of TiO2/graphene composites, including the sol–gel, solvothermal, and hydrothermal methods. The hydrothermal method offers good control over the growth of TiO2 crystal structures and can be carried out at a relatively lower temperature. Because GO can be thermally damaged at a temperature higher than 150 °C, the lower synthesis temperature can be of great advantage. Khalid and his team synthesized Cu–TiO2 nanoparticles using the sol–gel technique and composited Cu–TiO2 with graphene using a hydrothermal process. The composites exhibited a wide range of light absorption covering UV and visible light regions and provided better photoactivity than the Cu–TiO2 particles and TiO2/graphene. Song and his team synthesized a Pd–TiO2/graphene composite by incorporating TiO2 nanoballs on graphene using a poly (diallyl dimethyl ammonium chloride) linker. The composite was introduced to the palladium chloride along with ammonia solution and heated in a hydrothermal reactor. Katsarakis mixed P25 TiO2 with a silver nitrate precursor along with a dimethylamine borane reducing agent to obtain an Ag–TiO2 powder. The powder was dispersed in a water–ethanol mixture and introduced to GO in a hydrothermal reactor. The Ag–TiO2/GO composite showed outstanding photoactivity in decoloring methylene blue.
In this work, we synthesized Ag–TiO2/GO, Pd–TiO2/GO, and Cu–TiO2/GO composites using a one-step hydrothermal method. The method yielded high-quality composites with metal nanoparticles distributed on GO sheets. The composites were utilized in the photoreduction of CO2 to ethanol and were characterized for optical, geometrical, chemical, and crystallographic properties. Analytical instruments, including a UV–visible spectrophotometer, a scanning electron microscope (SEM), a transmission electron microscope (TEM), an energy dispersive X-ray spectrometer (EDS), an X-ray photoelectron spectrometer (XPS), a Fourier transform infrared spectrometer (FTIR), a thermogravimetric analyzer (TGA), an Autosorb surface analyzer for conducting Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) surface area measurements, an X-ray fluorescence spectrometer (XRF), and an X-ray diffractometer (XRD) were used."
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