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Interfacial Engineering of Low-dimensional nanostructures for Artificial Photosynthesis

Interfacial Engineering of Low-dimensional nanostructures for Artificial Photosynthesis
Alternative Author(s)
Issue Date
2021. 2
Solar energy, an infinite source of sustainable and alternative energy production, is the most important asset to achieve renewable energy to overcome energy crisis. Artificial photosynthesis is the one that can use solar energy as alternative energy production without external applied energy to produce hydrocarbon fuels. However, its low efficiency occurring from a photocatalyst is a fatal limitation in this system so that enhancing photocatalytic CO2 reduction reaction using efficient photocatalysts is a challenging issue. In order to fabricate efficient semiconductor photocatalyst materials, not only economical but also sustainable processes should be considered. Moreover, engineering design using low-dimensional nanostructures such as 1-dimensional or 2-dimensional is an important strategy to improve artificial photosynthesis as 0-dimensional nanoparticles show poor electron transfer movements due to agglomeration issues among the nanoparticles, reducing active sites. In this thesis, noble-metal-free semiconductor photocatalysts using facile and simplest methods were developed and designed with new strategies for artificial photosynthesis reaction: electron transportation, sufficient activation energy, and maximization of surface active sites. First, the appropriate semiconductor materials are selected without noble-metal to design heterostructures to evaluate photocatalytic activities. Heterostructure between TiO2 and graphitic carbon nitride (g-C3N4) were selected to broaden the solar spectrum of TiO2 using g-C3N4 which has lower bandgap than that of TiO2. We found that the optimized photocatalysts showed the highest photocurrent density of 19.7 μA/cm2, lowest impedance value of 475.6 ohm·cm2, and also the long-term stability for 15 hours. Moreover, 1-dimensional TiO2 nanorods were studied to identify the electron movements compared to 0-dimensional TiO2 nanoparticles (commercial P25). We have confirmed that 1-dimensional nanorods showed about 10 times higher photocurrent density and 90 times lower impedance value than that of 0-dimensional structures due to fast electron pathway. Second, the disordered black titania nanofibers with g-C3N4 were fabricated to achieve sufficient activation energy which is necessary to acquire better performances in artificial photosynthesis. As photocatalytic CO2 reduction of 1-dimensional black titania nanofibers was compared with P25 or H2-reduced P25, black titania showed higher CO2 selectivity, indicating that fast electron movements of 1-dimensional affect the photocatalytic CO2 reduction performance. The analysis in optical absorbance and CB/VB potential levels, g-C3N4/black titania nanofibers have produced higher CO or CH4 yields from sufficient 〖∆E〗_(〖a,CH〗_4 ) of 0.54 eV compared to that of g-C3N4 or black titania nanofibers, suggesting an effective separation of the photoinduced electron and hole pairs. Therefore, 1-dimensional nanofibers indicated higher photocatalytic CO2 conversion. For artificial photosynthesis system, CO2 selectivity is one of the crucial parts, producing a high yield of hydrocarbon with the suppression of H2. To increase the CO2 selectivity, we controlled low-dimensional nanostructures as 1-dimensional and 2-dimensional through interfacial engineering and thus, developing noble-metal-free heterostructures. Finally, TiO2/MoS2/g-C3N4 and TiO2/MoSe2 heterostructures which combines 1-dimensional and 2-dimensional nanostructures were fabricated, respectively. These 1-dimensional and 2-dimensional heterostructures showed a strong photocatalytic CO2 reduction redox ability at surfaces via their interfaces, reducing its recombination of pointless charge carriers due to internal electric field following S-scheme mechanism. As a result, these heterostructures the fast electron pathway due to low interface resistance with higher conductivity through well-connected interfaces among TiO2/MoS2/g-C3N4 and TiO2/MoSe2. In conclusion, low-dimensional heterostructures were successfully developed in this thesis. Through controlling its absorbance range, the sufficient kinetic overpotential, and expanding the active sites were achieved to promote hydrocarbon production reaction along with high CO2 selectivity. Finally, we have designated significant factors that influence artificial photosynthesis reaction using noble-metal-free semiconductor materials through simplest process to advance further system.
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