Synthesis Of Ultrathin TiO₂ Nanosheets & Their Applications As Photocatalysts For CO₂ Reduction

Due to the peculiar intrinsic properties of ultrathin TiO2 atomic layers and their potential applications for photocatalytic CO2 reduction, the ultimate goal of investigation elucidated in this dissertation was to reduce CO2 into hydrocarbon fuels via the photocatalysts. Therefore, using a facile and scalable lamellar hybrid strategy, novel atomic layers have been synthesized and hence showed enhanced photocatalytic activities in CO2 reduction to hydrocarbon fuels. The details of this dissertation are summarized concisely as follows:(1) In the first chapter, the author briefly interpreted the ultrathin two-dimensional nanosheets, because during the past few years the main research focus was on the two-dimensional ultrathin nanosheets after the impetus from the Nobel Prize-winning work of graphene. These peculiar properties such as, (i) quantum confinement, (ii) large surface area, (iii) ultrahigh level of active sites, and (iv) atomic level thickness, endow the ultrathin nanosheets with a new regime for delivering auspicious prospects to fulfill people’requirements regarding the generation of renewable hydrocarbon fuels from CO2 reduction. The hydrocarbon fuels generated by ultrathin nanosheets may be methane, methanol, formate etc., which could be utilized to mitigate future energy crises.(2) In the second chapter, the main fascination for the research work was manifested in detail. Actually, carbon dioxide (CO2) being the greenhouse gas is the most notorious gas released by natural and artificial processes. Unluckily, due to the escalation of industrial progress, this balance has progressively been messed up, generating more CO2 in environment and leading to the global warming phenomena. Therefore, great attention has been paid on the CO2 conversion into useful chemicals, which seems to be the main desire of current scenario. Hence, this environmental issue stimulates us to trail an appropriate material model for studying the CO2 reduction into valuable chemicals, in which the atomically thin two-dimensional TiO2 nanosheets could serve as an ideal model, owing to their (i) relatively large surface area, and (ii) the ultrahigh fraction of active sites for photocatalytic CO2 reduction into hydrocarbon fuels.(3) In the third chapter, the author first clarified the synthesis strategies used for TiO2 atomic layers fabrication. Basically, these novel materials have been synthesized by solvothermal methods using a lamellar hybridization strategy. Main benefits of these methods are (i) convenience, and (ii) cost effectiveness which can augment the probability of nanosheets synthesis up to the industrial level. Therefore, this approach has opened a new route in the world of materials science to tune the material performance. In the second part, characterizations of ultrathin nanosheets using various advanced techniques and the understanding of their clear structure-property correlation have been illustrated.(4) In the fourth chapter, the author first realized the synthesis of 1.66 nm thick TiO2 atomic layers by virtue of a lamellar TiO2-octylamine hybrid precursor, followed by the investigation of their photocatalytic CO2 reduction. Photoreduction of CO2 into fuels over TiO2 helps to relieve the increasing energy crisis and the worsening global climate, however, the low energetic efficiency impedes its large-scale applications. Herein, ultrathin TiO2 layers are first put forward to fully optimize their crucial CO2 photoreduction processes through affording abundant catalytically active sites and increased two-dimensional conductivity. Ultimately, the atomic thickness of 1.66 nm endows TiO2 with ultrahigh fraction of surface atoms, which ensures stronger UV light absorption and higher CO2 reduction ability compared with its bulk counterpart. Benefitting from the increased density of states near Fermi level and the vast majority of charge density concentrating on the surface, the TiO2 atomic layers show increased conductivities, which is confirmed by the temperature-dependent resistivities. The 3 times higher fluorescence lifetime, revealed by time-resolved fluorescence spectroscopy, accounts for the increased separate rate of photoexcited electron-hole pairs. As an outcome, the TiO2 atomic layers achieve a formate formation rate of 1.9 μmol g-1 h-1,450 times higher than that of bulk counterpart and also roughly 2 times higher than that of previously reported Ag-modified BaLa4Ti4O15.Briefly, this study will unlock many opportunities for designing efficient CO2 photoreduction.