Adjusting Nanostructures Of TiO₂-based Composites For Photocatalytic Reduction Of CO₂ Into Renewable Hydrocarbon Fuel

Photocatalytic reduction of CO2 to hydrocarbon fuel is a promising avenue to reduce CO2 emission and to produce value-added chemical fuels, which is one of the effective methods to realize the carbon cycle. Titanium dioxide (TiO2) is widely used in the photocatalytic fields including water splitting, degradation of organic contaminants, and CO2 reduction, because it is cheap, non-toxic, and light stability. However, the recombination of electron-hole pairs and the lack of visible light absorption limit its photocatalytic performance. The processes of photocatalytic reduction of CO2 involve multiple electrons to take part in. The adjusting of TiO2 nanostructures can affect the separation and transfer of photogenerated charge carrier, and thus affects photocatalytic performance. In this dissertation, a series of TiO2-based nanocomposites were fabricated via introducing materials such as graphene, CdS particles, and Au. The photocatalytic activity and mechanism of CO2 reduction of TiO2-based nanocomposites were studied. The details are summarized as follows:(1) Robust hollow spheres consisting of molecular-scale alternating titania (Tio.9102) nanosheets and graphene (G) nanosheets as building blocks are successfully fabricated by a layer-by-layer assembly technique on polymer beads as sacrificial templates using a microwave irradiation technique to simultaneously remove the template and reduce graphene oxide into graphene. The 9-time significant increase of photocatalytic activity of G-Ti0.91O2 hollow spheres was confirmed with photoreduction of CO2 into renewable fuels (CO and CH4), relative to commercial P25. The large enhancement of the photocatalytic activity benefits from:(1) the ultrathin nature of Tio 9102 nanosheets allows charge carriers to move rapidly onto the surface to participate in the photoreduction reaction; (2) The sufficiently compact stacking of ultrathin Ti0.91O2 nanosheets with G nanosheets allows the photogenerated electron to fast transfer from Ti0.91O2 nanosheets to G to enhance lifetime of the charge carriers; (3) The hollow structure potentially acts as a photon trap well to allow the multi-scattering of incidence light for the enhancement of light absorption(2) A novel in situ simultaneous reduction-hydrolysis technique was developed for fabrication of TiO2-graphene hybrid sheets in ethylenediamine (En) solvent. This method can simultaneously reduce graphene oxide (GO) into graphene and spread TiO2 nanoparticles onto graphene through chemical bonds (Ti-O-C bond) to form 2D sandwich-like nanostructure. In contrast with prevenient G-TiO2 nanocomposites, abundant Ti3+ was detected on the surface of TiO2 of the present hybrid, caused by reducing agent En. The Ti3+ sites on the surface can serve as sites for trapping photo-generated electrons to prevent recombination of electron-hole pairs. The synergistic effect of the surface-Ti3+ site and graphene favors the generation of C2H6. Our work may open a new doorway for new significant application of graphene for selectively catalytic C-C coupling reaction.(3) Au@TiO2 yolk-shell hollow spheres for photocatalytic reduction of CO2 were prepared by using Au-encapsulated carbon spheres as a template and adjusting the hydrolysis of tetrabutyl titanate to control the thickness of TiO2 shells. An electric field in a photocatalytic system consisting of Au@TiO2 yolk-shell hollow spheres is created to enhance the generation of electron-hole pairs and remit the charge-carrier recombination. Local surface plasmon resonance (LSPR)-mediated local electromagnetic field nearby Au nanoparticles can not only enhance the local generation and subsequent separation of electron-hole pairs in TiO2 shells to improve the photoreduction yield of CO2,but also facilitate chemical reactions involved multiple e-/H+ transfer processes to allow the formation of high-grade carbon species (C2H6), which was rarely observed in precedent CO2 photocatalytic reduction systems. The work may provide a new viewpoint for designing photocatalysts for artificial photosynthesis involved multiple reactions.(4) Hollow spheres consisting of alternating ultrathin Ti0.91O2 nanosheets and CdS nanoparticles were fabricated via exquisite layer-by-layer self-assembly to achieve a redox mediator-free artificial Z-scheme for photocatalytic reduction of CO2 into CH4, which exhibits 7-times enhancement of CH4-production rate relative to pure Ti0.91O2 hollow spheres, due to greatly prolonged lifetime of charge carriers. The indirect optical transition effect confirms the successful construction of artificial Z-scheme system where excited electrons in conduction band of Ti0.91O2 nanosheets recombine with holes in valence band of CdS nanoparticles through d-p conjugation, totally different from traditional TiO2-CdS systems. The Z-scheme system leads to the holes in the VB of Ti0.91O2 with a strong oxidation power and the electrons in the CB of CdS with a strong reduction power, which contributes to improved photocatalytic performance. Our results may open a new viewpoint for tailoring and constructing hybrid nanostructure of semiconductors for photocatalysis.