Exploring Molecular Nature Of Interfacial Heterojunctions Of TiO₂-based Semiconducting Oxide Nanocomposites

Design and synthesis of semiconducting composite materials is an effective way to obtain the photocatalysts with high performance. The formation of hetero-or homo-interfacial "junction" is often considered to be a key for enhanced photocatalysis of semiconductor coupling. However, the chemical composition and specific structure of junction as well as its molecular mechanism for understanding the generation, migration, and interfacial transfer of photoinduced electron-hole pairs has not been clarified. Therefore, revealing the light absorption of semiconducting oxide composite photocatalyst and microscopic mechanism of photocatalytic processes at the molecular level is the crucial mission in the realm of the photocatalytic scientific research.Aiming at these scientific problems, the thesis uses TiO2 based composites and photocatalytic biomass reforming for H2 evolution as research object and model reaction, respectively, and proposes a new idea to explore at molecular level the molecular mechanism of heterojunctions at the interface of nanocomposite photocatalysts via a strategy of designing and constructing single-core metal-oxygen-metal molecular at the surface of TiO2 semiconductor using surface organometallic chemistry, combined with the characterization and performance evaluation. We therefore performed systemic study as follows:(1) We studied the grafted reaction beween the tetramethyltin and acidic hydroxyl on surface of anatase TiO2 photocatalyst under the vacuum in-situ conditions. (2) The well-defined Tiâ…£-O-Snâ…£ heterojunctions were constructed on the TiO2 surface and the photocatalytic mechanism of SnO2/TiO2 composite was studied at the molecular level. (3) We designed the Pt/SnOx/TiO2 ternary nanoheterostructure and discussed the effects of composition and structure on the photocatalytic performance of photocatalyst. (4) We constructed the SnOx and RuO2 co-modified TiO2 composite photocatalyst and studied the effects of Tiâ…£-O-Snâ…£ and Tiâ…£-O-Ruâ…£ interfacial heterojunction formed at the TiO2-RuO2 and SnOx-TiO2 interface on the generation, migration, and interfacial transfer of photogenerated charge carries. (5) We presented the reaction process and path of photocatalytic biomass reforming over the TiO2-based composites.The main results and conclusions obtained in this work are presented as follows:(1) It was found that the grafting reaction of SnMe4 on surface hydroxyl species of TiO2 can occurrs and the Ti-OSnMe3 complex forms on TiO2 surface. The reaction equation can be described as Ti-OH+SnMeâ†'Ti-OSnMe3+ CH4↑. The reactivities of surface hydroxyl toward the SnMe4 were related to their acidities, the acidic intensity of these surface hydroxyls was well-established to follow a decreased order of 3735 cm-1≈3715 cm-1> 3688 cm-1≈3672 cm-1> 3644 cm-1. (2) We synthesized SnOx/TiO2 composites with well-defined Tiâ…£-O-Snâ…£ heterojunction by controlling the preparation conditions of Ti-OSnMe3 complexes and subsequent processing conditions of grafted species. It was found that the photocatalytic H2 evolution activity of this composite material much higher than that of pure TiO2 sample. We proposed the formation mechanism and photocatalytic mechanism of molecular junction. (3) We found that the co-modification of Pt and tin-oxo species can further improve the photocatalytic hydrogen evolution activity of TiO2. The photocataltic activity of sample was enhanced by loading Pt nanopraticles on SnOx/TiO2. However, grafting SnOx onto Pt/TiO2 sample decreased the photocatalytic activity. For the Pt/SnOx/TiO2 sample, the Pt and SnOx co-catalyst showed the synergistic effect for enhanced photocatalytic acitivity. The directional migration of photogenerated electrons of TiO2 can be controlled by changing the anchoring site of supported Pt nanoparticles. (4) It could be established that the modification of anatase TiO2 with RuO2 and SnOx co-catalysts improves the photocatalytic hydrogen production from biomass/water solutions, and RuO2 and SnOx show the synergistic effect. It Was indicated that the Tiâ…£-O-Snâ…£ molecular junctions were formed at SnOx-TiO2 interface and Tiâ…£-O-Ruâ…£ molecular junctions were formed at TiO2-RuO2. These two kinds of molecular junctions can promoted the electron and hole interfacial transfer, respectively, and consequently enhanced the charge separation efficiency and photocatalytic reaction rate. The photocatalytic biomass reforming over Sn, Ru co-modification TiO2 nanocomposites was a general free radical reaction pathway triggered by the hole oxidations. The CxHyOz renewables underwent one or more processes for the sequential oxidation of alcohol to aldehyde, acid, and finally CO2 and CO. The activity results clearly revealed that the proton reduction reaction of H++e-â†'1/2H2 is the controlling-rate step of the overall photo-reforming reaction. The hydrogen evolution rate was directly related to the a-dehydrogenation and oxidation of the reacting substrates over the RuO2 cocatalyst. (5) The IR results showed that both chemisorbed and physisorbed states of 2-propanol were present on the Pt/TiO2 surface. The rapidly deprotonation reaction of chemisorbed 2-propoxide occurred by the attack of photogenerated h+ was the initial step of reforming reation for H2 production. The OH groups in 2-propanol played key roles in the adsorption of 2-propanol and formation of charge transport ways.These results illustrated that the heterojunctions in composite photocatalyst are essentially the molecular junction. It provides new insights into understanding of photocatalytic microcosmic mechanism at atomic and molecular level, and offers a reference strategy for constructing and designing the efficient doped photocatalyst and semiconducting composite heterostructure photocatalysts. This work has the most important signification for enriching the photocatalytic theory of foundations and promoting the sustainable development of photocatalytic science.