Studies On Construction Of Ultra-thin TiO₂（B） Based Composites And Their Photocatalytic H₂ Evolution Performance

The development of novel renewable sustainable energy is highly demand due to it can resolve the crisis caused by the the rapid depletion of fossil fuels and pollution.As a clean and renewable energy,hydrogen is an ideal substitution for fossil fuels.Photocatalytic H₂ evolution can convert solar energy into valuable chemical energy,which is considered as one of the most effective routes to solve the energy and environmental problems.However,the low-efficiency,expensiveness and unstableness of most photocatalysts limited their large-scale application.Among various semiconductor photocatalyst,TiO₂ has been deemed as a promising practical photocatalyst due to its high chemical stability,non-toxicity,and low cost.However,the conventional bulk TiO₂ show low photocatalytic efficiency due to the rapid recombination of electron hole pairs.The ultra-thin 2D materials can boost photocatalytic performance due to their large surface area,abundant active surfaces.The ultra-thin thickness considerably decreases the distance needed for the separation and migration of electron-hole pairs reducing their recombination.In this doctoral thesis,the ultra-thin TiO₂（B）based composite were constructed in order to achieve high effciency H₂ evolution and their photocatalytic performance were studied.The main research works are as follows:（1）Ni₂P quantum dot was used as a substitute for noble metal cocatalyst to promote the photocatalytic performance and stability.A novel 0D/2D architecture,fabricated by anchoring Ni₂P quantum dots on ultra-thin TiO₂（B）nanosheets,was successfully synthesized via a solvothermal method.The optimum photocatalytic H₂ evolution rate with 10 wt%Ni₂P/TiO₂（B）(3.966 mmol?g^-1?h^-1)was superior to Pt loaded TiO₂（B）(3.893 mmol?g^-1?h^-1),which was 15times higher than pure TiO₂（B）nanosheets.Moreover,the Ni₂P/TiO₂（B）catalyst showed high stability and reusability.Only 5%of performance degradation was found in multiply cycled H₂production runs for a 30 h period,indicating the stability of Ni₂P/TiO₂（B）is excellent.In addition,the H₂ production rate can be considerably increased furthered by using synergistic photothermal H₂ evolution(20.129 mmol?g^-1?h^-1 at 90?C).The Ni₂P exhibits excellent conductivity,which can promote charge transfer.A Schottky junction exists between the Ni₂P and TiO₂（B）.The photo induced charge carriers in TiO₂（B）transfer to Ni₂P,which can inhibit the charge recombination and accelerate surface reaction rate,results in the enhancement of photocatalytic performance.（2）Hybridization of CdS with TiO₂（B）was performed to extends the light response range and promote the stability of photocatalysts.Tandem CdS/TiO₂（B）nanosheet architectures with visible light H₂ evolution were constructed by photo-deposition and solvothermal methods to realize visible light H₂ evolution.A type II heterojunction of hexagonal CdS/TiO₂（B）was constructed by photo-deposition and CdS dots,with size about 6 nm,were anchored on TiO₂（B）nanosheets.A second type I heterojunction of cubic CdS/TiO₂（B）was fabricated by a hydrothermal method and CdS nanoparticles,with size about 50 nm,were dispersed on TiO₂（B）nanosheets.Both heterojunction systems display remarkable enhancement of H₂ evolution activity under full spectrum irradiation,with highest H₂ evolution rate of 1.776 mmol?g^-1?h^-1 for the photo-deposited catalyst and 1.494 mmol?g^-1?h^-1 for the hydrothermal process.The different heterojunction systems shown very different H₂ evolution under just visible light irradiation.The highest H₂ evolution(1.577 mmol?g^-1?h^-1)for the type II system was far much higher than the type I(48μmol?g^-1?h^-1).The band structure analysis shows that the conduction band edge of CdS in type II system is higher than TiO₂（B）.Therefore,photo excited electrons of CdS can transfer to TiO₂（B）in the case of visible-light irradiation structure,results in the type II system can achieve visible light H₂ evolution.（3）A Ti_0.5Ru_0.5O₂/TiO₂（B）/TiO₂（A）three-phase-junctions are constructed via a hydrothermal method and subsequent calcination to further promote the charge separation and accelerate the surface reaction speed.At the optimal calcination temperature of 350°C,with 2%Ru loaded,the highest H₂ evolution rate reaches 27.817 mmol?g^-1?h^-1at 25 ^oC,which is further increased up to 62.357 mmol?g^-1?h^-1 at 70°C by synergistic photothermal catalysis.The photocatalytic activity of the optimized catalyst was further enhanced by loading with Pt as a co-catalyst,60.919 mmol?g^-1?h^-1 at room temperature,which is further increased up to 106.685mmol?g^-1?h^-1 at 70°C by synergistic photothermal catalysis.It is found that,in addition to helping charge separation,migration,and inhibiting the charge recombination,the three-phase-junctions structure is a key to present strong oxidation and reduction ability.The photo induced electrons transfer to TiO₂（A）,while the photo induced holes transfer to Ti_0.5Ru_0.5O₂.Ti_0.5Ru_0.5O₂ has excellent co-catalytic ability,much better than Ru oxide and metallic Ru.It works as hole acceptor,which can accelerate the generation and quench photo induced peroxyl radicals,and thus significantly enhancing the H₂ generation rate.In this thesis,in order to develop novel robust photocatalysts,several ultra-thin TiO₂（B）based photocatalysts were constructed in order to accelerate the surface reaction speed,realize the visible light H₂ evolution and promote the charge separation.Their enhanced photothermal catalytic methods,mechanism and applications were studied in details.This research work will serve as a demonstrator for the design and synthesis of high performance 2D photocatalysts.