Study On The Fabrication Of Quantum Dots Sensitized TiO₂-based Photocathodes And Their Performance In Photoelectrocatalytic Reduction Of CO₂

With the development of the global economy and the increasing number of population,the demand for energy in human society has also improved rapidly.The energy crisis and environmental problems have become the most serious challenges facing the world in the 21st century.The excessive consumption of fossil fuels leads to more emissions of CO₂,the main kind of green gas,and eventually causes the global warming.The key to solving the problem is to seek and develop clean renewable energy that can promote the sustainable development of the whole society.Compared with the traditional fossil energy,solar energy is a clean,inexhaustible and renewable energy.Photosynthesis in nature is the most representative way of converting solar energy.So in the carbon dioxide cycle of the earth,it will be a powerful solution to the crisis of energy and environment if we can introduce solar energy to transform carbon dioxide into fuels.Photoelectrocatalytic reduction of CO₂ is considered as one of the most promising strategy for converting CO₂ into value added fuels.TiO₂ semiconductor is commonly used for photoelectrocatalytic reduction of CO₂,but its poor ability response to visible light severely limits its applications.Quasi-zero-dimensional quantum dots are attractive for photocatalysisapplications because of their very desirable optical and electronic properties.Depositingquantum dots onto the surface of the semiconductor have been certified as an efficient strategy for improving the light harvesting property.In the present study,efficient and stable TiO₂ based photocathodes have been designed and fabricated by quantum dots sensitization and functionalization for catalytic reduction of CO₂.?1?A new photoelectrocatalytic system for reduction of CO₂ composed of Cd SeTe and/or Mn:Cd S QDs sensitized TiO₂ photocathode and FeOOH/NiOOH/BiVO₄ photoanode has been developed aimed to mimicking the natural photosystem I?Calvin cycle?and photosystem II,respectively.ThephotocathodesCdSeTe/TiO₂/FTO,CdS/CdSeTe/TiO₂/FTOand Mn:Cd S/CdSeTe/TiO₂/FTO were obtained by successive ionic layer adsorption and reaction?SILAR?strategy.The morphological,structural,optical and electrochemical properties of the resultant materials were investigated by various physical techniques,such as XRD,SEM,TEM,AFM,XPS,EDX,UV-vis,LSV and EIS.We found that the system had a highly efficient catalytic performance for CO₂ reduction into methanol in 0.1 M[APMIm]Br ILs at-0.9 V bias potential?vs.SCE?under illuminated(200 mW·cm^-2),the yield of methanol can reach to 90?M·cm^-2·h^-1,the total selectivety of methanol and ethanol is 97.54%,and the light quantum efficiency?QE?is up to0.58%that is higher than 0.4%of plants in nature.?2?TiO₂ nanotubes have attracted much attention in many fields because their large surface area and vertical alignment afford to upmost electrochemical,optical and electrical performance.Considerring of the advantages,a new CdTe/CdS/TiO₂ nanotube photocathode was prepared by sonication assisted successive ionic layer adsorption and reaction method?S-SILAR?and pulse electrodeposition technique?PET?.The experiments results indicated that the as prepared photocathode can absorb visible light in the range of 400⁸50 nm?E_g=1.88 eV?.Due to the formed heterostructure that can promote the charge transfer and enhance the electron-hole saparation,the CdTe/CdS/TiO₂ nanotube photocathode can efficiently photoelectrocatalytic reduction of CO₂ into methanol accompanying with little CO and H₂ in gas phase.The yield of methanol is up to 66.7?M·cm^-2·h^-11 with a high selectivety of 81.2%in 0.1 M KHCO₃ at-0.8 V bias potential?vs.SCE?under illuminated(200 mW·cm^-2).?3?The element copper is regarded as a kind of catalyst with high activity and selectivity to forming hydrocarbon compounds in photoelectrocatalytic reduction of CO₂.Multiwalled carbon nanotube?MWCNT?is evaluated as the most efficient cocatalyst considering its excellent electron capture and transport properties.Inspired by the potential advantages of the materials,a new Cu₂O/MWCNT/TiO₂/FTO heterostructure photocathode was fabricated aimed to enlarge the yield of alcohols.For enhancing the stability of the elactrode,ZnTe QDs were diposited on the surface of the electrode as protection layer and light absorption layer.With the combination of the potential advantages of the each components,ZnTe/Cu₂O/MWCNT/TiO₂/FTO photocathode presented as a visible-light-driven CO₂ reduction catalyst,in which MWCNT may act as an electron conductor to accelerate the electron transfer from excited ZnTe and Cu₂O to TiO₂ for boosting charge separation.The results show that the photocathode can efficiently and selectively convert CO₂ into liquid methanol and ethanol.Under illuminated with applied-1.0 V bias,the yield of methanol and ethanol can up to 18.5?M·cm^-2·h^-11 and 7.1?M·cm^-2·h^-1,with a high selectivety of 74.4%and 14.3%,respectively.