Design Of Composite Photocatalysts Based On Several Sulfur/Oxide Semiconductors And Study On Photocatalytic Properties Of Formic Acid Decomposition And Water Oxidation

The depletion of traditional fossil fuels and the accompanying environmental pollution make people pay more attention to the development of clean energy.Solar energy is the key to solve these problems.Photo（electro）catalysis,as the main research direction of converting solar energy to chemical energy,has been widely concerned.The production of hydrogen,syngas,and other high value-added chemicals from solar energy is a promising way of energy conversion.In this thesis,I designed and synthesized a variety of non-noble metal catalysts,combined with different modification strategies,and carried out a series of research work on their properties in the photocatalytic formic acid decomposition and photoelectrocatalytic water splitting.The main contents are summarized as follows:1.Iron phosphide（FeP）nanoparticles（5～10 nm）as an efficient cocatalyst were loaded on CdS nanorods to realize efficient photocatalytic H2 evolution from formic acid.Under optimal conditions,the H2 production rate driven by visible light can reach 556 μmol·h^-1（pH 3.5）,which is 37 times higher than that of pure CdS nanorods.The apparent quantum yield of the hybrid sample is-54%under 420nm monochromatic light irradiation.In addition,the CdS@FeP photocatalysts demonstrate excellent stability.After continuous testing for more than 4 days,there was no significant decrease in catalytic activity.The experimental results show that FeP as cocatalysts can effectively promote the migration and separation of photogenerated carriers and inhibit charge recombination.Moreover,FeP has good hydrogen adsorption capacity and reduction ability,electrons quickly transfer from CdS to FeP and reduce the H+ adsorbed by FeP to produce H2.2.A photocatalytic system of homogeneous Fe salen molecular catalyst mixed with CdS nanorods was designed and synthesized to directly convert formic acid into syngas（H2+CO）at ordinary pressure and room temperature.The H2 evolution rate of the photocatalytic system is 300 μmol·h-l under optimal conditions,and the apparent quantum efficiency at 420 nm is 16.8%.It is one of the most active noble metal free photocatalytic systems for formic acid decomposition to produce H2 under visible light irradiation.Meanwhile,Fe salen molecular catalysts also show significant enhancement of CO evolution and excellent stability.The mechanism of Fe salen molecules in the decomposition of formic acid was studied.The results show that FeⅠ formation is a key step during the process.3.W2N3 nanosheets were prepared by solvothermal and high-temperature treatment in an ammonia atmosphere,then combined with CdS nanorods to construct an efficient photocatalytic composite catalyst for formic acid decomposition to syngas with an adjustable ratio.Under optimal conditions,the H2 production rate can reach 262μmol·h^-1 with 207 μmol·h^-1 for CO generation under visible light illumination.Meanwhile,the apparent quantum yields of H2 and CO under 420nm monochromatic light irradiation are 17.6%and 16.9%,respectively.Further experimental results show that the traditional type Ⅱ heterojunction formed between CdS and W2N3 can effectively facilitate interfacial charge transfer and separation,and suppress the recombination of photogenerated charge.W2N3 can also be used as a cocatalyst to accelerate the surface catalytic reaction kinetics,thus achieving efficient syngas production.4.A novel organic-inorganic hybrid photoanode was designed and synthesized by simulating natural photosynthesis.Ultrathin NiFe MOF nanolayers and Cobaloxime complex molecules were successively deposited on the surface of Ti-doped porous hematite（Ti-PH）by solvothermal and drop coating methods,which were further used as photoanodes for photocatalytic water splitting.Under AM 1.5 G irradiation,the composite photoanode exhibits a photocurrent density as high as 2.45 mA cm-2 with good stability.The photoanode shows an incident photon current efficiency（IPCE）of 83.0%at 365 nm,and the surface charge injection efficiency（ηinj）increases to 87.5%at 1.23 V vs RHE.The detailed research shows that NiFe MOF,as a hole transport layer,can effectively facilitate the transfer and separation of photogenerated charge and restrain the recombination of electrons and holes.Cobaloxime molecular cocatalyst can accelerate the surface water oxidation reaction kinetics and passivate the surface trap states.This sandwich structure can effectively improve the photoelectrocatalytic performance of materials.5.A nanoporous BiVO4 photoanode modified with nickel hydroxide/borate（NiOEC）and molecular Cobaloxime as cocatalysts was designed and synthesized,and applied in photoelectrocatalytic water splitting.The cocatalyst layers were loaded on the anode by photoelectrodeposition and drop coating methods.The optimal composite photoanode shows a photocurrent of 5.1 mA cm-2（1.23 V vs RHE）under AM 1.5 G irradiation,which is about 3.19 times higher than that of pure BiVO4,and the cathodic shift is improved to～480 mV at 0.5 mA cm-2.Further spectral characterizations and electrochemical tests show that the dual molecular/heterogeneous cocatalysts can effectively reduce the interface charge transfer resistance,restrain the surface charge recombination.These cocatalysts can also greatly accelerate the water oxidation kinetics,and improve the catalytic stability.