Preparation Of The Composite Films Containing Manganese,Cobalt Polyoxometalate And Study On Photoelectrocatalytic Water Oxidation Activity

At present,the world’s energy consumption is serious,and the problem of environmental pollution needs to be solved urgently.Photoelectrochemical（PEC）decomposition of water to produce hydrogen（H₂）is the most environmentally friendly and direct method of converting solar energy into chemical energy,which can meet the green sustainability concept and environmentally friendly international strategic needs.However,the slow four-electron oxidation reaction to produce oxygen（O₂）is much more difficult than the H₂ collection in PEC water splitting.Therefore,scholars all over the world have also carried out many explorations to find more effective green photoelectric water oxidation catalysts.Polyoxometalates（POMs）are clusters composed of inorganic metal-oxygen compounds.They perform well in many fields and have become a research hotspot in cross-integration of many fields.POMs contain multiple metal centers and have the ability to stabilize various oxidation states of metals.It has been found that some POMs have both photocatalytic and electrocatalytic water oxidation activities.Therefore,POMs are considered as promising for constructing effective water oxidation catalysts.POM clusters have a strong electron-accepting ability,which can capture photo-generated electrons on the semiconductor conduction band,inhibit the recombination of photo-generated carriers,and can promote the migration of photogenerated carriers,thereby improving the photoelectric conversion efficiency of semiconductor materials.Therefore,the research on the preparation of water oxidation photocatalysts by combining POM clusters with semiconductors has great application prospects.If we expand and study in depth,we will surely gain more in-depth understanding of water splitting catalysts and provide ideas for the development of water oxidation catalyst materials.In this paper,POMs containing manganese and cobalt are used as the charge collection and transport media,and the inorganic semiconductor cobalt tetroxide and organic porphyrin-based photosensitive materials with absorption capacity in the visible region are used to construct composite films through layer-by-layer self-assembly strategies.Efficient photoelectric catalyst for water oxidation.According to the basic principles of photogenerated electrons,the photoelectric catalytic water oxidation activity is studied under the control of an external electric field and a fixed light source.The specific work content is as follows:First,themanganese-containingsilicotungstate Na_3.5K_2.5[Mn₃^ⅢMn^ⅣO₃（CH₃COO）₃(A-ɑ-Si W₉O₃₄)]·20H₂O·Na CH₃COO·0.5KCH₃COO（Na K-Mn4）was synthesized,which has good oxidation Reduction properties and photoelectric responsiveness.With the black light-absorbing material Co₃O₄,the photoelectric catalytic water oxidation activity was studied after self-assembly to construct a composite film.The influence of light time on catalytic activity was investigated,and the best light time was found.The photoelectric catalytic water oxidation activity of each film assembled on the ITO electrode was studied under the optimal light time condition,and compared with the catalytic current under dark conditions.The results showed that the film containing the two active components of Mn4 and Co₃O₄ has good performance.Photoelectric catalytic effect,the average increase of water oxidation current by 25%under light conditions.Second,thecobalt-containingpolyoxometalateK₈Na₈[(A-a-Si W₉O₃₄)₂Co₈（OH）₆（H₂O）₂（CO₃）₃]·52H₂O（Na K-Co8）was synthesized.With the aid of layer-by-layer assembly method,it is assembled with Co₃O₄ into a composite film.The film infrared,XRD,XPS and AFM characterization methods proved that Co8 and Co₃O₄ were successfully assembled on the electrode substrate.Ultraviolet-visible spectroscopy was used to monitor the film growth process,which showed that the contents of the two components increased uniformly with the increase of the number of assembled layers.Under the control of incandescent light and external power supply,the activity of thin-film photoelectrocatalytic water oxidation was studied.The results showed that the water oxidation onset potential of the film was significantly advanced,and the photoelectrocatalytic current increased by 32%,and the film was stable and could be reused repeatedly.Third,porphyrin tetramine（TPPA）and porphyrin ion compounds[H2TPP][Cl O₄]₂（cation abbreviation H2TPP）,which are more sensitive to light,were synthesized,and combinedwithNa_3.5K_2.5[Mn₃^ⅢMn^ⅣO₃（CH₃COO）₃(A-ɑ-Si W₉O₃₄)]·20H₂O·Na CH₃COO·0.5KCH₃ COO（anion abbreviated as Mn4）for assembly.The composition and morphology of the film were characterized by infrared spectroscopy,XPS and AFM;the growth process of the film was monitored by ultraviolet-visible spectroscopy.Under the control of incandescent light and external power supply,the activity of thin-film photoelectrocatalytic water oxidation was studied.The results show that the composite film shows high photoelectric conversion efficiency,and the photoelectric catalytic current increase can reach 77%.The reason is that porphyrin has good visible photosensitivity,which can make up for the lack of polyacids that can only absorb light in the ultraviolet region.The assembly of the two can convert more light energy into electrical energy,and realize light capture and photoexcited electron migration.Promote the directional flow of electrons under the effective interaction between the two components,thereby speeding up the water oxidation process.As we all know,photoconductivity is produced by the competition between photogenerated electrons（charge carriers）and electron-hole recombination.The assembly of polyacids,cobalt tetroxide and organic porphyrins is complementary to each other.The synergy between them can produce higher photoelectric catalytic activity,which provides reference and reference for the development and application of photoelectric catalytic water oxidation catalysts as soon as possible.