Synthesis Of Porphyrin And Carbon Dots-based Composites For Photocatalytic Water Splitting

At present,with the rapid development of the global economy,the excessive use of fossil fuels has caused the continuous rise of atmospheric CO₂ content,and the development of human society is facing many challenges such as environmental pollution,climate change,energy shortage and food safety.How to effectively solve the energy crisis has become a worldwide challenge.It is an effective way to achieve the goal of"carbon peak and carbon neutrality"to develop a clean,efficient and pollution-free low-carbon energy.Therefore,it is urgent to develop renewable energy and improve energy utilization rate.As a kind of green and sustainable clean energy,solar energy has attracted wide attention all over the world.Photocatalytic water splitting（involves H2 evolution reaction and O2 evolution reaction）and carbon dioxide reduction through solar energy is one of the effective ways to solve energy problem.Among them,the construction of efficient and stable hybird photocatalyst is the core of realizing ideal photocatalytic water splitting and carbon emission.Porphyrins and their derivatives have become key materials in energy conversion systems because of their special conjugated structures and flexible metal active sites,which can realize reversible conversion of light energy,electrical energy and chemical energy.In addition,as a kind of multifunctional photocatalyst,carbon dots has attracted extensive attention of researchers due to its unique optical and electronic properties as well as its rich surface groups.Herein,a series of porphyrin and carbon dots-based hybrid photocatalysts(such as CDs@CoO_x,BiVO₄/CQDs/TPP,Zn_0.5Cd_0.5S/MOF-545CoandP-Mn_0.5Cd_0.5S/Co TCPPOMe)were chosen as the research object for enhanced water splitting and CO₂ reduction.Through controlled morphologies or crystallinity,crystal facet engineering and construction of heterojunction to further improve light absorption,photogenerate carrier separation efficiency and the surface catalytic reaction of these semiconductors.Furthermore,the photocatalytic reaction mechanism was also systematically studied.The main contents include the following aspects:（1）Photocatalytic performance can be controlled by controlling its crystallinity.As well known,cobalt oxide is one of the most promising candidates for photocatalytic reaction.However,the relationship between crystallinity and photocatalytic performance of cobalt oxide is not clear at present.In order to explore the relationship between crystallinity and photocatalytic performance of cobalt-based oxides,a series of carbon dots coupled CoO_x as bifunctional photocatalysts CDs@CoO_x from crystal state to amorphous state were constructed.It is found that a spongy porous amorphous catalyst CDs@CoO_x-300 calcined at 300℃ exhibits higher photocatalytic water oxidation and CO₂ reduction performance than Co₃O₄（crystalline state）without CDs coupling.In addition,the boosted photocatalytic behavior is attributed to the introduction of CDs,which changes the morphology and crystallinity of CDs@CoO_x-300,and the hybrid photocatalyst not only exposes more active sites,but also promotes charge transfer.This work offers a novel strategy to fabricate a high performance bifunctional photocatalyst for water oxidation and CO₂ reduction.（2）Crystal engineering is closely related to photocatalytic performance.The photocatalytic activity of BiVO₄ is different with different exposed crystal planes.The（010）crystal face of BiVO₄ is a highly active surface,but it is not easily to obtain.By controlling the morphology of BiVO₄ and exposing the crystal（010）surface of BiVO₄,we introduced 5,10,15,20-tetraphenylporphyrin（TPP）and carbon quantum dots（CQDs）in the process for synthesizing BiVO₄,and effectively regulated the morphology of BiVO₄ and changed its exposed crystal surface,which construct an efficient oxygen production photocatalyst BiVO₄/CQDs/TPP.The structure,morphology,chemical state,photocatalytic activity and charge transfer path of the composite photocatalyst were studied.Importantly,the X-ray electron spectroscopy（XPS）,surface photovoltage spectroscopy（SPV）and transient absorption spectroscopy（TAs）confirmed that CQDs,as an ideal electron transporter,accelerated the transfer of electrons from the conduction band of BiVO₄ to TPP,which effectively improved the activity of photocatalytic water oxidation.This strategy dramatically boosts photocatalytic water oxidation over bare BiVO₄/CQDs/TPP.Under visible light irradiation（λ=420 nm）,the oxygen production rate of BiVO₄/CQDs/TPP-60 reached 352.5μmol g^-1 h^-1,which is 3.1 times than that of bare BiVO₄.After four times of recycling,the photocatalytic activity and structure did not change significantly,indicating that the composite catalyst has good stability.Furthermore,DFT calculation results demonstrated that water dissociation on the exposed（010）facets are more energetically favorable for BiVO₄/CQDs/TPP,which change the rate-determining step of water oxidation reaction from*OH to*O species rather than from H₂O to*OH.This work provides a reference for the design of photocatalysts with efficient solar energy conversion.（3）Zirconium based porphyrin MOF-545 has been widely research in the field of photocatalysis because of its strong light absorption,efficient light capture and fast energy/electron transfer ability.A series of Zr-based metalloporphyrins MOF-545M（M=Co,Ni,Cu,Zn）containing different metals were firstly synthesized,and a series of solid solutions Zn_xCd_1-xS were synthesized by adjusting the ratio of zinc and cadmium.Then a novel composite photocatalytic material Zn_xCd_1-xS/MOF-545M was successfully fabricated by mechanically mixing.The photocatalytic performance of the catalysts and their photocatalytic hydrogen production were analyzed by means of various measurements and characterization.The introduction of MOF-545M effectively improves the corresponding range of visible light of Zn_xCd_1-xS and enhances the separation efficiency of photogenerated carriers.Under visible light irradiation,the hybrid photocatalyst Zn_0.5Cd_0.5S/MOF-545Co exhibits a high H₂ evolution activity of29.6 mmol g^-1 h^-1 using lactic acid as a sacrificial agent,which is about 52.8,22.9 and6.5 times more than that of bare Zn S,Cd S and Zn_0.5Cd_0.5S,respectively.These results indicate that MOF-545Co can significantly improve the photocatalytic activity of inorganic semiconductor photocatalyst for hydrogen production.（4）The photocatalytic hydrogen production mentioned above is achieved by using lactic acid as sacrificial reagent.In order to avoid the use of sacrificing reagents,in this work,a series of solid solutions Mn_xCd_1-xS were synthesized by regulating the ratio of manganese and cadmium,and then P-doped Mn_xCd_1-xS was prepared by in-situ phosphatization strategy.P-Mn_0.5Cd_0.5S/MTCPPOMe（M=Fe,Co,Ni）heterostructure based on the coupling of P-Mn_0.5Cd_0.5S nanorods and MTCPPOMe was obtained by mechanically mixing.The P-Mn_0.5Cd_0.5S/Co TCPPOMe exhibited excellent H₂evolution rate of 410μmol g^-1 h^-1 under visible light irradiation（λ=420 nm）without adding any sacrificial reagent,which is nearly 9.1 and 4.2 times more than the bare Mn_0.5Cd_0.5S and P-Mn_0.5Cd_0.5S,respectively.The direction of charge transfer from P-Mn_0.5Cd_0.5S to Co TCPPOMe was confirmed by experiments and theoretical calculations,which effectively enhanced the separation efficiency of photogenerated carriers.In all,this work provides an alternative approach to photocatalytic water decomposition without the adding external electron sacrifice agents.