Synthesis And Photocatalytic Properties Of Lead-Free Double Perovskite Materials Modified By Transition Metal Phosphide

The photocatalytic technology driven by sunlight is expected to realize the conversion of solar energy to hydrogen energy,and alleviate the problems of energy shortage and environmental pollution to a certain extent.Lead-halide perovskite materials have attracted the extensive attention of researchers because of their promising photoelectric properties,such as easily adjustable bandgap,high absorption coefficient,and high carrier mobility.However,the toxicity of lead hampers their practical application in the future,so exploring new lead-free perovskite materials has been the research hotspot nowadays.Lead-free Cs₂AgBiBr₆（CABB）double perovskite has the advantages of ideal band gap width（1.95 eV～2.30 eV）and good air stability,which can be used as a new-type photocatalytic material alternative to lead halide perovskites,but the recombination of photogenerated carriers caused by intrinsic defects leads to the poor photocatalytic hydrogen evolution activity of CABB,which restricts its further industrial applications.Providing transport channels for charge transfer through modification of earth-abundant cocatalysts is an ideal strategy to enhance the photocatalytic performance of CABB.Transition metal phosphides（TMPs）are excellent cocatalysts for promoting charge transfer due to their eminent electrons transport performance,structural stability and low cost.Therefore,CABB-based composite photocatalytic materials modified by TMPs were designed and synthesized.Based on the activity and stability of photocatalytic hydrogen evolution,the crystal structure,microscopic morphology,element valence state,optoelectronic properties,and the mechanism of photocatalytic hydrogen evolution were investigated.The specific reserch content is as follows:（1）CABB modified by single metal phosphide CoP for efficient photocatalytic hydrogen evolutionThe CoP-modified composite photocatalysts CoP/CABB were synthesized via dissolution-recrystallization for the first time,and the hydrogen evolution activity of the composite photocatalysts in saturated HBr/H₃PO₂ aqueous solution were performed under visible light（λ≥420 nm）irradiation.When the loading of CoP nanoparticles was 7.5%,the best hydrogen evolution rate was 137.69μmol g^-1 h^-1,which was 33 times higher than that of pure CABB(4.23μmol g^-1 h^-1),and was even better than that of CABB/5%Pt(7.78μmol g^-1h^-1).Additionally,the CoP/CABB composite displayed excellent photocatalytic stability during a 48 h cycle tests.The characterization results showed that the CoP/CABB composite had stronger light absorption capacity,smaller charge transfer resistance,lower electron-hole recombination rate and higher charge separation efficiency.The study on mechanism of photocatalytic hydrogen evolution revealed that these excellent properties were mainly ascribed to the successful construction of the Type-I heterojunction.CoP rapidly captured photogenerated electrons from CABB via the transport channel built by the Type-I heterojunction,which effectively suppressed the recombination of photogenerated carriers in CABB,thereby improving the overall activity of photocatalytic hydrogen generation.（2）CABB modified by bi-metallic phosphide NiCoP（NCP）for efficient photocatalytic hydrogen evolutionThe NCP-modified composite photocatalysts NCP/CABB were synthesized through the efficient electrostatic coupling method,and applied to visible-light-driven photocatalytic hydrogen generation from HBr splitting.The results indicated that the synergistic effect of Ni and Co elements optimized the electron capture ability of NCP compared to single metal phosphides.When the molar ratio of Ni and Co was 7:3,it exhibited the best synergistic catalytic effect.When the loading amount of NCP was 12.5%,the optimal hydrogen generation rate was as high as 373.16μmol g^-1 h^-1,it was approximately 88 times higher than that of pure CABB(4.23μmol g^-1 h^-1),which could rival the performance of other reported CABB-based photocatalysts.The optoelectronic performance measurements reflected that NCP/CABB had a longer photogenerated carrier lifetime,beneficial from the band bending caused by the carriers transfer,which provided the driving force for hole transport and greatly improved the electron-hole pairs separation efficiency,thus the fabricated NCP/CABB composite exhibited excellent and stable hydrogen evolution performance.