Carrier Dynamics Of Lead-free Metal Halide Perovskites And Their Applications For Photocatalytic CO₂ Reduction

The exploitation and utilization of fossil energy has contributed to the rapid development of human civilization while also leading to a serious energy crisis and eco-environmental crisis,which has resulted in the active development of renewable and green new energy sources.Among them,the solar-driven CO2 reduction reaction using semiconductor materials as a carrier can store solar energy as chemical energy,while solving ecological and environmental problems such as the greenhouse effect caused by CO2.Lead-free metal halide perovskites are expected to be a new generation of single-component photocatalysts due to their tunable energy band structure,high optical absorption coefficient,simple preparation method and non-toxicity.At present,there are very few examples about the application of lead-free metal halide perovskites for photocatalytic CO2 reduction,which are mainly 3D Cs2AgBiBr6,Cs2NaBiCl6 and 2D Cs3Bi2Br9.However,their CO2 reduction activities as single-component photocatalysts are low due to their low photogenerated carrier separation efficiency and poor structural stability caused by the distortion of octahedra in the materials.In order to improve their photocatalytic efficiency,researchers usually prepare complex semiconductor heterojunctions or load precious metal particles to improve their carrier separation efficiency and thus their photocatalytic performance.However,the complicated preparation process and high preparation cost of heterojunctions will seriously limit the wide application of lead-free chalcogenide materials as photocatalysts.In addition,low-dimensional halide perovskites,especially zero-dimensional perovskites,exhibit excellent air and moisture stability,which makes them favorable candidates for photocatalysts.Unfortunately,due to the intrinsic high exciton binding energy of zero-dimensional halide perovskites leading to their severe carrier recombination phenomenon,and the low carrier separation and migration efficiency will seriously affect their activity as photocatalytic materials.On the basis of the above analysis,we have improved the performance of lead-free halide perovskites as single-component catalysts for photocatalytic CO2 reduction by modulating the microstructure of lead-free perovskites through component engineering,which resulted in modulating the photogenerated carrer behavior in the materials.Compared with the complicated preparation process of heterojunction materials,the preparation process of component-engineered lead-free metal halide perovskites is simple and hopefully can be realized in mass production.The main contents of this paper are as follows:(1)Pb-free Sb-alloyed all-inorganic quadruple perovskite Cs4Mn(Bi1-xSbx)2Cl12(0≤x≤1)is synthesized as efficient photocatalyst.By Sb alloying,the undesired relaxation of photogenerated electrons from conduction band to emission centers of[MnCl6]4-is greatly suppressed,resulting in a weakened PL emission and enhanced charge transfer for photocatalyst.The ensuing Cs4Mn(Bi1-xSbx)2Cl12 photocatalyst accomplishes efficient conversion of CO2 into CO,accompanied by a surprising production of H2O2,a high value-added product associated with water oxidation.By optimizing Sb3+ concentration,a high CO evolution rate of 35.1 μmol g-1 h-1 is achieved,superior to most other Pb and Pb-free halide perovskites.Our findings provide new insights into the mixed-cation alloying strategies for improved photocatalytic performance of Pb-free perovskites and shed light on the rational design of robust band structure toward efficient energy transfer.(2)By controlling the concentration of B-site heteroion(Te4+),we achieved the behavior of modulating the radiative complexation and separation of photogenerated carriers in the zerodimensional lead-free halide chalcogenide Cs2SnCl6,which resulted in the modulation of its luminescence and photocatalytic CO2 reduction performance.When low concentrations of Te4+were introduced into the main body of Cs2SnCl6,it exhibited bright yellow light with a quantum efficiency of 76%;further increasing the Te4+concentration,its fluorescence performance was signifi cantly weakened and its photocatalytic CO2 reduction performance was enhanced,and its optimal catalytic activity was 28.6 μmol g-1 h-1 for CO and 3.3μmol g-1 h-1 for CH4.This is due to the formation of[TeCl6]2-octahedra that tend to segregate from each other at low concentrations of Te4+,leading to strong charge localization,which makes the photogenerated carriers more susceptible to radiative recombination;further increasing the concentration of Te4+weakens the charge localization and thus reduces the exciton binding energy,as well as the electron-phonon coupling,promoting charge separation and thus charge transfer to CO2 for efficient photocatalytic reduction.This work systematically investigates the regulation of the excitation energy relaxation pathway in perovskites by B-site heteroion concentration and its impact on its application.(3)Well-stabilized A2TeBr6(A=Rb+,Cs+)nanocrystalline materials(NCs)were prepared as efficient single-component photocatalysts by proposing the use of low-polarity and lowliganding ability propionic acid as an antisolvent and surface ligand in combination with hydrothermal synthesis and propionic acid-assisted antisolvent method.Among them,the activity of Rb2TeBr6 was as high as 72.5 μmol g-1 h-1 CO precipitation rate and 0.96 μmol g-1 h-1 CH4 precipitation rate,which was higher than that of Cs2TeBr6 with 33.83 μmol g-1 h-1 for CO and 1.21 μmol g-1 h-1 for CH4.And the CO selectivity of the former is 98.69%,which is higher than that of the latter at 96.55%.The smaller size of A2TeBr6 nanocrystalline material relative to the microcrystalline material increases the probability of photogenerated carriers migrating to the surface in the zero-dimensional halide perovskites.Moreover,the larger comparative area facilitates the exposure of more active sites.In addition,the smaller size of Rb+ ions than Cs+ ions make the cell of Rb2TeBr6 smaller than that of CS2TeBr6,and the shorter length of the Te-Br bond in the former with the latter makes it more electron dispersive.Surface photo voltage and transient surface photovoltage analyses revealed that Rb2TeBr6 has stronger photogenerated carrier separation and migration ability than Cs2TeBr6.This may be the reason for the higher photocatalytic CO2 reduction performance of the former than the latter.