Synthesis And Optical Properties Investigation Of Zirconium/Hafnium/Vanadium-Based Lead-Free Perovskite Nanocrystals

Lead-free perovskites are a class of semiconductor materials that have emerged in recent years and have broad application prospects in photovoltaic power generation,electroluminescence,photocatalytic reduction or organic synthesis,X-ray imaging,and light detection.Lead-free perovskite nanocrystals are small crystals with sizes ranging from a few nanometers to tens of nanometers.Compared with macroscopic materials(perovskite single crystals or microcrystals),quantum-confined perovskite nanocrystals have the advantages of size-tunable band gaps and higher absorption cross-sections.On the other hand,although single crystals reflect the intrinsic properties of the material,in most cases,thin films are used in devices.Unfortunately,the metal halide precursors used to synthesize perovskites have poor solubility,which makes it difficult to prepare high-quality crystalline films using solution processing methods.In this regard,preparing perovskite nanocrystals using a colloidal method and then spin-coating the nanocrystals into thin films seems to be an attractive alternative.Therefore,lead-free perovskites in the form of colloidal nanocrystals have received more and more attention,and the development of high-performance stable lead-free perovskite nanocrystals has become a hot research topic.Lead-free vacancy-ordered perovskite nanocrystals have attracted much attention in recent years due to their low toxicity,high stability,and unique optical properties.However,at present,there are very few examples of successful preparation of Cs2M4+X6-type nanocrystals.This results in very limited research on the luminescence mechanism and light color regulation of such nanocrystals.In addition,in the currently widely reported protocols for preparing perovskite nanocrystals through hot-injection methods,metal halides or metal acetates are often used as metal precursors.However,for the synthesis of many novel perovskite nanocrystals,the inability of these two metal salts to be ionized in organic solvents is the main factor causing the preparation failure.Therefore,the preparation of perovskite nanocrystals by finding new metalloid precursors has become increasingly important.Based on these,the main contents and conclusions of this dissertation include:(1)We developed a method for the ionization of Zr4+ by dissolving zirconium carbonate in acetic acid,and synthesized phase-pure vacancy-ordered Cs2ZrCl6 perovskite nanocrystals through hot injection methods.Cs2ZrCl6 nanocrystals exhibit broadband blue light emission based on self-trapped excitons with a photoluminescence quantum yield as high as 60.37%.The thermally activated delayed fluorescence emission mechanism of Cs2ZrCl6 nanocrystals was elucidated by spectroscopic characterizations combined with density functional theory calculations.On this basis,the effects of halogen anion composition and metal ion doping on the luminescence properties of vacancy-ordered perovskite nanocrystals were explored.By synthesizing Cs2ZrBrxCl6-x(0<x≤1.5)nanocrystals,the fluorescence color of the nanocrystals can be tuned from blue to green.By doping Bi3+ions,the excitation spectrum of the nanocrystals can be extended to 365 nm.The blue fluorescence of Bi3+-doped Cs2ZrCl6 nanocrystals under 365 nm excitation originates from the localized excitons of Bi3+ and is attributed to the 3Pn→1S0(n=0,1)transitions.(2)We explored metal acetylacetonates as metal precursors for the synthesis of lead-free perovskite nanocrystals.Firstly,we synthesized vacancy-ordered Cs2HfCl6 perovskite nanocrystals by hot injection methods using hafnium acetylacetonate as the hafnium source,and elucidated their optical properties.Cs2HfCl6 nanocrystals exhibit defect-intolerant properties.Its absorption spectrum is characterized by strong exciton absorption at 240 nm and 310 nm,where 240 nm is the band-edge absorption of the Cs2HfCl6 host,while 310 nm is the absorption of sub-bandgap defect states induced by Hf4+ or Cl-vacancies.The photoluminescence properties of Cs2HfCl6 nanocrystals exhibit excitation wavelength-dependent emission characteristics,and the emission bands at 378 nm,444 nm and 540 nm originate from[HfCl6]2-octahedrons,impurity[ZrCl6]2-octahedrons and sub-band gap defect states,respectively.Aiming at the sub-bandgap defect states inside Cs2HfCl6 nanocrystals,we propose a Sb3+-assisted passivation strategy.At the same time,Sb3+doping achieves bright orange emission,and the photoluminescence quantum yield is as high as 40.7%.Then,using rare earth acetylacetonate as the rare earth ion source,we successfully doped four rare earth ions(Pr3+,Tb3+,Eu3+,Ho3+)into the Cs2HfCl6 nanocrystals’ host lattice and obtained multicolor emission(from blue to green to pink)based on the energy transfer process of self-trapped excitons to rare earth ions.This provides a feasible countermeasure for the tunable multicolor emissions of vacancy-ordered perovskite nanocrystals.(3)Using vanadium acetylacetonate as the vanadium source,we synthesized two vanadium-based perovskite nanocrystals with different crystal structure dimensionity,namely three-dimensional Cs2NaVCl6 and zero-dimensional CS3V2Cl9 perovskite nanocrystals using hot-injection methods,which demonstrates the universality of metal acetylacetonates as the metal precursors for perovskite nanocrystals.Then,we investigated the optical properties and band positions of Cs2NaVCl6 and Cs3V2Cl9 nanocrystals.The Cs2NaVCl6 and CS3V2Cl9 nanocrystals have visible and near-infrared light absorption capabilities.Due to the synergistic absorption effect of the[VCl6]3-octahedron,the Cs3V2Cl9 nanocrystals have significantly stronger visible and near-infrared light absorption than the Cs2NaVCl6 nanocrystals.Due to the synergistic absorption effect of[VCl6]3-octahedrons,Cs3V2Cl9 nanocrystals have significantly stronger visible and near-infrared light absorption than Cs2NaVCl6 nanocrystals.Moreover,Cs3V2Cl9 nanocrystals possess suitable band positions for photocatalytic CO2 reduction.Moreover,the Cs3V2Cl9 nanocrystals have suitable band positions for photocatalytic CO2 reduction.Finally,we investigated the photocatalytic CO2 reduction performance of Cs3V2Cl9 nanocrystals.Under simulated sunlight irradiation,the average formation rates of CO and CH4 in Cs3V2Cl9 nanocrystals were 3.03 μmol g-1 h-1 and 1.35 μmol g-1 h-1,respectively.