Design,Preparation And Sensing Properties Of Organic-Inorganic Hybrid Perovskites

Organic-inorganic hybrid perovskites,as one type of emerging semiconductor optoelectronic materials,possess the unique advantages,such as tunable direct bandgap,large light absorption coefficient,and high quantum efficiency.They have been widely used in the fields of solar cells,light displays,photodetectors,and lasers.It is worth noting that the environmental sensitivity of perovskites makes them also exhibit great application potential in sensing and monitoring.In recent years,many research groups studied the applications of perovskites for toxic and harmful gas sensing,and achieved relatively excellent sensing performances.Nevertheless,there are still controversial conclusions about an interaction mechanism between perovskite materials and target gas molecules;besides,the gas sensitivity requires to be further improved.Therefore,in this thesis,we will rationally design the organic-inorganic hybrid perovskite materials for the gas sensing applications,and study the interaction mechanisms between perovskite crystals and target molecules,in order to promote the research and applications of perovskite materials in the field of sensing.The main contents are as follows:(1)In view of the poor stability of organic-inorganic hybrid perovskite materials,we introduced a long-chain ammonium salt ligand(Tetrabutylammonium bromide,TBA)into the CH₃NH₃Pb Br₃(MAPb Br₃)perovskite for enhancing the perovskite fluorescence intensity and stability,and then constructed a fluorescence ammonia(NH₃)gas sensor.In this MAPb Br₃-TBA-based sensor,the fluorescence intensity decreases by 62.5%upon exposure to 100 ppm NH₃ with short response and recovery times.Moreover,the sensor exhibits excellent gas selectivity and humidity stability.Hereafter,we studied the NH₃ sensing mechanism of MAPb Br₃-TBA film by means of a series of characterization and measurements.The results demonstrate that NH3 molecules can penetrate the TBA ligand passivation layer onto the MAPb Br₃ surface,and then cause structural transformation of the internal MAPb Br₃ crystals.In this process,the protonated NH₃ molecules can replace the methylamine cations(CH₃NH₃⁺)of MAPb Br₃,leading to a formation of NH₄Pb Br₃·CH₃NH₂ intermediate phase.This work indicates that this highly stable fluorescent MAPb Br₃-TBA film is potential for the NH₃sensing application.(2)An interfacial layer composite(MAPb Br₃/mp-Ti O₂)was fabricated by growing the MAPb Br₃ film onto the mesoporous titanium dioxide layer,which exhibited excellent NH₃ sensing performance.Upon NH₃ exposure,the fluorescence intensity of the sensor increases rapidly with relatively high gas response values(72%at 5 ppm NH₃).Besides,the sensor possesses superior cycle response,gas selectivity,humidity stability and reproducibility.In addition,this MAPb Br₃/mp-Ti O₂-based sensor can be applied to detect different types of amine vapors,such as methylamine,ethylamine,and propylamine,etc.The fluorescence-enhanced sensing mechanism can be explained that NH₃ or amine molecules are preferentially adsorbed onto the surface and interface of MAPb Br₃ film,and thus generating the NH₄Pb Br₃·CH₃NH₂ or alkylammonium salt passivation layers,which could hinder an electron transfer process between MAPb Br₃and mp-Ti O₂.This process can facilitate the electron-hole recombination in the MAPb Br₃,thereby exhibiting the NH₃/amine induced fluorescence enhancement phenomenon.(3)Compared with fluorescence-typed sensors,resistance-typed sensors possess the advantages of device simplification and low-cost.Here,we studied a resistive NH₃sensing application of the halide perovskite CH₃NH₃Pb I₃(MAPb I₃).It is observed that NH₃ gas can induce an abnormal increase in resistance of perovskite films,which is inconsistent with previous reports.In order to elucidate this anomalous NH₃ sensing behavior,we investigated and analyzed the NH₃-induced microscopic electrical properties and structural evolution processes of MAPb I₃ films.The results indicate that this anomalous NH₃ sensing behavior is dominated by perovskite grain boundaries.The NH₄Pb I₃·CH₃NH₂ insulating layers preferentially appear at the perovskite grain boundaries,thereby increasing the resistance of perovskite film.Finally,we constructed a gas sensor based on the MAPb I₃ film,which presents an excellent gas response(472%at 30 ppm NH₃).This work can provide the experimental and theoretical guidance to develop the high-performance resistive perovskite-based gas-sensing materials.(4)The lead toxicity in lead-halide perovskites is a major obstacle for their gas sensing applications.Herein,we prepared a lead-free bismuth-based perovskite(PEA)3Bi2Br9(PEA-Bi-Br,PEA=?-phenylethylamine)film for NH3 sensing.In this sensing film,upon 100 ppm NH₃ exposure,the gas response value can reach 2.78(R₀/R_g)with an excellent repeatable response.According to a series of characterization methods,it is found that the NH3 sensing mechanism of this PEA-Bi-Br is different from that of lead-based perovskite MAPb X3.In brief,the electron-donating NH3 molecules can be preferentially adsorbed onto the surface of(PEA)3Bi2Br9 crystals for electron injection.Then these NH3 molecules penetrate into the crystals,dissociating of the[Bi2Br9]^3-double octahedrons,thereby yielding a new NH4Br phase.The above results demonstrate that the environment-friendly bismuth-based perovskites are promising for gas sensing applications.