Study On The Electronic And Photocatalytic Properties Of Novel Bi-based Oxyhalides

In modern society,the problem of environmental pollution and energy shortage is becoming increasingly serious,and it is urgent to find sustainable,environmental protection and efficient solutions.As an emerging technology,photocatalysis can convert solar energy into chemical energy to promote chemical reactions,especially in the areas of pollutant degradation,water splitting,CO₂ reduction,etc.,with broad prospects for application.However,the serious limitations of traditional photocatalytic materials,such as wide bandgap,poor stability,and high carrier-recombination rate,severely restrict their potential applications.The emergence of first-principles calculations can design materials with good photocatalytic performance,helping to solve problem in photocatalytic materials,such as the separation and transfer of photogenerated charge,and providing strong theoretical support for developing efficient and stable photocatalytic materials.Therefore,the combination of first-principles calculations and experimental methods can accelerate the development of new photocatalytic materials and provide effective solutions for environmental pollution and energy shortage.New Bi-based oxyhalides with layered structures can regulate the band structure by changing the stacking layers.Additionally,due to the existence of the Bi 6s lone pair electrons,the new Bi-based oxyhalides exhibit unique physical and chemical properties.In this study,a series of design strategies for new Bi-based oxyhalides were carried out using the first-principles calculation software package VASP.The following research topics were explored:（1）investigating the replacement of Sill（?）n phase halogen anions and divalent metal cations to regulate the band structure;（2）predicting new Ti-based materials Bi₅Ti₂O₁₁X（X=Cl,Br,and I）to fill the Sill（?）n-Aurivillius phase blank and studying the changes in band structure after halogen anion replacement;（3）predicting new Zr-based and Mn-based Sill（?）n-Aurivillius phase materials Bi₅Zr（Mn）₂O₁₁Cl and studying the changes in band structure after cation replacement;（4）providing a detailed comparative analysis of the band structure calculated and experimental results for Fe-Ti-based Sill（?）n-Aurivillius phase Bi₇Fe₂Ti₂O₁₇X（X=Cl,Br,and I）.The above work provides direction for a deeper understanding of band regulation in new Bi-based oxyhalides.The main research achievements are as follows:（1）Firstly,the band structures of Sill（?）n phase Bi OX（X=F,Cl,Br,and I）were investigated,and it was found that the substitution of F,Cl,Br,and I not only changes the bandgap value but also alters the contribution of halogen p orbital on the valence band.In addition,Bi OBr exhibits the smallest effective masses for both photo-generated electrons and holes,indicating that carriers are more easily migrate and participate in surface reactions.Subsequently,divalent metal cations were introduced to study the band structures of ABi O₂X（A=Ca,Sr,Ba,Zn,Cd,and Pb;X=F,Cl,Br,and I）.It was ultimately found that the substitution of cations from the same main group lead to a decrease in the bandgap value and a significant change in the orbital contribution of elements in the valence band.Among them,Cd Bi O₂X（X=F,Cl,Br,and I）altered the orbital contribution of elements in the conduction band.The bandgap values of Pb Bi O₂X（X=F,Cl,Br,and I）were the smallest among all structures（2.96e V,2.96 e V,2.81 e V,and 2.75 e V）,and Pb Bi O₂Cl and Pb Bi O₂Br exhibited small effective masses for both electrons and holes and the most suitable materials for photocatalytic applications.（2）The instability of the Bi₅Ti₂O₁₁F structure and the stability of the Bi₅Ti₂O₁₁Cl,Bi₅Ti₂O₁₁Br,and Bi₅Ti₂O₁₁I structures were successfully predicted using phonon spectra calculation.Then,the band structures of Bi₅Ti₂O₁₁Cl,Bi₅Ti₂O₁₁Br,and Bi₅Ti₂O₁₁I were investigated,and it was found that their bandgaps increased gradually,and they were all indirect-bandgap semiconductor materials.The band structure of Bi₅Ti₂O₁₁I underwent significant changes,with the VBM primarily occupied by I 5p,making it susceptible to self-oxidation deactivation.The charge density at the valence band maximum（VBM）and conduction band minimum（CBM）of Bi₅Ti₂O₁₁Cl showed excellent charge space separation effect.The photocarrier mobility of Bi₅Ti₂O₁₁Br was higher than that of Bi₅Ti₂O₁₁Cl.Finally,the stability of eleven Bi₅Ti₂O₁₁Cl surface structures and nine oxygen vacancy defect structures were studied.The surface structure with a Bi-rich layer on the top and bottom surfaces was found to be the most stable,and the defect structure with an oxygen vacancy in the[Bi₂O₂]²⁺layer was the most stable.（3）By calculating the phonon spectrum of Bi₅Zr（Mn）₂O₁₁Cl,the stability of the Bi₅Zr（Mn）₂O₁₁Cl structure is successfully predicted from a dynamical perspective.The electronic band structure of Bi₅Zr（Mn）₂O₁₁Cl is compared with that of Bi₅Ti₂O₁₁Cl.The results show that the bandgap of Bi₅Zr₂O₁₁Cl is 0.11 e V smaller than that of Bi₅Ti₂O₁₁Cl.Bi₅Mn₂O₁₁Cl is ferromagnetic and has spin-up and spin-down bandgaps of 0.46 e V and2.80 e V,respectively,both of which are indirect bandgaps.The charge separation effect of Bi₅Mn₂O₁₁Cl in the spin-up and spin-down states is not as excellent as that of Bi₅Ti₂O₁₁Cl.The ferromagnetism of Mn ion contribute to recycle and reuse the photocatalytic materials.（4）The crystal structure of Bi₇Fe₂Ti₂O₁₇X（X=Cl,Br,and I）are theoretically predicted,and their XRD spectra are calculated.The theoretical results agree well with the XRD data obtained by experimental testing.Finally,the differences in CO₂reduction ability of Bi₇Fe₂Ti₂O₁₇X（X=Cl,Br,and I）were compared.