Synthesis And Properties Of KNbO₃ Based Narrow Band Gap Perovskite Oxides

Perovskite materials have attracted great attention due to their application in solar energy harvesting and photocatalysis.The power conversion efficiency?PCE?of solar cells based on the organic-inorganic hybrid CH₃NH₃PbI₃ perovskite materials have soared from 3.8%when first reported in 2009 to over 20%in 2017 and still have the potential to be increased.This new type solar cell has become a star in photovoltaic research.However,the organic-inorganic hybrid perovskite CH₃NH₃PbI₃ contains toxic element Pb and suffers from severe stability problems,which greatly hindered practical application of this new type perovskite solar cell based on it.Perovskite oxides usually have excellent physical and chemical stability.Perovskite oxides materials with good optical and electrical properties suitable for perovskite photovoltaic application may provide a possible solution to overcome the shortcomings of CH₃NH₃PbI₃.Research on perovskite oxides based solar cells has become a very important subject in material sciences.Besides,applications of perovskite oxides in photocatalytic water splitting and photodegradation of contaminant demand perovskite materials with good optical absorbing properties.Tailoring the optical band gap of perovskite oxides to increase their light absorption is the first step to obtain perovskite oxide materials suitable for perovskite photovoltaic or photocatalytic applications.By taking the strategy of doping at B-site of ABO₃ perovskite to tune its band gap,researchers have successfully modified the band gap of double perovskite Bi2FeCrO6 and perovskite-double perovskite solid solution[KNbO₃]1-x[BaNii/2Nb1/2O₃-s]x and other B-site doped perovskite materials.Their applications in light harvesting devices have also been reported.Based on these researches,this dissertation studied the band gap tuning effect of different transition metal ions of Ni,Fe and Co in the KNbO₃ based perovskite-double perovskite solid solution system.Band gap narrowing of KNbO₃ has been achieved.The main research work and innovations of this dissertations are listed in details as follows:?.Ba,Ni co-doped KNbO₃ have been synthesized at relative lower sintering temperature with Sol-Gel method.Structural transition and optical absorption of Ba,Ni co-doped KNbO₃ have been investigated.Experiments of photodegration of methylene blue have been conducted to evaluate the photocatalytic performance of Ba,Ni co-doped KNbO₃.?a?Ba,Ni co-doped KNbO₃ perovskite oxides have been successfully synthesized with Sol-Gel method at sintering temperature of 700?,much lower than the typical 1100? sintering temperature in solid state reaction.?b?Band gap of Ba,Ni co-doped KNbO₃ have been decrease by 0.2 eV from 3.3 eV of pure KNbO₃ to 3.1 eV.Unusual absorption peaks in UV-vis-NIR absorption spectra of Ba,Ni co-doped KNbO₃ have been observed and interpreted.The absorption peaks are assigned to energy levels of Ni2+ ions in NbO6 octahedral crystal field.A possible energy band scheme of Ba,Ni co-doped KNbO₃ has been proposed.?c?Evaluation of photocatalytic degradation of methylene blue shows that Ba,Ni co-doped KNbO₃ with x=0.2 exhibits the highest photodegradation reactivity,which is 5 times higher than that of KNbO₃.?.Ba,Fe co-doped KNbO₃ have been synthesized with Sol-Gel method.Structural transition,optical and magnetic properties of prepared samples were investigated.Energy band structure and visible light water splitting application of Ba,Fe co-doped KNbO₃ were studied.?a?Structural transition from orthorhombic to cubic crystal structure similar to that in Ba,Ni co-doped KNbO₃ was observed in Ba,Fe co-doped KNbO₃ synthesized with Sol-Gel method.?b?Depending on the ratio of doping concentration,optical band gap values of Ba,Fe co-doped KNbO₃ were decreased to as low as 2.41 eV.?c?Ultra-violet Photoelectron spectra show that positions of valence band and conduction band of Ba,Fe co-doped KNbO₃ can be both lowered by Ba,Fe co-doping.The band gap narrowing effect is largely due to the down shift of conduction band.However,reactivity of photocatalytic visible light water splitting was relatively low because the valence band and conduction band of Ba,Fe co-doped KNbO₃ fails to straddle the water oxidation and reduction potentials.?d?KNbO₃ was transformed from diamagnetic to ferromagnetic by Ba,Fe co-doping.The origin of ferromagnetism was explained with bound magnetic polaron?BMP?theory.?.Ba,Co co-doped KNbO₃ were synthesized with Sol-Gel method.Structural transition,optical absorption and band gap modification of Ba,Co co-doped KNbO₃ were investigated.A possible energy band scheme of Ba,Co co-doped KNbO₃ was proposed.?a?The effect of different Co sources in Sol-Gel precursors were found to have great impact on the synthesis of Ba,Co co-doped KNbO₃.Co3+ doped Ba,Co co-doped KNbO₃ can be synthesized with cobalt nitrate as Co source in the precursors at a sintering temperature of 800?.?b?Orthorhombic to cubic crystal structure transition was also observed in Ba,Co co-doped KNbO₃.Intrinsic optical band gap of KNbO₃ can be reduced to a minimum value of 2.45 eV depending on different doping ratios.Intermediate energy levels between the Nb 4d conduction band and O 2p valence band were introduced by Ba,Co co-doping,which lead to the 1.3 eV and 0.7 eV sub energy band gaps in the absorption spectrum and hence the strong visible and infrared light absorptions.This Ba,Co co-doped KNbO₃ materials may have potential in photovoltaic and photocatalysis applications due to their excellent optical properties.?.Energy band structure of KNbO₃ was investigated by first principle calculation.Electronic energy level structures of orthorhombic KNbO₃ was obtained by first principle calculation.It is confirmed that the conduction band and valence band of KNbO₃ are composed by Nd 3d and O 2p energy levels respectively.Possible reason for the band gap narrowing of Ba,Fe co-doped KNbO₃ was also discussed.