Synthesis Of Highly Efficient Visible-light Active Perovskite-type Oxide Nanophotocatalysts And Mechanism

Nowadays,with the rapid development of economy and the remarkable improvement of living standard,there is a wealth of environmental issues facing people across the globe day by day.Current environmental problems make us vulnerable to disasters and tragedies now and in the future.Unless we address various issues prudently and seriously,we are surely doomed for disaster.As a promising environmental remediation technology,semiconductor photocatalysis has received much attention owing to its highly efficient,low cost and environmental friendly merits.Among the widely investigated semiconductor photocatalysts,Ti O2 has attracted much attention owing to its low cost,non-toxicity and suitable conduction and valence band positions for redox reactions.However,the large intrinsic band gap? 3.2 e V?of Ti O2 is unfavorable for harvesting visible-light photons that comprises ca.46% of the solar spectrum.Therefore,it is highly desirable to develop cheap,stable and narrow band gap photocatalysts for effective solar energy utilization.Among the widely investigated narrow band gap oxide photocatalysts,?-Fe2O₃?Eg = 2.2e V?and Bi VO4?Eg = 2.4 e V?have attracted much attention in the field of photocatalysis owing to their low cost,natural abundance,stability,non-toxicity,environmentally friendly and positive valence band edge potential.Nevertheless,the photocatalytic performance of these materials is still limited due to their low absorption coefficient,high charge recombination rate and low conduction band bottom for reduction reactions.Recently,perovskite-type La Fe O₃?LFO?has attracted much attention in the field of photocatalysis owing to its unique characteristics such as cheapness,high stability,non-toxicity and suitable band gap? 2.0 e V?.However,it still exhibits low photocatalytic activity which is attributed to the weak photogenerated charge separation due to the low conduction band bottom,limited visible-light absorption and small surface area.To overcome these drawbacks,we have carried out few modification strategies to improve the photocatalyticperformance of LFO for effective degradation of environmental pollutants and CO2 reduction.Firstly,we have fabricated Ti O2/porous-LFO?T/P-LFO?nanocomposites via a wet chemical process.It is investigated that the resultant T/P-LFO nanocomposites with a proper mole ratio percentage of Ti O2 exhibits enhanced visible-light activities for gasphase acetaldehyde and liquid-phase phenol degradation compared to the unmodified PLFO.Based on the steady-state surface photovoltage?SS-SPS?responses,Photoluminescence?PL?spectra and Fluorescence?FS?spectra related to the amount of·OH radical produced,it is confirmed that the enhanced visible-light activities of the nanocomposites are attributed to the improved charge transfer and separation by coupling Ti O2.This strategy is also applicable to Ti O2/porous Bi Fe O₃ nanocomposites.Secondly,we have fabricated Ti O2 coupled N-doped LFO-AC?T/N-LFO-AC?nanocomposites via a wet-chemical method.It is shown that the amount-optimized 6T/6NLFO-AC nanocomposite exhibits exceptional visible-light activities for 2,4-dichlorophenol?2,4-DCP?degradation and CO2 conversion compared to the LFO-AC sample with rather high photoactivity due to its large surface area.It is clearly demonstrated by means of BET,valence band XPS,UV-vis DRS,SS-SPS,FS and PEC results that the obviously improved visible-light activities of the nanocomposites are attributed to the improved specific surface area by introducing pores,to the extended visible-light absorption by doping N to create surface states and to the promoted charge transfer and separation by coupling Ti O2.Moreover,it is confirmed by means of radical trapping experiments that the photogenerated holes are the predominant oxidants involved in the photocatalytic degradation of 2,4-DCP over T/N-PLFO nanocomposite.In addition,a possible degradation pathway for 2,4-DCP is proposed mainly based on the radical trapping experiments and liquid chromatography tandem mass spectrometry analysis of the intermediate products.Thirdly,we have synthesized Bi,Zn co-modified porous LFO?XZn/Bi-PLFO?samples via a wet-chemical solution method.Based on the experimental results,it is confirmed that the modified Bi3+cations entered into the crystal lattice and substituted La3+,while the comodified Zn2+ is coupled to LFO in the form of ultrafine Zn O with diameter of 15 nm.It is shown that the amount-optimized 5Zn/7Bi-PLFO sample exhibits 3-time improved visible-light activity for 2,4-DCP degradation compared to the unmodified PLFO.Based on the measurements of valence band XPS spectra,SS-SPS spectra,TR-SPV spectra,photoelectrochemical?PEC?measurements,FS spectra related to produced ·OH amount and photocurrent action spectra,it is clearly demonstrated that the significantly improved visible-light activity of the 5Zn/7Bi-PLFO sample for 2,4-DCP degradation is attributed to the extended visible-light absorption and to the enhanced charge separation by doping Bi to create surface states and then further to the enhanced utilization of visible-light-excited high-level energy electrons by coupling Zn O.Interestingly,it is investigated that under UV-vis-light irradiation,the amount-optimized 5Zn/7Bi-PLFO sample exhibits much higher photoactivity for 2,4-DCP degradation compared to the commercially available P25 Ti O2.Moreover,the detail photocatalytic mechanisms and pathways for 2,4-DCP degradation over the amount-optimized 7Bi-PLFO and 5Zn/7Bi-PLFO samples are proposed mainly based on the radical trapping experiments and liquid chromatography tandem mass spectrometry analysis of the intermediates.This research work would help us to deeply understand the photophysical and photochemical processes and also provide feasible routes to improve the photocatalytic performance of visible-light responsive semiconductor oxide photocatalysts for pollutant degradation with clear mechanisms and solar fuel production.