Synthesis And Characterization Of Z-scheme BiOBr/MIL-125(Ti) Composite For Nitrate Reduction With Advance Nitrogen Selectivity

The quest to mollify human’s needs due to the hysterical increase in population has resulted in cumulative anthropogenic activities which are radically deteriorating the environment by causing severe pollution.Nitrate（NO₃^?）,a highly stable and a common contaminant present in both surface and groundwater,is extremely difficult to eradicate in its aqueous form.The excessive concentration of nitrate in water can lead to the eutrophication and poses major threats to human health when it enters the human body.The photocatalytic reduction of nitrates into harmless nitrogen has been widely considered among scientists due to its mild conditions,easy operation,high efficacy,and environment friendly attribute.However,there are several drawbacks associated with traditional photocatalysts,which hinder the successful application of the photocatalytic technology.Therefore,developing a novel material with high photocatalytic performance is a major challenge.In our work,considering the advantages of MOFs’large surface area,high reduction potential and BiOBr’narrow bandgap,herein,we constructed a series of novel BiOBr/MIL-125（Ti）composites and employed it for nitrate reduction.The properties and enhanced photocatalytic activities of synthesized materials were reasonably explained,and the plausible photocatalytic mechanism was speculated.（1）A series of Z-scheme BiOBr/MIL-125（Ti）composite was prepared via solvothermal treatment,considering different BiOBr ratios（30,40 and 50 wt%）,and synthesis temperature（80°C,120°C,160°C）.The crystallinity of the prepared samples was characterized using X-ray diffraction（XRD）,the morphology was observed from the transmission electron microscopy（TEM）,the chemical compositions were examined by Fourier transformation infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）,the optical properties were analyzed by UV-vis diffuse reflection（UV-vis DRS）and Photoluminescence（PL）spectra,respectively.The XRD,XPS and FT-IR results proved that BiOBr/MIL-125（Ti）composite was successfully prepared.The TEM images revealed that the flakes of BiOBr were uniformly distributed onto the surface of disc-like MIL-125（Ti）in the composite.The UV-vis DRS analysis showed that pure MIL-125（Ti）could adsorb light only in ultraviolet region,however,after the incorporation of BiOBr,the light adsorption ability expanded to visible region for BiOBr/MIL-125（Ti）composite.（2）The photocatalytic experiment results showed that,after 60 min of UV light irradiation,the BiOBr/MIL-125（Ti）composite（40%BiOBr,prepared at 160°C）exhibited highest nitrate reduction ability（81.1%）and excellent nitrogen selectivity（95.3%）as compared to the pristine BiOBr and MIL-125（Ti）(0.1 mol L^?1 formic acid as scavenger).The incorporation of BiOBr into MIL-125（Ti）not only broaden the light adsorption ability but also greatly enhanced the separation of interfacial charge carriers by forming direct Z-scheme heterojunction with proper band alignments among BiOBr and MIL-125（Ti）,thus improving the nitrate reduction activity with extraordinary nitrogen selectivity.The results of PL spectra,transient photocurrent response and Electrochemical impedance spectroscopy further confirmed that BiOBr/MIL-125（Ti）exhibited lowest recombination and most effective transfer of photoinduced charge carriers.Moreover,the BiOBr/MIL-125（Ti）composite showed high recyclability,even after three consecutive cycles the nitrate conversion rate was above 81%.The inhibitory effect of different anions on nitrate reduction efficiency was in the order of PO₄^3->CO₃^2->Cl^?,where PO₄^3-with highest repressive effect lowered the nitrate reduction rate to 64.9%.The possible mechanism of photogenerated electrons/holes transfer and photocatalytic nitrate reduction over MIL-125（Ti）Z-scheme heterojunction was proposed by following the radical species capturing experiment combined with Mott Schottky.Henceforth,our work gives a new perception into the construction of Z-scheme semiconductor/MOFs hybrids and encourages the further research to expand utilization of MOFs for photocatalytic nitrate reduction on the practical level.