Ultrathin FeOOH Nanosheets As An Efficient Cocatalyst For Photocatalytic Water Oxidation

Photocatalytic water splitting and CO₂ reduction with H₂O are important applications of photocatalysis and have great potential in solving energy crisis and climate change issues.Water oxidation involving a complex four-electron transfer process is an extremely challenging or even rate-determining step in these two processes,which causes a serious concern on significantly improving water oxidation performance.Generally,a suitable cocatalyst is required to solve this kinetically slow process and much effort has been carried out.Ruthenium/iridium oxides are representative oxygen evolution cocatalysts among them.However,their use is limited by high cost and the general cocatalyst effect,which makes them far from the requirements of practical applications.Developing novel and nonprecious oxygen evolution cocatalysts will be the key to obtain highly active water-oxidized composites.Co,Ni,Fe-based oxide/(oxy)hydroxide are several low-cost and effective cocatalysts for the photocatalytic/photoelectrochemical(PEC)O₂-production.However,the development of Fe-based water oxidation cocatalysts still faces a predicament,particularly for pure Fe(oxy)hydroxides due to low conductivity for charge carriers.With the discovery and development of two-dimensional(2D)materials,more and more interesting properties of 2D materies have been discovered.2D nanosheets with ultrathin thickness to accelerate the conduction of charge carriers and possessing the maximum number of active surfaces are promising structural material for realizing a great number of active sites.Meanwhile,the ultrathin nature and confinement effect of two-dimensional materials leads to obvious lattice distortion and low-coordinated surface atoms with numerous dangling bonds.Such materials are thus suitable candidates for the realization of highly efficient catalytic process.Herein,2D ultrathin FeOOH nanosheets(FeOOH NSs)rich in oxygen vacancies(Vo)were synthesized by a bottom-up method.AFM test showed that the thickness of the nanosheet was about 1.8 nm,which is equivalent to four atomic layer thicknesses.For comparison,FeOOH quasi-two-dimensional materials(FeOOH q-NSs)and FeOOH bulk materials(FeOOH bulk)was also synthesized.The FeOOH NSs cocatalyst was composited with BiVO₄ via electrostatic adsorption method,and HR-TEM showed that FeOOH NSs and BiVO₄ had tight interfacial contact.FeOOH NSs/BiVO₄ demonstrated a remarkable enhancement in visible-light photocatalytic/photoelectrochemical oxygen evolution,approximately 2-fold and 9-fold higher than that of pristine BiVO₄.The prominent cocatalyst effect of FeOOH NSs was further validated in Rh B degradation with an 8-fold apparent rate constant enhancement.Moreover,the mechanism of photocatalytic activity enhancement was explored through a series of characterizations.PL and EIS studies showed that FeOOH NSs effectively promote charge carrier separation and transfer.XPS,ESR and EXAFS indicated the existence of abundant Vo defects in FeOOH NSs and the relationship between Vo defects and photocatalytic activity was established by oxidation treatment comparison experiments.Furthermore,the DFT calculation results revealed that Vo in FeOOH NSs facilitated the adsorption of water molecules,which may be an important reason for a 90%decrease in apparent activation energy for photocatalytic water oxidation.In summary,effective charge carrier separation and transfer,facilitated H₂O adsorption by the presence of Vo defects and decreased apparent activation energy were cruical to the enhancement of photocatalytic activity.