| Photoelectrochemical(PEC)water splitting provides a "green" approach for hydrogen production.However,due to the involved complex multiple electron/proton-coupled reaction in the water oxidation and the sluggish reaction kinetic,the design and fabrication of highly efficient photoanodes have become an effective strategy to improve the efficiency of the PEC water splitting reaction.In recent years,hematite has become one of the most potential photoanode materials owing to its suitable band gap(?2.2 eV),abundance,outstanding chemical stability in alkaline aqueous solutions and excellent theoretical photocurrent density of 12.6 mA/cm2.However,due to the inherent shortcomings of hematite,including short carrier diffusion distance(2-4 nm)and sluggish water oxidation kinetics,the performance of PEC water splitting has been severely limited.Therefore,the modification and optimization of ?-Fe2O3 to obtain higher photocatalytic efficiency has very important research significance in solving the energy crisis.Herein,hematite nanobelts(denoted as ?-Fe2O3 NBs)containing highly ordered oxygen vacancies were fabricated by Pd catalyzed oxidation under precise control of oxygen partial pressure.The introduction of highly ordered oxygen vacancies can increase the carrier concentration and shorten hole migration distance for PEC water oxidation.(1)Iron foils were annealed at suitable oxygen partial pressure to prepare ?-Fe2O3 NBs containing highly ordered oxygen vacancies by Pd catalyzed oxidation with concave Pd,cube Pd,and octahedron Pd as catalysts at 500? for 2 h,which were recorded as ?-Fe2O3 NBs(concave Pd),?-Fe2O3 NB s(cube Pd)and ?-Fe2O3 NB s(octahedron Pd).The results show the direction of ordered oxygen vacancies is consistent with the growth direction of ?-Fe2O3 NBs,which are both parallel to the[110]direction.The long-range ordered oxygen-vacancy structure is observed in ?-Fe2O3 NBs(concave Pd)with a periodicity of 1.48 nm corresponding to ten times(112)lattice spacing;Two long-range ordered oxygen-vacancy structures with different periodic structures in ?-Fe2O3 NBs(cube Pd)and ?-Fe2O3 NBs(octahedron Pd),one being a periodicity of 1.37 nm corresponding to five times(112)interplanar spacing,and the other being a periodicity of 1.51 nm corresponding to four times(110)interplanar spacing,respectively.When the oxygen partial pressure is too high or too low during the annealing process,?-Fe2O3 nanoflakes(denoted as ?-Fe2O3 NFs)and Fe3O4 nanoflakes(denoted as Fe3O4 NFs)will be prepared,respectively.However,no ordered oxygen-vacancy structure was observed in either ?-Fe2O3 NFs or Fe3O4 NFs.(2)Using ?-Fe2O3 NFs as control sample,the PEC water oxidation performance of?-Fe2O3 NBs was investigated.The ?-Fe2ONFs photoanode exhibits photocurrent density of about 1.28 mA·cm-2 at 1.6 V vs.RHE under illumination(AM 1.5G,100 mW·cm-2),which is comparable to that of previously reported hematite nanoflakes.?-Fe2O3 NBs with ordered oxygen vacancies have achieved higher performance in terms of reducing onset potential and increasing photocurrent density.The photocurrent density of ?-Fe2O3 NBs(cube Pd)is approximately twice than that of ?-Fe2O3 NFs,reaching 2.4 mA·cm-2 counterpart at 1.6 V vs.RHE.Most importantly,?-Fe2O3 NBs(cube Pd)presented an onset potential(Von)as low as 0.437 V vs.RHE,which was the lowest value ever reported for the hematite-based photoanodes.The ?-Fe2O3 NBs(concave Pd)photoanode shows one of the best performances ever reported only by doping,giving rise to a photocurrent density of about 3.5 mA·cm-2 at 1.6 V vs.RHE and Von of 0.58 V vs.RHE.Furthermore,the hydrogen evolution rate of?-Fe2O3 NBs(concave Pd)can reach to 29.46 ?mol·cm-2·h-1,which is about twice as high as that of ?-Fe2O3 NFs film(10.38 ?mol·cm-2·h-1).(3)On the basis of experiments,we further explored the growth process and the PEC water oxidation performance improvement mechanism of highly ordered oxygen vacancies in?-Fe2O3 NBs.An obvious layered structure,FeO,Pd layer,Fe3O4 columnar crystal layer,Fe2O3 small crystal grain and Pd nanoparticles layer and ?-Fe2O3 nanobelts layer from bottom to top,can be observed.The corresponding force and lattice distortion between Fe3O4 columnar crystals and Fe2O3 small crystal grains together lay the foundation for the ordered oxygen vacancies.The reaction rate is controlled by the O2-generation rate so that the ordered oxygen vacancies are retained during the growth of ?-Fe2O3 NBs.Due to the catalytic activities difference of Pd nanoparticles with different morphologies,the layer thickness and grain sizes of the Fe3O4 columnar crystal layer and Fe2O3 small crystal grain layer have slight differences during the annealing process,which affects the corresponding forces and lattice distortions and result in the periodicity differences of ordered oxygen vacancies.The introduction of ordered oxygen vacancies gives the photoanode significant advantages:?The doping of ordered oxygen vacancies results in a higher carrier concentration(?1020 cm-3);? The ordered oxygen vacancies can form a directional potential field for promoting the separation of photogenerated carriers;? The ordered oxygen vacancies can be used as hole transport channels to accelerate the holes surface migration;?The ordered oxygen vacancies provide more surface reaction active sites,which greatly promotes the progress of water oxidation reactions. |
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