| Since the discovery of graphene,ultrathin two-dimensional(2D)nanomaterials(also called ultrathin nanosheets)have been developed rapidly and played a key role in many nanotechnology research fields.Because of their ultrathin atomic thickness and ultrahigh specific surface area,ultrathin 2D nanomaterials exhibit excellent physical and chemical properties which are different from their bulk phase and other nanostructures.Therefore,they have been widely applied in the magnetic materials,catalysts.energy storage and conversion,sensors,electronic and optoelectronic equipments,and biomedical devices.However,most of their synthetic methods are only applicable to the layered materials,while the effective synthetic methods for non-layered materials are still limited.Thus,it's still faced with great difficulties and challenges to synthesize the ultrathin 2D nanomaterials.Therefore.to develop a new synthetic method of ultrathin 2D nanomaterials,which can be applicable in different research fields,could not only solve the synthesis problems of ultrathin 2D nanomaterials.but also greatly enrich the 2D nanomaterials system and bring excellent physical and chemical properties in many research fields.Meanwhile,as one of the most attractive functional materials,metal oxide based ultrathin 2D nanomaterials exhibit unique electronic transition,electrical and optical properties,so it is of great significance to develop new metal oxide based ultrathin 2D nanomaterials.This work mainly focuses on the synthesis based on a simple ionic layer epitaxial method(ILE)of three new ultrathin metal oxide 2D nanomaterials,whose applications in magnetism,electrocatalysis and photoelectrocatalysis were also investigated.These results indicated that the ILE method can be used as a general synthesis strategy of ultrathin 2D nanomaterials in different research fields,and also provide new ideas and new ways for the design and development of high-performance ultrathin 2D nanomaterials in a variety of research fields.The conclusions of this paper are as follows:The ultrathin cerium oxide based organic-inorganic hybrid nanosheets(hy-CeO2-x),as one kind of ultrathin 2D magnetic materials were successfully synthesized by ILE.ILE method empolyed the mixed stearic acid(SA)and oleylamine(OAM)organic surfactant molecules at the water-air interface to guide the growth of nanosheets.Finally,the hexagonal ultrathin nanosheets with thickness ranging from 0.67 nm to 3.01 nm were successfully obtained.A significantly enhanced room-temperature ferromagnetism was observed when their thickness was reduced to 0.67 nm.The saturation magnetization(Ms)of these 0.67 nm nanosheets was up to 0.149 emu/g,and this Ms value was almost 5-fold larger than that of reported CeO2 thin films and more than 20 times higher than that of reported CeO2 nanoparticles.This robust magnetization was attributed to the hybridization of the organic surfactant layer with the inorganic CeO2-x nanosheets layer.which stabilized high concentration of oxygen vacancy(Vo)and enabled electron transfer between the organic surfactant layer and cerium oxide layer.Atomic layer deposition(ALD)coating of Al2O3 layer further increased the saturation magnetization to 0.1 9 emu/g by raising Vo concentration from 15.5%to 20.6%.Annealing the nanosheets in different atmospheres(e.g.,H2.Ar,and air)tuned Vo concentration and organic-inorganic binding situation.and further confirmed that the strong magnetization in hy-CeO2-x nanosheets was attributed to the hybridization of the organic surfactant with the CeO2-xnanosheets.The application of ILE method was extended to the synthesis of ultrathin 2D electrocatalyst materials.The ultrathin La2O3 ultrathin nanosheets were successfully synthsized by ILE method.After subsequent annealing treatment which caused the precipitation of nanoparticles at La2O3 nanosheets surface,the nanoparticle-decorated ultrathin La2O3 nanosheets(La2O3@NP-NS)was finally obtained.AFM and TEM results confirmed that La2O3@NP-NS was a nanohybrid where many tiny crystalline La2O3 nanoparticles were embedded on the surface of amorphous La2O3 ultrathin nanosheet.The 2.27 nm thick La2O3@NP-NS exhibited a low overpotential of 310 mV at 10 mA cm-2,a small Tafel slope of 43.1 mV dec-1,and charge transfer resistance of 38 ?.At an overpotential of 3]OmV.the very large mass activity(6666.7 A g-1)of 2.27 nm thick La2O3?NP-NS was more than three orders of magnitude higher than benchmark IrO2(4.4 A g-1)and RuO2(2.05 A g-1),and five orders of magnitude higher than commercial La2O3(0.048 A g-1).The nanosheets also exhibited an impressive stability for about 11-hour continuous OER operation with maintenance of 90%current density.The ILE method was further extended to the synthesis of ultrathin 2D photoelectrocatalysis materials.The ultrathin In2O3 nanosheets with thickness of 2.47 nm were successfully synthesized by ILE method.When these ultrathin In2O3 nanosheets were introduced to the surface of Si based photoelectrode Si NW/TiO2 as cocatalyst,and the new composite photoelectrode Si NW/TiO2/2.47 nm In2O3 showed excellent photoelectric catalytic performance.Si NW/TiO2/2.47 nm In2O3 showed a low onset potential of 0.6 V vs RHE(reversible hydrogen electrode)and its solar-to-hydrogen efficiency was up to 2.25%.Furthemore,at the potential of 1.6 V vs RHE,Si NW/TiO2/2.47 nm In2O3 showed a saturated photocurrent density as high as 36.9 mA cm-2.This photocurrent density value was respectivly about 16 times and 335 times larger than that of Si NW/TiO2 photoelectrode and Si NW/TiO2/In2O3 NP with In2O3 nanoparticles as surface cocatalyst.Moreover,Si NW/TiO2/2.47 nm In2O3 showed a good stability of 5 hours.The excellent photoelectric catalytic performance of Si NW/TiO2/2.47 nm In2O3 is mainly due to the fact that when ultrathin In2O3 nanosheets acts as the surface cocatalyst of the photoelectrode Si NW/TiO2,it can significantly improve the charge separation efficiency and charge injection efficiency of the photoelectrode without reducing the light absorption efficiency.Moreover,presence of ultrathin In2O3 nanosheets on the surface of the photoelectrode Si NW/TiO2 could effectively protect the Si based photoelectrode from being corroded in the electrolyte solution,thus enhanced the stability of the photoelectrode.Three new high-performance ultrathin metal oxides 2D nanomaterils were successfully developed by ILE method in this work,which successfully extended ILE methods to the synthesis of new ultrathin 2D nanomaterials.These results confirmed that ILE method is an effective method and can be used as a general synthetic strategy for new ultrathin 2D nanomaterials in different research fields.All these reasearch results are of great significance in resolving synthesis problems of ultrathin 2D nanomaterials and further enriching theoretical system of 2D nanomaterials. |
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