Designed Synthsis And Property Studies Of Mesoporous Metal Oxide And Their Nanocomposites

Since the discovery of mesoporous silica materials in the 1990s,mesoporous materials have attracted the researchers5 interests due to their large pore size,high specific surface area and their wide applications in various fields.Currently,a number of methods have been developed to prepare mesostructured silica nanomaterials,while the preparation of non-silicon based mesoporous nanomaterials still faces enormous challenges.It is mainly due to the fact that the silica precursor(such as tetraethyl orthosilicate/TEOS in water)is easily to be well controlled and the product is thermally stable;in contrast,the hydrolysis and condensation processes of the non-silicon based precursor especially the metal alcohol salts are extremely difficult to control.Under the circumstance,it is more challenging to design and synthesize mesoporous metal oxides and their composite nanomaterials,which have been widely applied in energy conversion and storage,photoelectric response,catalysis,adsorption and separation,etc.In this work,we have focused on designing and synthesizing mesoporous metal oxides and their composite nanomaterials through three simple,gentle and controllable synthetic strategys:1)synthesize four kinds of mesoporous metal hydroxides and oxide nanospheres via 'chelation-induced'cooperative self-assembly strategy;2)synthesize hierarchically mesoporous NiO microspheres of different morphologies by mild 'one-pot' hydrothermal method;3)synthesize with defect-rich ZnO@ZIF-8 nanocomposites through simple in situ 'self-sacricfed'template.The major achievements are described as following.A 'chelation'-induced cooperative self-assembly strategy is proposed.In the synthetic system,cetyltrimethylammonium bromide(CTAB)is used as a template and a chelating ligand(citric acid/ascorbic acid)acting as a co-template to strength the weak interaction bwtween metal ions and surfactants.Both MMHO(including Cu(OH)2,Zn(OH)2,Fe(OH)3,Ni(OH)2)and MMO(including CuO,ZnO,Fe2O3,NiO)nanospheres and their nanocomposites with tunable diameters,high specific surface areas,and different pore sizes are synthesized via this strategy.The particle size of the prepared MMHO nanospheres is tunable by simply modulating the volume ratio of ethanol/water in the system.The as-synthesized mesoporous ZnO nanospheres exhibit good photoelectric performance.As a highly efficient oxygen evolution reaction(OER)catalyst of low cost,the calcined Cu(OH)2 nanospheres exhibit one of the best activities for OER.Moreover,this cooperative method gives rise to an alternative to'classical' self-assembly methods for the preparation of mesostructured nanomaterials and,in some cases,the only viable synthetic route toward MMHO and MMO nanostructures.A simple 'one-pot' hydrothermal method is employed to synthesize core-shell and hollow structured hierarchically pore Ni(OH)2 and NiO microspheres.The obtained NiO microspheres possess numerous mesopores,a high specific surface area,and 2D nanosheet-like shells,which largely promote mass/charge transportation and reduce ion transport pathways.By increasing the molar ratio of HMTA/Ni2+,the thickness of the outer nanosheet-like shell can be varied in the range of 0-35 nm.In particular,the NiO-300/r=2 microspheres possess excellent properties for OER(overpotential of 360 mV to reach 10 mA cm-2),compared with previously-reported Ni-based materials.Furthermore,improved activity with overpotential of 340 mV was reached by introducing Co.The simple'ne-pot' method and the excellent OER catalytic activity exhibited by the prepared nickel oxide microspheres provide more possibilities for the application of nickel-based materials in water splitting.A convenient in situ 'self-sacrificed' template method is used to synthesize core-shell structured defective ZnO@ZIF-8 nanocomposites,combining mesoporous ZnO cores and microporous ZIF-8 shells.The defect amount,varied crystalinity,and ZIF-8 shell thickness are controllably adjusted by changing the solvent components in the system.The in situ 'self-sacrificing'template induces insufficient zinc ions in the synthetic system,which leads to defective active sites—ordered arrangement of uncoordinated nitrogen atoms in the skeleton of ZIF-8.The nanocomposites exhibit superior photocurrent response to ZnO(an increase of up to 348%).The advantages on photoelectrochemistry of the nanocomposites are mainly attributed to that well-defined micropores provide more contact areas for media,and that the uncoordinated N atoms and H2O molecules form hydrogen bonds,thus producing proton-conductive pathways to promote the transmission of photogenerated electrons and preventing recombination reactions to some extent.The defective nanocomposites and their improved performance may inspire more interesting synthesis of ZnO based photoanode materials and has an important effect on understanding defect chemistry in MOFs.