Ionic Liquids-modulated Synthesis Of Noble Metal Micro/nanomaterials In Aqueous Solution At Room Temperature

Noble metal nanomaterials have attracted much attention due to their unique physical and chemical properties.Unlike bulk counterparts,nanomaterials have the features of small size,large specific surface area,a large number of surface atoms,defects and steps,which endow them excellent properties in many applications.Moreover,their properties are also closely related to their composition,structure and morphology.Because of ensemble effect,electronic effect and geometric effect in bimetals,they exhibit superior properties than single ones in catalysis,electrocatalysis,surface enhanced Raman scattering,photothermotherapy and biosensing.However,due to the smaller size,nanomaterials have larger surface energy,which make them prone to aggregate and deactivate during use.In this regard,it is very important to exploit suitable stragety for the delicate construction of hierarchical structures with nano-sized units as building blocks,which not only can they maintain the morphology and activity during application,but also can be easily recycled.In this paper,based on the characteristics of adjustable structure and properties of ionic liquids(ILs),several of them were purposefully selected and then obtained by purchase or synthesis.After that,under the modulation of ILs,Au nanoworms and Au Pd,Au Pt bimetal micro/nanomaterials with different morphologies were prepared in aqueous solution at room temperature,and their SERS and catalytic properties were evaluated.(1)First,tetrapropylammonium glycine([N₃₃₃₃][Gly])was synthesized and pured.Then,porous worm-like Au micro/nanomaterials were grown under the control of[N₃₃₃₃][Gly]in aqueous solution at room temperature.It is shown that the as-prepared porous worm-like Au has the length about 0.3-0.6?m and the width of approximately0.1-0.15?m,and it is composed of lots of small nanoparticles about 6-12 nm in diameter.BET tests show that the specific surface area of porous worm-like Au is as high as 71.16 m²·g^-1,which is significantly higher than most reported values in literatures.It is mainly dominated by 2.4 nm mesopore.The results show that[N₃₃₃₃][Gly]play a crucial role in the formation of Au worm-like structures.Using Rhodamine 6G as a probe molecule,worm-like Au has strong SERS response signals(detection limit<10^-13)and extremely high reproducibility(average standard deviation<2%).Moreover,using p-aminothiophenol and pyridine as molecular probes,worm-like Au also have strong SERS enhancement.At the same time,in the catalytic reduction of p-nitrophenol and p-nitroaniline,worm-like Au also has high catalytic activity,and the reaction can be completed in 100 s and 150 s.(2)With the assistance of ionic liquid 1-hydroxyethyl-3-methylimidazolium chloride([HEmim]Cl),the porous sponge-like Au Pd micro/nanomaterials was quickly one-pot synthesized in aqueous solution at room temperature.The spongy-like Au Pd micro/nanomaterials of different components are formed by the aggregation and fusion of nanoparticles with the size of about 30-40 nm.The sponge-like structure is fluffy and porous.Au₃Pd₁,Au₁Pd₁,and Au₁Pd₃ all have large specific surface areas,which are 51.544 m²·g^-1,45.142 m²·g^-1,and 47.533 m²·g^-1,respectively.There are also a large number of mesopores.The ionic liquid plays an important role for the construction of Au Pd sponge-like structures.Futhermore,all the as-obtained Au Pd nanosponges exhibit excellent catalytic performance than commercial Pd/C in the reaction of p-nitrophenol reduction,in which the Au₁Pd₃ nanosponges have the highest catalytic activity.In the presence of Au₁Pd₃ nanosponges,the reaction can be quickly finished within only 98 s and the reaction rate constant is calculated to be 0.0143 s^-1,which is 2.3 times higher than commercial Pd/C.Yet,the current protocol can be used to grow and assemble other bi-(such as Pd Cu and Pt Cu)and multi-metallic nanosponges.(3)Poly(ionic liquid)s(PILs)poly(1-vinyl-3-ethylimidazole bromide)(Poly(Vi Et Im)Br,shorthand for PVEIB)was first synthesized,pured and characterized.Then,with the assistance of PVEIB,the well-defined snowflake-like Au Pd micro/nanomaterials with multi-level structures were grown in aqueous solution at room temperature.The primary structure of Au Pd nanosnowflake is the snowflake-like structure with the diameter of about 1.4?m;the secondary unit is the 2D sheet-like branch structure with a length of about 0.6-0.9?m;the tertiary unit is the side branch structure growing from the 2D sheet-like structure;the fourth-order is the small spherical nanoparticles that build up the tertiary structure.From the observation of high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM),it can be seen that Au element is mainly distributed in the branch part,and Pd mainly in the sheet structure part.They have the specific surface area of as high as31.943 m²·g^-1,and mainly pore size of about 2.6 nm.The results show that PILs act as the structure directing agent guiding the formation of snowflake-like morphology.The metal precursor ratios,the concentrations of PILs,the concentrations of reducing agent,and the reaction temperature have effects on the morphology of the products to some extent.In the experiment of p-nitrophenol reduction,Au Pd nanosnowflakes display better catalytic activity than pure Au,Pd and commercial Pd/C.Au Pd nanosnowflakes also show excellent selectivity and catalytic performance in selective hydrogenation of p-nitrophenylacetylene.(4)In aqueous solution and room temperature,flower-like Au Pt nanospheres were prepared in the presence of PILs 1-butyl-3-methylimidazole polystyrene sulfonate([Bmim][PSS]).The Au Pt nanospheres are very uniform in morphology with the diameter is about 500 nm and are built up by a large number of nanoplates with about20 nm in thickness.All the Au Pt samples with different molar ratios(such as 3:1,1:1and 1:3)have similar flower-like structures.The results of elemental mapping and X-ray diffraction show that the Au Pt nanosphere has alloy structure.[Bmim][PSS]plays a key role in directing the formation of flower-like nanospheres.The reducing agent concentrations also have a certain effect on the morphology of the product.With R6G as a molercule probe,flower-like Au Pt nanosheres exhibit high SERS sensitivity and reproducibility,and with the increase of Au content in the alloy,the SERS sensitivity increases correspondingly.