The Chargeability Of Kaolinite And Its Application In Ion Transport And Energy Conversion Systems

Clay minerals are widely used in many fields such as chemistry,biology,nanoscience,environmental science and energy because of their special layered structure,excellent thermal stability,great chemical stability,and high absorption properties.Many natural clay minerals can be exfoliated into two-dimensional nanosheets and then reassembled into a variety of functional membranes/films with microscopic layered structures.Kaolinite is a typical 1:1 layered clay mineral,however,traditional intercalation method has not been able to exfoliate it into single layers,which greatly limits the application of kaolinite in functional materials.Furthermore,since the Si-tetrahedron and the Al-octahedron have different surface chemical activities,the role of the natural two-dimensional heterostructure in nanoscience cannot be maximized.Therefore,in order to deeply excavate the kaolinite structure and surface reactivity characteristics,fully exhibit the role of kaolinite-based functional membrane in energy-related application,a two-dimensional kaolinite membrane with ion channels was prepared by a self-assembly method based on the analysis of the crystallinity and the chargeability of kaolinite.The properties of the membrane,transmembrane ion transport behavior,and transmembrane energy conversion performance were studied.And the following main results and understanding were obtained:(1)The crystal structure and surface characteristics of different genesis kaolinite in four typical regions of China(Hebei Zhangjiakou,Fujian Longyan,Shanxi Pingshuo,and Anhui Huaibei)were studied.By comparison,the crystallinity and surface potential of the above four regions are in descending order:Zhangjiakou,Pingshuo,Longyan,Huaibei.The surface potential of kaolinite is not necessarily related to the type of kaolinite deposit.The higher the crystallinity,the stronger the surface potential of the kaolinite in the solution.The absolute advantages of crystallinity and surface potential of Zhangjiakou kaolinite in the functional membrane and energy conversion are clarified.(2)Kaolinite/dimethyl sulfoxide intercalation compounds was prepared by intercalating kaolinite with polar organic species dimethyl sulfoxide,and the structural features and thermal performance of intercalation compounds was discussed.The kaolinite particles were exfoliated into few-layer nanosheets by intercalation-modification-ultrasonication combined exfoliation method.Bis-(?-triethoxysilylpropyl)-tetrasulfde(Si-69)was grafted onto the Al-OH group of kaolinite al-octahedron by selective surface modification to form heterogeneous nanosheets for the preparation of Janus-like building blocks.About 70%of the nanosheets have a lateral dimension of 800-1000 nm.The thickness of the nanosheets is between 2-20 nm,and more than 75%of them have a thickness below 12 nm.The thinnest kaolinite nanosheets have a thickness of 2.1 nm and a lateral dimension of about 320 nm.All the nanosheets show a high aspect ratio and a smooth surface topography.(3)The effect of surface modification on the chargeability of kaolinite was studied.The kaolinite particles before modification have a negative charge in the range of pH?3-11,and the zeta potential of-27.8 mV was measured at neutral pH.After covalent modification with Si-69,the zeta potential generally moveed in the positive direction.but the surface charge still exhibited a negative charge under neutral pH conditions.It was caused by the consumption of hydroxyl groups exposed to the aluminum octahedral surface and structural defects in the chemical reaction.(4)A reconstructed kaolinite membrane with a layered microstructure was prepared by a bottom-up self-assembly method.The surface of the membrane was hydrophobic with a contact angle of 112.7°.Both of the evaporation and vacuum filtration in the assemble process are indispensable for the formation of highly ordered layered structure.The stability in water of the reconstructed kaolinite membranes were tested by the immersion method.The membranes could maintain stable in water for a long time due to the hydrophobic properties.And the structure didn't collapse which could be caused by swelling.Moreover,the membranes were also structurally intact in other harsh chemical environments(strong acid,strong alkaline,and high-concentration saline solutions,etc.).(5)The effect of membrane formation on the crystal structure and surface characteristics of kaolinite was studied.Two new(001)diffraction peaks appeared in the XRD pattern and the original kaolinite(001)diffraction peak almost disappeared,indicating that the crystal structure of the raw kaolinite was seriously damaged.Due to the two-dimensional heterostructure of kaolinite nanosheets,the assembly unit showed two new stacking modes during the reassembly process:face-to-face(AOS-AOS combination)corresponded to a larger layer spacing of 21 A,back-to-back(STS-STS combination)corresponded to a smaller layer spacing of 14 A.Ion transport channels with two sizes of 13.8 and 6.8 A were obtained.The ratio of the two types of extended layered structures(A/S)is about 3:1.(6)The ions transport behavior in the nanochannels of the reconstructed kaolinite membrane was studied.The ion transport of the nanochannel was regulated by the surface charge of the channel,which is mainly because the thickness of the electric double layer formed on the inner wall of the channel is equivalent to the size of the channel,resulting in the superposition of the electric double layer,thereby evacuating the oppositely charged ions.Due to the negative charge on the surface of the kaolinite,the channels exhibit a very strong cation selectivity.(7)The reconstructed kaolinite membranes were integrated into the nanofluidic circuit to construct two-dimensional nanofluidic generators.Driven by a transmembrane hydraulic flow,a flow current of several hundred nanoamperes was observed,which exceeded nearly 300%of the graphene oxide membrane under the same conditions.Under the difference of transmembrane concentration,the osmotic current was observed and the output power density is ca.0.15 W/m2,which is very close to the output power density obtained by the graphene oxide membrane.The conversion efficiency is up to 44.2%which is very close to commercial ion exchange membrane.However,the raw material price of kaolinite membrane is 5 orders of magnitude lower than the graphene oxide membrane and the commercial ion exchange membrane.