Mxene (Ti₃C₂)-Based 2-Dimensional Nanochannel Ionic Transportation Properties And Their Applications

Two-dimensional nanosheets by a way of self-assembled to form a layered membrane that consists of a plenty of sub-nanoscale channels.By studying the mechanism of ion transport in nanochannels and further realizing the regulation of ion transport,which can greatly expand the application of two-dimensional layered membrane materials in many fields.MXene is a novel two-dimensional nanomaterial,which has abundant physical and chemical properties and provides a natural platform for modulate ion transport.Therefore,Ti₃C₂,as the most representative material in the MXene,is selected for the research of how ions transport inside the Ti₃C₂ membrane in its intrinsic state.Besides,the excellent photothermal conversion performance of Ti₃C₂membrane can realize controllable ion transportation and can be used as switchable ion channels,light-heat-electric conversion and biomimetic ion pumps.Firstly,the Ti₃C₂ nanosheets was prepared by chemical liquid phase stripping,and then the lamellar films constructed from Ti₃C₂ nanosheets were obtained by the self-assembly via vacuum filtration.The classical nanofluidic test device assembled by Ti₃C₂ films proved the surface-charge-govern ionic transport when the ions pass through membrane channels under the electric field force.The negative surface charge of nanosheets leads to the cation enrichment and conductivity increases.The surface-charge-govern ion transport characteristic of the Ti₃C₂ membrane can remains unchanged in different states.Moreover,because of the strong van der Waals force and the weak charge repulsive force inside Ti₃C₂ membrane,it is more stable and does not disperse in the system of pure water,compared with graphene oxide and vermiculite membranes.Based on these two features,by using the intrinsic photothermal conversion of Ti₃C₂ membrane to control the moisture gasification inside the ion channels,the photo-controlled ion transportation can be realized.Secondly,the Ti₃C₂ membrane has an outstanding light to heat conversion property,especially for the near-infrared region.Under the asymmetric irradiation of the film by near infrared 808 nm monochromatic light,the asymmetric water evaporation caused by thermal gradients that can drive internal water flow through nanochannels and also push the ion transport along the water flow.Furthermore,since the surface of Ti₃C₂nanosheets is negatively charged,cations will be adsorbed on them.Therefore,based on the electrokinetic theory,the induced current is about 12?A when the illumination power is 100 m W/cm².The magnitude of the induced current is directly related to the rate of water evaporation rate.For practical applications,an output current can be obtained under outdoor sunlight and 1 M Na Cl solution.Most importantly,the current varies with the intensity of sunlight during testing.For example,in the rising stage of the sun,the current increases as the light intensity increases gradually;after sunset,natural evaporation becomes the main way of generating electricity.In addition,asymmetric evaporation can be achieved not only by asymmetric illumination,but also by changing the geometric construction of the device.The Ti₃C₂membrane is cut into a triangular shape for the device construction.The shape-induced asymmetric evaporation can also realize the active transport of ions and achieves the ion pump effect that against concentration gradient transport in the absence of light.Based on this effect,we designed a semi-wet membrane region with certain ion selectivity,which shows obvious difference fluxes for Na⁺and K⁺that with similar hydration radius.By analyzing the experimental results of a variety of other ions,it is proved that the process of ion screening is mainly based on interlayer spacing.Combined with the difference of hydration radius and crystal radius of various ions,ion screening is carried out to realize that different regions allow ions of different radius to pass through.On basis of selectivity ion pump model,an artificial ion pump was designed to response to light stimulus,the potential balance was broken and signal formed.The ion selectivity still remained in this reaction process.In conclusion,the surface-charge-govern effect of Ti₃C₂ membrane is proved by studying the transport characteristics of the nano-ion channels in the inherent Ti₃C₂membrane.Lower surface charge density and stronger van der waals force are unique advantages for the practical application of Ti₃C₂ membrane in the nanofluidic field.At the same time,based on the intrinsic and excellent photothermal property of Ti₃C₂,the device can realize three-step conversion of light-heat-electricity under asymmetric illumination.Moreover,through a kirigami mothed,a selectivity artificial ion pump can be achieved to response to light stimulus.