Using transmission electron microscopy (TEM) to directly observe a dynamic chemical reaction process in liquid-phase environment is a big challenge because it is difficult to keep liquid reactants under vacuum.In this work,we reported a liquid-based cell that enables in-situ observation of a solution reaction process under TEM.The novel liquid cell design not only realizes the self-alignment of top/bottom windows,but also achieves an adjustable liquid layer thickness down to nanometer scale.The cell consists of top and bottom frames,both of which are fabricated from silicon wafers using conventional micro-fabrication techniques.The transparent observation windows are made of silicon nitride (SiNx) membranes.In a typical assembling process,an ethanol solution containing silver nanowires and an ethanol solution containing saturated sulfur were sequentially dropped into the liquid tank of the bottom-frame of a liquid cell by using 1 mL syringe.Then,the liquid tank was covered by the top-frame with a window direction of 90 degrees,and an epoxy was used to seal the edge between the top-frame and bottom-frame.Using this assembled liquid cell,we performed in-situ TEM observation of the chemical reaction between Ag nanowires and sulfur in ethanol solution.Additional ex-situ X-ray diffraction (XRD) and Raman spectroscopy were also performed to study the reaction intermediates and final products.Instead of a simple reaction process in which sulfur diffuses into Ag to form the final product of Ag₂S,we found that the real reaction process involves the formation of a soluble intermediate phase (Ag₂S₄),which led to a partial dissolution of Ag nanowires in ethanol solution during reaction.These results well demonstrate that the in-situ TEM technique is a powerful tool to reveal the "real" chemical reaction mechanism.The experimental technique developed here could also be used to study a broad range of dynamic phenomena in liquid environment.

Key words: in-situ transmission electron microscopy (TEM), liquid-based cell, Ag-S reaction process, silver polysulfide, Raman spectrum