用原位电镜技术研究银纳米线与硫溶液的液相反应机理

In-situ TEM Study of the Liquid-Phase Reaction of Ag Nanowires with a Sulfur Solution

为了研究纳米材料在液态环境下的反应机理,开发了一组新型的液体透射电镜(TEM)芯片.该组液体基芯片不仅可以实现上下窗口自对准的功能,而且可以实现在纳米级别控制液体层的厚度.利用该组液体芯片我们成功地观测了液态环境下Ag纳米线与S溶液的化学反应过程.实验结果表明,该反应过程不是简单的一步化合反应形成最终产物硫化银(Ag_2S),而是在反应过程中先形成可溶解的中间产物多硫化银(Ag_2S_4).这个中间产物会导致一小部分反应物Ag纳米线的溶解,而大部分Ag纳米线转变成Ag_2S纳米颗粒的聚集物.我们相信这一液体TEM芯片技术可以被广泛应用到液相环境下各种化学反应过程的研究.

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（Si Nx） 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 m L 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 Ag_2S, we found that the real reaction process involves the formation of a soluble intermediate phase（Ag_2S_4）, 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.