Monitoring of Galvanic Replacement Reaction between Silver Nanowires and HAuCl₄ by In Situ Transmission X-ray Microscopy

G replacement reactions between solid metal nanoparticles with oxidizing metal precursors represent a versatile strategy for synthesizing hollow metal nanostructures with tailored properties that are difficult (or even impossible) to achieve from their solid counterparts. For example, reactions of silver nanocubes with aqueous solutions of HAuCl4 enable the synthesis of hollow nanoboxes with seamless walls and nanocages/nanoframes with porous walls that exhibit absorption bands tunable in the range of both visible and near-infrared (NIR) regions and have been used in biological imaging, medical therapy, drug delivery, catalysis, sensing, etc. Similar reactions can also convert silver nanowires into nanotubes made of other metals with unique properties. 12 Extensive studies have been done to investigate the mechanism involved in the nanoscale galvanic replacement reactions, but direct observation of morphological evolution of the nanoparticles has not been reported due to the lack of in situ imaging techniques that are compatible with solution-based reactions. Most recently, in situ transmission electron microscopy (TEM) combined with a special thin cell has been used to study the growth of Pt nanoparticles from a solution of Pt(acetylacetonate)2 in a mixture of o-dichlorobenzene and oleylamine through the reduction of Pt(II) with solvated free electrons generated from inelastic scattering of the incident electron beam. However, the in situ TEM cell is not suitable for studying the galvanic replacement reaction between silver nanowires and HAuCl4 solution because mixing them together in the cell immediately initiates the reaction before the cell can be loaded in the TEM chamber. In addition, the spacing between the two window membranes of the TEM cell cannot exceed 200 nm in order to allow the electron beams to pass through the loaded solution for imaging. The small reaction chamber makes it difficult to load silver nanowires with diameters larger than 100 nm into the cell. Herein, we report the use of in situ transmission X-ray microscopy (TXM) in combination with a flow cell for realtime monitoring the reaction between silver nanowires and HAuCl4. The use of a flow cell allows us to trigger the reaction by delivering a HAuCl4 solution into the cell when the TXM is ready for imaging. The large penetration length of hard X-rays in solution makes it possible to use reaction chambers as thick as hundreds of micrometers that allow easy manipulation of nanowires as well as potential in situ three-dimensional (3D) imaging with computed tomography (CT) technique. The flow cell TXM technique can overcome the limitations of in situ TEM for successfully observing the real-time morphological evolution of silver nanowires in the course of a galvanic replacement reaction with an aqueous solution of HAuCl4 at room temperature. A transmission X-ray microscope system works similarly to a conventional optical microscope. Figure 1 gives the schematic configuration of a typical transmission X-ray microscope that consists of a condenser lens, a beam stop, a pinhole, an objective lens (i.e., Fresnel zone plate), and a charge-coupled device (CCD) detector. The diffraction limited resolutions (R) is determined by 0.61λ/NA, where λ is the X-ray wavelength and NA is the numerical aperture of the objective lens. With current lens fabrication technology, the