Observation of materials processes in liquids by electron microscopy

Materials synthesis and the functioning of devices often involve liquid media. However, direct visualization of dynamic processes in liquids, especially with high spatial and temporal resolution, has been challenging. For solid materials, advances in aberration-corrected electron microscopy have made observations of atomic-level features a routine practice. Here, we discuss the extent to which one can take advantage of the resolution of modern electron microscopes to image phenomena occurring in liquids. We describe the fundamentals of two different experimental approaches that use closed and open liquid cells. We illustrate the capabilities of each approach by considering processes in batteries and nucleation and growth of nanoparticles from solution. Liquid-cell electron microscopy appears to be duly fulfilling its role and promise for in situ studies of nanoscale processes in liquids, revealing physical and chemical processes that are otherwise difficult to observe.