In Situ Analysis of Growth Behaviors of CuO Nanocubes in Liquid Cell Transmission Electron Microscopy.

Metal oxides have attracted substantial attention over the years and are commonly used in the semiconductor industry because of their excellent physical and chemical properties. Among the various metal oxides, cuprous oxide (CuO) is regarded as a promising material. It is inexpensive, earth-abundant, and nontoxic; therefore, it can be used in catalysis, sensors, solar cells, and p-type semiconductors. However, the redox reaction of CuO is still uncertain. The size, morphology, and structure of CuO strongly influence its properties. In this work, we developed a new synthesis method of CuO that involves reducing the precursor by an electron beam without reducing agent. The growth process of CuO nanocubes was observed via in situ liquid cell transmission electron microscopy (in situ LCTEM). The nucleation kinetics, oscillating growth behavior, and redox reaction of the CuO nanocubes in the liquid phase were systematically studied. CuO exhibited a round shape at the beginning and transformed into a cubic shape afterward. Interestingly, the CuO nanocubes grew clearly under long-term observation; however, their diameters increased and fluctuated during the short-term observation. The electron beam not only stimulated the solution to reduce the nanocubes but also caused electron radiation effect to the nanocubes. During the CuO growth and dissolution, the cubic shape evolved with specific planes in the {100} family. Our direct observation sheds light on the preparation of CuO by a reduction method, extending the study of reaction kinetics and providing a new way to synthesize metal oxides.