Visualizing Redox Dynamics of a Single Ag/ AgCl Heterogeneous Nanocatalyst at Atomic Resolution Yimin A. Wu, Liang Li, Zheng Li, Alper Kinaci, Maria K. Y. Chan, Yugang Sun, † Jeffrey R. Guest, Ian McNulty, Tijana Rajh, and Yuzi Liu* Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States * S Supporting Information ABSTRACT: Operando characterization of gas-solid re- actions at the atomic scale is of great importance for determining the mechanism of catalysis. This is especially true in the study of heterostructures because of structural correlation between the different parts. However, such experiments are challenging and have rarely been accom- plished. In this work, atomic scale redox dynamics of Ag/ AgCl heterostructures have been studied using in situ environmental transmission electron microscopy (ETEM) in combination with density function theory (DFT) calculations. The reduction of Ag/AgCl to Ag is likely a result of the formation of Cl vacancies while Ag + ions accept electrons. The oxidation process of Ag/AgCl has been observed: rather than direct replacement of Cl by O, the Ag/AgCl nanocatalyst was first reduced to Ag, and then Ag was oxidized to different phases of silver oxide under different O 2 partial pressures. Ag 2 O formed at low O 2 partial pressure, whereas AgO formed at atmospheric pressure. By combining in situ ETEM observation and DFT calculations, this structural evolution is characterized in a distinct nanoscale environment. KEYWORDS: environmental TEM, in situ TEM, Ag/AgCl heterogeneous nanocatalyst, density functional theory A tomic scale details in heterogeneous gas-solid catalyst are important for understanding the reactions and activity. 1,2 In situ monitoring and controlling the gas- solid catalyst reactions at the nanoscale adds indispensable insight for the development of catalysts for a wide range of energy and environmental technologies. 3 Although many in situ and operando X-ray spectroscopy techniques are available, 4-8 it remains a great challenge to obtain atomic scale information on structural transformations and reactivity of nanocatalysts under actual reaction conditions. Measurements using X-ray spec- troscopy cannot provide direct information on a single nanocatalyst due to the limited spatial resolution of X-ray techniques. 9 In situ environmental transmission electron microscopy (ETEM) has the capability to monitor the gas- solid interaction on a single nanoparticle due to its high spatial and temporal resolution. 10-12 Extensive efforts have been devoted to developing in situ ETEM techniques by modifying the transmission electron microscope (TEM) to accommodate an environmental cell using the di fferential pumping strategy; 13-17 however, the column pressure was limited to millibars, 13-18 which is well below the atmospheric pressure often used in general catalysis. The recent development of nanoreactors using nanofabrication techniques allows for the realization of in situ ETEM operating at atmospheric pressure. 9,19,20 Hence, in situ visualization of gas-solid reactions on a single nanocatalyst at atmospheric pressure is achievable. Here, we report a direct visualization of redox process of a single Ag/AgCl heterostructure at the nanoscale using an in situ TEM equipped with a nanoreactor under atmospheric pressure. By combining in situ ETEM and DFT calculations, we were able to capture the local phase transition. This work focused on Ag/AgCl heterogeneous plasmonic photocatalysts, which has attracted substantial interest owing to two distinct characteristics. One is the Schottky junction between the metallic Ag nanoparticle and semiconductor AgCl interface. 21 This Schottky junction leads to an intensive electric field inside the space charge region, which can enhance the separation of the photogenerated charge carriers within the photocatalyst. 21 The other one is the localized surface plasmon resonance (LSPR) on the Ag nanoparticle, which enhances visible light harvesting due to the dipole formation and collective oscillation of surface plasmons under illumina- Received: January 15, 2016 Accepted: March 2, 2016 Article www.acsnano.org © XXXX American Chemical Society A DOI: 10.1021/acsnano.6b00355 ACS Nano XXXX, XXX, XXX-XXX