Surface Structural Evolution of Pd Nanosheets during Aging and Their Effect on (Electro-)Catalytic Activities

Advances in probing the correlations between surface atomic structures of noble metal nanocrystals and their catalytic activities are vital for the development of effective catalysts. In general, the nanocrystals with well-preserved morphology over time is believed to present stable and consistent physicochemical properties. However, little attention has been paid to their surface structural evolution during aging, which might significantly alter their catalytic activities. In this work, hexagonal Pd nanosheets with an average thickness of ca. 0.93 nm (around four atomic layers) were synthesized and studied before and after aging as stable colloidal suspension at room temperature. Upon exposure to the electron beam (e-beam) of high-magnification transmission electron microscopy (TEM, > 150k, 200 kV), the fresh Pd nanosheets with (111) facet dominated on the surface immediately developed into poriferous and quasi-polycrystalline nanosheets. However, the Pd nanosheets became much more stable under e-beam after aging in water/ethanol at room temperature for two months. In addition, diverse defects were observed on the surface of the aged Pd nanosheets, including dislocations, lattice distortions, twin boundaries and vacancies, indicating the atomic reconstruction during aging. The driving force of structural transformation was found to originate from the gradual removal of surfactants and thus induced localized strain during aging, as evidenced by the greatly improved electrochemical active surface areas (ECSA) after aging. Electrochemical evaluation further demonstrated that the fresh Pd nanosheets possessed high activities for ethanol oxidation, formic acid oxidation and the reduction of 4-nitrophonel. The results suggest that the surface structural evolution during aging, which tends to be ignored, has a critical effect on catalytic properties. Our studies might provide insight on the design and processing of advanced nano-catalysts.