2, Jozef Stefan Institute, Ljubljana, , Slovenia
Recently, materials science benefits largely from in-situ transmission electron microscopy (TEM) using specialized holders, which separate small reactor volumes of (the sample and surrounding) liquids or gases from the high vacuum of the microscopy column. With these novel approaches, the mechanism of dynamical process can be revealed, reaction kinetics can be quantified, and even electrochemical processes can be studied in-situ in the microscope. Here, we report on the electrochemical deposition of iron on amorphous carbon electrodes in a liquid cell. The experiments were performed on a JEM-2100 (JEOL Ltd., Tokyo, Japan) microscope operated at 200 kV. The microscope is equipped with an energy dispersive X-ray spectrometer (EDXS). A Poseidon 510 TEM (Protochips Inc., Raleigh, NC, USA) holder was used together with a Gamry Reference 600 potentiostat (Gamry Instruments, Warminster, PA, USA) for conducting the electrochemical experiments (chronoamperometry). The liquid cell is comprised of two chips with 50 nm thin amorphous SiNx windows, where the smaller one has a spacer height of 50 nm thereby controlling the total thickness of the liquid layer between the two windows. The upper, larger chip contains the amorphous carbon working electrode, the Pt reference and the counter electrodes. An iron sulphate containing solution with pH = 2 was used as electrolyte. Upon applying a potential of -1.2 V vs. Pt, the electrodeposition of an amorphous layer of iron is stimulated, which subsequently crystallizes to Fe and FeOx, respectively. The nucleation of the crystal growth occurs instantaneously upon reaching a critical potential. The resulting deposition is discontinuous. In order to minimize artifacts such as the beam-induced radiolysis of water or secondary radical chemistry, the electron beam was blocked during the electrodeposition. The growth mechanism and the composition of the amorphous and crystalline deposits as derived from supporting EELS measurements will be discussed.