In Situ Optical Microscopy Studies of Spontaneous Oscillatory Growth of Zinc Dendrites
Here, we present results from in situ optical microscopy studies of Zn dendrite growth during electrodeposition of Zn in zinc sulfate (ZnSO4) solutions. All of our experiments are carried out using an electrochemical Hele-Shaw cell. The cell consists of a circular (10 cm in diameter) Zn wire (99.994% purity) anode sandwiched between two Plexiglas plates, separated by a polydimethylsiloxane (PDMS) spacer and filled with 100-μm-thick solution of ZnSO4. A vertical Zn electrode (0.25 mm in diameter) placed at the center of the cell serves as the cathode. In order to prevent bubble formation, the electrolyte is introduced slowly into the cell. In our experiments, we observe an interesting phenomenon: oscillatory growth of the dendrites, during electrodeposition in 0.1 M ZnSO4 with pH = 3.0. Upon applying a constant potential across the electrodes, the dendrites start to grow with a self-similar structure (see Figure 1). From the images of the dendrites obtained as a function of time, we find that the dendrite growth rate changes periodically. Concomitant with this oscillatory growth, we observe a periodic variation in deposition current.
Scanning electron microscopy (SEM) images (see Figure 2(a)) of the dendrites reveal that they have self-similar morphology. Tilted (Figure 2(b)) and cross-sectional images (not included here) also show that the dendrites are made of micro-crystallites sticking out of a main arm, and that the dendrite thickness varies periodically along the length of the dendrites. Transmission electron diffraction patterns acquired from the sample show that the growth structures are single-crystalline and with the same crystal orientation. Based upon these initial results, we propose a growth model to explain the oscillatory growth.
We gratefully acknowledge funding from the National Science Foundation (NSF-GOALI: DMR-1310639).
Figure 1. In situ optical micrograph acquired during electrodeposition of Zn in 0.1 M ZnSO4 solution with pH = 3 at Ewe = -2.5 V for 20 minutes. The scale bar is 100 µm in length.
Figure 2. (a) Top and (b) 58°-tilt view scanning electron microscopy images of the dendrites extracted from the solution. The layered morphology is characteristic of oscillatory growth. The scale bars are 40 µm in length.