In Situ Electrochemical Transmission Electron Microscopy of Transient Electrodeposition and Coarsening on Graphene

Field: Materials Science (Physical Sciences)

For heterostructures composed of metal nanoparticles and 2D materials, improving the precision of electrochemical fabrication requires a deep understanding of the interplay of kinetic and thermodynamic phenomena. Here, we explore the deposition of copper (Cu) metal on a graphene electrode using liquid cell transmission electron microscopy (TEM), using the temporal and spatial resolution of this technique to explore the nucleation and growth of nanocrystals. The technique reveals an unexpected transient phenomenon under certain applied voltage conditions, and we discuss possible mechanisms. We created working electrodes from few-layer graphene by transferring the crystals onto liquid cell chips patterned with platinum (Pt) electrodes. Pt was used as counter and quasi-reference electrodes. We then performed electrochemical deposition of Cu on graphene by controlling the deposition potential in an acidified copper sulfate solution. In situ movies reveal that during application of the potential, Cu nanocrystals nucleate and grow following the expected kinetics of progressive nucleation and diffusion-limited growth: larger numbers of smaller nanocrystals grow at more negative overpotentials. When the applied potential is low (-40 mV, -60 mV, -80 mV, and -100 mV with respect to Pt reference electrode ), particles immediately dissolve as the potential is turned off. However, we observe that when more negative potentials (-180 mV, -220 mV, -240 mV and -260 mV) are used for deposition, the growth of Cu nanocrystals continues after the voltage goes off. Typical results are shown in the Figure. These images were extracted from a movie recorded during and after pulse voltammetry at -260 mV for 10 s in a liquid cell filled with 0.1 M CuSO 4 + 0.1 M H 2 SO 4 . In the first row of the Figure, Cu islands nucleate and grow on the graphene electrode during the ‘V on’ period, while the later images show continued growth of Cu islands after the voltage is turned off at t=10 s. Eventually all islands disappear after tens of seconds. Tracking the size of the individual particles shows that smaller particles disappear earlier while the largest particles grow to large sizes. The decrease in the number of islands with time is particularly apparent at higher applied potentials, which is consistent with an Ostwald ripening process, and the size threshold that separates crystals that grow or shrink appears is also consistent with the model. The eventual dissolution of Cu nanocrystals in all experiments would not ordinarily be expected at zero applied potential. The comparison of cyclic voltammetry curves among different configurations of graphene electrodes suggests that there is an intrinsic potential between the graphene and Pt electrodes which is high enough to dissolve the deposited Cu even when the applied potential is 0 mV. We consider several mechanisms that may be responsible for the unexpected transient deposition. One possible explanation for the additional growth could be that Cu is deposited during the potential pulse but does not form into nanocrystals until later. This could arise if Cu is intercalated in the graphene during the applied potential. However, the measured charge that flows during the potential pulse is not strongly dependent on voltage, in particular not varying enough to account for the large deposited volume at high potentials. A second possible explanation may arise from the capacitance of graphene. Graphene has high capacitance based on electrochemical double-layer capacitance (EDLC) measurements, and it has been shown that the electrochemical interfacial capacitance increases for larger (negative) applied potential, which is consistent with our observation of the islands grown at larger potential last longer. A final mechanism for the effect could be that the application of low cathodic potential may cause the reduction of various oxygen functional groups forming species such as -OH. These charged species could sustain additional growth by reducing Cu ions from the solution. We will discuss this possible role of graphene to drive morphological changes during electrochemical processes, and consider wider opportunities that may arise for controlling electrodeposition to tailor thin film and nanoscale structures. Figure 1