Peng Chen, Cornell University
Graphene and the related graphene oxide (GO) are attractive catalytic materials for electro-conversion reactions. Understanding their activity-structure correlation in catalysis is important for helping guide their applications and tailor them for better performances. We have been using a fluorescence microscopy approach to study graphene and GO electrocatalysis at the single-sheet level and in a spatially resolved manner. We deposit individual graphene and GO sheets on an indium tin oxide substrate, apply an electrochemical potential to carry out electrocatalytic reactions and monitor a fluorescent product of a probing electrocatalytic reaction through an optical microscope. Below I summarize some of our recent progresses.
1) When we applied cyclic potentials on GOs, the fluorescent intensity (which reports the amount of catalytic product formed and thus the activity) of a single GO showed multi-stage, periodic changes, which can be correlated with conventional cyclic voltammetry curves. However, different regions on a single GO sheet respond differently to the applied cyclic potentials, indicating different activities of the associated sites on the GO sheet. By analyzing the fluorescence responses pixel by pixel in the images, we were able to reconstruct an activity map of individual GO sheets. This approach not only provided us a novel and inexpensive way of visualizing GOs, but also allowed us to spatially resolve the electrochemical reactivity on GOs. We found that the activity-based reconstructed image could well reveal the morphology of GOs on substrate. By detecting local activity jumps, fine structures were observed on reconstructed images, including single sheet, folded sheet regions, wrinkles, and possibly defect lines. The reconstructed image will be correlated with fluorescent quenching microscopy (FQM, developed by others) and AFM to confirm our findings on reactivity determined fine structures. These correlations will allow us to quantitatively understand the electronic properties of different regions on GOs.
2) GOs are rich in oxygen containing functional groups. Past studies have shown that the electrochemical reactivity is related with the abundance of oxygen containing functional groups on GOs. We deposited pristine graphene on Au-electrode and measured the cyclic voltammetry for the same probing reaction. The resulting reactivity of pristine graphene sheet was low, probably because of lacking defect sites and slow electron transfer rate along basal plane. However, after exposing the pristine graphene sheet to UV/ozone, the reactivity increased significantly. This observation implied that oxygen containing functional groups were the main contributor to the electrochemical reactivity. Naturally, the next question one would ask is how the oxygen containing functional groups are distributed on the GOs? Do they appear more on one region over the other region? To address these questions, we performed multiple-staged electrochemical treatments on GOs. Here fluorescence microscopy coupled cyclic voltammetry was performed first to generate the activity map of individual GO sheet. A negative potential was then applied to partially reduce the GOs. This electrochemical reduction was followed by another fluorescence microscopy coupled cyclic voltammetry, which enabled us to spatially resolve, determine, and compare the activity changes on GOs. We discovered that in order to significantly change the activity of GOs, at least -0.8V (vs. Ag/AgCl electrode) must be applied to GOs. At -0.7V or lower potential, the activity and its spatial distribution pattern did not change much. Locations initially shown higher activity tended to lose more activity during the electrochemical reduction.
3) We investigated the spatially-resolved activity changes upon the electrochemical reduction. We found that multiple populations were present in GOs’ boundary, wrinkle and sheet regions. These sub-regions, as their division lines connect with each other, merge together and form domains on GOs. This observation is consistent with the grains in pristine graphene sheets studied by Dark-field-TEM. In-situ Raman will be used in future to study the fundamental chemical nature of different domains on GOs.
4) We also examined the temporal behaviors of the electrocatalytic activity of GO sheets upon applying a negative potential. We observed that some localized sites within each domain dropped in activity before the activity of entire domain decreased. This observation indicated that the electro-reduction of GOs likely started from a few seeding (i.e., nucleation) sites. As the reduction potential was continuously applied to GOs, these seeding sites propagated and expanded until the whole domain eventually reached some homogenous activity spatially. We plan to apply a reducing potential on GOs for short periods of time and repeat this process for a few times to progressively reduce GOs. This will enable us to understand the kinetics of manipulating GOs' activity through electrochemical reduction.