Aqueous Proton Transfer Across Single Layer Graphene (1411.1034v4)

Abstract: Proton transfer across single layer graphene is associated with large computed energy barriers and is therefore thought to be unfavorable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here, we subject single layer graphene supported on fused silica to cycles of high and low pH and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid-base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.68 to 0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavorable energy barriers to helium and hydrogen transfer indicate the transfer process is selective for aqueous protons.

• The paper demonstrates that naturally occurring hydroxyl-terminated defects reduce the energy barrier for proton transfer from over 3.8 eV to approximately 0.68–0.75 eV.
• The paper employs SHG experiments and DFT simulations to validate the proton transfer process across single-layer graphene without external modifications.
• The paper highlights the potential of engineered graphene for proton-selective membranes in fuel cells and advanced electrochemical applications.

Aqueous Proton Transfer Across Single Layer Graphene

This paper presents a detailed investigation of the proton transfer process across single-layer graphene, emphasizing the potential implications for material science and nanotechnology. The paper challenges the prevalent understanding that graphene, typically impermeable to protons, can facilitate proton transfer without the need for nanoscale modifications or external potential biases.

Experimental and Computational Analysis

The experimental aspect involved placing a well-characterized single layer of graphene on a fused silica substrate. The researchers cycled the aqueous solution's pH above the graphene layer between acidic and basic at room temperature, monitoring proton transfer through Second Harmonic Generation (SHG). The methodological design effectively ruled out proton diffusion through macroscopic pinholes, implying that proton transfer occurs through naturally occurring atomic defects in the graphene.

Compellingly, computational simulations supported these experimental findings. Density Functional Theory (DFT) calculations demonstrated that the energy barriers for proton transfer through hydroxyl-terminated defects ranged from 0.68 to 0.75 eV. This is notably lower than previously thought necessary for such transfer across a pristine graphene layer, which presented prohibitive energy barriers exceeding 3.8 eV. The Grotthuss-type relay mechanism, effective in proton shuttling, was found to play a crucial role in this transfer across hydroxyl-terminated defects, whereas defects terminated by pyrylium-like groups hindered the process.

Key Findings

1. Proton Selectivity: The research establishes that the transfer mechanism selectively favors aqueous protons, as considerable energy barriers are observed for helium and hydrogen molecules, indicating that graphene's defects are specialized for proton conduction.
2. Atomic Defects in Graphene: The presence of rare atomic defect sites, primarily hydroxyl-terminated, facilitates proton transfer. These defects form intrinsic pathways across the graphene layer, challenging the view of graphene's complete impermeability to protons.
3. Proton Diffusion Dynamics: Computational insights and SHG data collectively reveal that proton diffusion dynamics are significantly slower in the presence of surface anionic species due to proton trapping, emphasizing the complexity of surface interactions at the atomic scale.

Implications and Future Directions

This paper opens avenues for the deployment of single-layer graphene as a selective proton transport medium, potentially serving in applications such as fuel cell technologies, where proton exchange membranes are crucial. The identification of low-energy pathways for proton transit through atomic defects lays the groundwork for the engineering of graphene for desired permeability properties.

Furthermore, this research prompts further exploration of graphene's potential in various electrochemical applications, including energy storage and molecular sieving. The confluence of experimental and theoretical methods highlights the importance of interdisciplinary approaches in advancing our understanding of material behaviors at the nanoscale.

In conclusion, the findings presented in this paper redefine the boundaries of graphene's applications in proton transport, elucidating the critical role of atomic-scale defects in facilitating this process. Future studies might explore controlled introduction or modification of these defects to optimize graphene for specific applications, thereby expanding its utility in innovative technological domains.