Vibrational spectroscopy at electrolyte/electrode interfaces with graphene gratings

Atomic dynamics of electrified solid-liquid interfaces in liquid-cell TEM

Electrified solid-liquid interfaces (ESLIs) play a key role in various electrochemical processes relevant to energy1-5, biology6 and geochemistry7. The electron and mass transport at the electrified interfaces may result in structural modifications that markedly influence the reaction pathways. For example, electrocatalyst surface restructuring during reactions can substantially affect the catalysis mechanisms and reaction products1-3. Despite its importance, direct probing the atomic dynamics of solid-liquid interfaces under electric biasing is challenging owing to the nature of being buried in liquid electrolytes and the limited spatial resolution of current techniques for in situ imaging through liquids. Here, with our development of advanced polymer electrochemical liquid cells for transmission electron microscopy (TEM), we are able to directly monitor the atomic dynamics of ESLIs during copper (Cu)-catalysed CO2 electroreduction reactions (CO2ERs). Our observation reveals a fluctuating liquid-like amorphous interphase. It undergoes reversible crystalline-amorphous structural transformations and flows along the electrified Cu surface, thus mediating the crystalline Cu surface restructuring and mass loss through the interphase layer. The combination of real-time observation and theoretical calculations unveils an amorphization-mediated restructuring mechanism resulting from charge-activated surface reactions with the electrolyte. Our results open many opportunities to explore the atomic dynamics and its impact in broad systems involving ESLIs by taking advantage of the in situ imaging capability.

Structure evolution at the gate-tunable suspended graphene-water interface

Graphitic electrode is commonly used in electrochemical reactions owing to its excellent in-plane conductivity, structural robustness and cost efficiency1,2. It serves as prime electrocatalyst support as well as a layered intercalation matrix2,3, with wide applications in energy conversion and storage1,4. Being the two-dimensional building block of graphite, graphene shares similar chemical properties with graphite1,2, and its unique physical and chemical properties offer more varieties and tunability for developing state-of-the-art graphitic devices5-7. Hence it serves as an ideal platform to investigate the microscopic structure and reaction kinetics at the graphitic-electrode interfaces. Unfortunately, graphene is susceptible to various extrinsic factors, such as substrate effect8-10, causing much confusion and controversy7,8,10,11. Hereby we have obtained centimetre-sized substrate-free monolayer graphene suspended on aqueous electrolyte surface with gate tunability. Using sum-frequency spectroscopy, here we show the structural evolution versus the gate voltage at the graphene-water interface. The hydrogen-bond network of water in the Stern layer is barely changed within the water-electrolysis window but undergoes notable change when switching on the electrochemical reactions. The dangling O-H bond protruding at the graphene-water interface disappears at the onset of the hydrogen evolution reaction, signifying a marked structural change on the topmost layer owing to excess intermediate species next to the electrode. The large-size suspended pristine graphene offers a new platform to unravel the microscopic processes at the graphitic-electrode interfaces.

Ultrasensitive Mid-Infrared Biosensing in Aqueous Solutions with Graphene Plasmons

Identifying nanoscale biomolecules in aqueous solutions by Fourier transform infrared spectroscopy (FTIR) provides an in situ and noninvasive method for exploring the structure, reactions, and transport of biologically active molecules. However, this remains a challenge due to the strong and broad IR absorption of water which overwhelms the respective vibrational fingerprints of the biomolecules. In this work, a tunable IR transparent microfluidic system with graphene plasmons is exploited to identify ≈2 nm-thick proteins in physiological conditions. The acquired in situ tunability makes it possible to eliminate the IR absorption of water outside the graphene plasmonic hotspots by background subtraction. Most importantly, the ultrahigh confinement of graphene plasmons (confined to ≈15 nm) permits the implementation of nanoscale sensitivity. Then, the deuterium effects on monolayer proteins are characterized within an aqueous solution. The tunable graphene-plasmon-enhanced FTIR technology provides a novel platform for studying biological processes in an aqueous solution at the nanoscale.

Non-Local Electrostatic Gating Effect in Graphene Revealed by Infrared Nano-Imaging

Electrostatic gating lies in the heart of field effect transistor (FET) devices and modern integrated circuits. To achieve efficient gate tunability, the gate electrode has to be placed very close to the conduction channel, typically a few nanometers. Remote control of a FET device through a gate electrode located far away is highly desirable, because it not only reduces the complexity of device fabrication, but also enables the design of novel devices with new functionalities. Here, a non-local electrostatic gating effect in graphene devices using scanning near-field optical microscopy (SNOM)-a technique that can probe local charge density in graphene-is reported. Remarkably, the charge density of the graphene region tens of micrometers away from a local gate can be efficiently tuned. The observed non-local gating effect is initially driven by an in-plane electric field induced by the quantum capacitance of graphene, and further largely enhanced by adsorbed polarized water molecules. This study reveals a non-local phenomenon of Dirac electrons, provides a deep understanding of in-plane screening from Dirac electrons, and paves the way for designing novel electronic devices with remote gate control.

Water dynamics at electrified graphene interfaces: a jump model perspective

The reorientation dynamics of water at electrified graphene interfaces was recently shown [J. Phys. Chem. Lett., 2020, 11, 624-631] to exhibit a surprising and strongly asymmetric behavior: positive electrode potentials slow down interfacial water reorientation, while for increasingly negative potentials water dynamics first accelerates before reaching an extremum and then being retarded for larger potentials. Here we use classical molecular dynamics simulations to determine the molecular mechanisms governing water dynamics at electrified interfaces. We show that changes in water reorientation dynamics with electrode potential arise from the electrified interfaces' impacts on water hydrogen-bond jump exchanges, and can be quantitatively described by the extended jump model. Finally, our simulations indicate that no significant dynamical heterogeneity occurs within the water interfacial layer next to the weakly interacting graphene electrode.

Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces

The properties of water at an electrified graphene electrode are studied via classical molecular dynamics simulations with a constant potential approach. We show that the value of the applied electrode potential has dramatic effects on the structure and dynamics of interfacial water molecules. While a positive potential slows down the reorientational and translational dynamics of water, an increasing negative potential first accelerates the interfacial water dynamics before a deceleration at very large magnitude potential values. Further, our spectroscopic calculations indicate that the water rearrangements induced by electrified interfaces can be probed experimentally. In particular, the calculated water vibrational sum-frequency generation (SFG) spectra show that SFG specifically reports on the first two water layers at 0 V but that at larger magnitude applied potentials the resulting static field induces long-range contributions to the spectrum. Electrified graphene interfaces provide promising paradigm systems for comprehending both short- and long-range neighboring aqueous system impacts.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Microwave-Assisted Synthesis of Highly-Crumpled, Few-Layered Graphene and Nitrogen-Doped Graphene for Use as High-Performance Electrodes in Capacitive Deionization

Capacitive deionization (CDI) is a promising procedure for removing various charged ionic species from brackish water. The performance of graphene-based material in capacitive deionization is lower than the expectation of the industry, so highly-crumpled, few-layered graphene (HCG) and highly-crumpled nitrogen-doped graphene (HCNDG) with high surface area have been introduced as promising candidates for CDI electrodes. Thus, HCG and HCNDG were prepared by exfoliation of graphite in the presence of liquid-phase, microwave-assisted methods. An industrially-scalable, cost-effective, and simple approach was employed to synthesize HCG and HCNDG, resulting in few-layered graphene and nitrogen-doped graphene with large specific surface area. Then, HCG and HCNDG were utilized for manufacturing a new class of carbon nanostructure-based electrodes for use in large-scale CDI equipment. The electrosorption results indicated that both the HCG and HCNDG have fairly large specific surface areas, indicating their huge potential for capacitive deionization applications.