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# Asymmetric response of interfacial water to applied electric fields

## Abstract

Our understanding of the dielectric response of interfacial water, which underlies the solvation properties and reaction rates at aqueous interfaces, relies on the linear response approximation: an external electric field induces a linearly proportional polarization. This implies antisymmetry with respect to the sign of the field. Atomistic simulations have suggested, however, that the polarization of interfacial water may deviate considerably from the linear response. Here we present an experimental study addressing this issue. We measured vibrational sum-frequency generation spectra of heavy water (D2O) near a monolayer graphene electrode, to study its response to an external electric field under controlled electrochemical conditions. The spectra of the OD stretch show a pronounced asymmetry for positive versus negative electrode charge. At negative charge below 5 × 1012 electrons per square centimetre, a peak of the non-hydrogen-bonded OD groups pointing towards the graphene surface is observed at a frequency of 2,700 per centimetre. At neutral or positive electrode potentials, this ‘free-OD’ peak disappears abruptly, and the spectra display broad peaks of hydrogen-bonded OD species (at 2,300–2,650 per centimetre). Miller’s rule1 connects the vibrational sum-frequency generation response to the dielectric constant. The observed deviation from the linear response for electric fields of about ±3 × 108 volts per metre calls into question the validity of treating interfacial water as a simple dielectric medium.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

This research was supported by Air Force Office of Scientific Research grant nos FA9550-15-1-0184 and FA9550-19-1-0115 (A.M., C.D., M.M., S.B.C., A.V.B.), Army Research Office award no. W911NF-17-1-0325 (B.Z.), US Department of Energy, Office of Science, Office of Basic Energy Sciences award DE-SC0019322 (B.H.), and National Science Foundation award no. CHE-1708581 (H.S.).

## Author information

Authors

### Contributions

H.S., B.H., B.Z. and S.B.C. manufactured graphene electrodes. A.M., H.S., B.H. and S.B.C. performed electrochemical measurements and Raman spectroscopy. A.M., C.D., M.M., D.B. and A.V.B. performed VSFG measurements. A.M., C.D., M.M. and A.V.B. contributed VSFG spectral analysis.

### Corresponding author

Correspondence to Alexander V. Benderskii.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Franz Geiger, Dusan Bratko, Poul Petersen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Extended data figures and tables

### Extended Data Fig. 1 Raman spectrum of the graphene electrode.

For a high-quality, defect-free graphene monolayer, the ratio of intensities of the 2D to G band is about 2. A.U., arbitrary units.

### Extended Data Fig. 2 Electrochemical current with D2O in the cell versus applied voltage to the graphene electrode versus Ag/AgCl.

The fact that the current magnitude does not exceed 100 μA for the voltages that were applied implies that water splitting does not occur to an appreciable degree.

### Extended Data Fig. 3 SFG spectra of the graphene–D2O interface at 0.3 V (versus Ag/AgCl).

Spectra at 0.3 V were taken several times throughout the experiment to verify that there was no drift in the SFG signal with time.

### Extended Data Fig. 4 SFG spectra of graphene–D2O following dilution with H2O.

Isotopic exchange weakens the peak at ~2,700 cm−1, confirming that D2O is responsible for this signal. A.U., arbitrary units.

### Extended Data Fig. 5 Linear dependence of G-band Raman shift on applied voltage in both the two-terminal and three-terminal configurations.

A two-terminal voltage (applied versus glassy carbon) can be converted to a voltage versus Ag/Cl in the three-terminal configuration by exploiting the linearity of the G-band shift with respect to the applied voltage.

### Extended Data Fig. 6 Electric-field-dependent SFG spectra of the graphene–D2O interface.

ac, Spectra are shown for the graphene–D2O interface (a), and its decomposition into the $${\chi }_{{\rm{s}}}^{(2)}$$ (surface; b) and $${\chi }_{{\rm{S}},{\rm{DL}}}^{(2)}\,$$ (bulk; c) contributions. With the bulk contribution known from the literature, the surface contribution is found through a spectral fitting procedure.

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Reprints and Permissions

Montenegro, A., Dutta, C., Mammetkuliev, M. et al. Asymmetric response of interfacial water to applied electric fields. Nature 594, 62–65 (2021). https://doi.org/10.1038/s41586-021-03504-4

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• DOI: https://doi.org/10.1038/s41586-021-03504-4

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