Surface-specific vibrational spectroscopy of interfacial water reveals large pH change near graphene electrode at low current densities

Molecular-level insight into interfacial water at buried electrode interfaces is essential in elucidating many phenomena of electrochemistry, but spectroscopic probing of the buried interfaces remains challenging. Here, using surface-specific vibrational spectroscopy, we probe and identify the interfacial water orientation and interfacial electric field at the calcium fluoride (CaF2)-supported electrified graphene/water interface under applied potentials. Our data shows that the water orientation changes drastically at negative potentials (<-0.03 V vs. Pd/H2), from O-H group pointing down towards bulk solution to pointing up away from the bulk solution, which arises from charging/discharging not of the graphene but of the CaF2 substrate. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the applied potentials change the local pH (more than five pH units) near the graphene electrode even at a current density below 1 microamp per square centimeter. Our work provides molecular-level insights into the dissociation and reorganization of interfacial water on an electrode/electrolyte interface.


Introduction
Water at a potentiostatically controlled electrode surface is relevant for a wide range of scientific and technological systems. [1][2][3][4][5] Homogeneous dielectric continuum models assumed in traditional mean-field theories often fail at the electrode/electrolyte solution interface 1,6-8 in which arrangements of interfacial water molecules are heterogeneous, 9 the water response to the electric field is supposed to be asymmetric, 10,11 and dissociation of water molecules, also not contained in continuum models, plays an important role. 2,12 For instance, the reorganization of interfacial water molecules and their spatial arrangement are closely linked with capacitive charge storage, 3,13 and with the electron transfer mechanism across electrode/water interfaces, such as in electrocatalytic water splitting. 2,12,14 Furthermore, changes in the local pH near the electrode with the applied potential affect the chemical reactions [15][16][17] because the changing local pH can significantly perturb the interfacial electric field, 2,18 and, in turn, the arrangement of interfacial water. As such, elucidating these local molecular events is essential for understanding the electrochemistry at electrode/water interfaces.
To probe the interfacial water structure, heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy is ideal, owing to three advantages: interface specificity, molecular specificity, and orientation specificity. 19,20 The SFG signal can only originate from the interface owing to its selection rule that centro-symmetry must be broken for non-zero SFG signals. As such, it naturally excludes contributions from the bulk, allowing us to probe the interfacial response selectively. 19,21 An SFG signal is enhanced when the infrared (IR) frequency is resonant with the molecular vibration, providing molecular specificity. Furthermore, HD-SFG signals provide (i) the absolute orientation of molecules (up-/down-orientation), 22,23 and (ii) the surface charge, and thus the interfacial electric field. 24,25 However, applying HD-SFG spectroscopy to buried electrode/water interface is challenging because IR often cannot reach the buried interfaces. [26][27][28] This is enabled by using graphene as an electrode; graphene, an atom-thickness metal-like material, can serve as the electrode while allowing the IR light to reach the electrode surface. 11,[29][30][31][32] As such, combining SFG with a graphene electrode allows us to explore the molecular-level insight into the interfacial conformations under electrified conditions.
Here, we perform HD-SFG measurements at the CaF2-supported electrified graphene/water interface. We observe that the Im#$ !"" ($) % spectra of the O-H stretching mode of water drastically change in between -0.63 V and -0.03 V versus the Pd/H2 electrode. We find that it is not due to the variation of charges on the graphene, but due to the drastic surface charge variation of the CaF2 substrate, likely accommodated by water trapped in between the CaF2 and the graphene. From HD-SFG measurements at various pH conditions, we identify that the surface charge of the CaF2 substrate varies substantially even at 1 μA/cm 2 current density due to the HER-induced local pH change at the CaF2/graphene interface. This work provides molecular-level insights into the dissociation and reorganization of interfacial water on a potentiostatically controlled electrode surface.

Experimental
Sample preparation. The schematic diagram of the experimental setup for the in-situ HD-SFG measurement under controlled electrochemical conditions is shown in Fig. 1a

Results and discussion
To understand the structure of the interfacial water on the electrified graphene surface, we measured Im#$ !"" ($) % spectra in the O-H stretching mode frequency region (2900-3700 cm -1  This negative band contribution is insensitive to the variation of the applied potential in the range of +0.57 V to -0.03 V. When we decreased the potential from -0.03 V to -0. V, the Im#$ !"" ($) % spectra were found to be again insensitive to the applied potentials. (c) The differential Im #Δ$ !"" ($) % spectra obtained from taking the difference between $ !"" ($) recorded at 1 mM and 100 mM NaClO4 aqueous solutions. (d) Potential-dependent charge density of the graphene (from Raman spectra), the total surface charge at the CaF2-supported graphene/water interface (from Im(Δ$ !"" ($) )) and where $ (() represents the third-order nonlinear susceptibility originating from bulk water, . ) is the electrostatic potential, / the inverse of Debye screening length, c electrolyte concentration, and Δ3 * the phase-mismatch of the SF, visible, and IR beams in the depth direction. To obtain ' &!' , we measured the $ !"" ($) spectra at c = 1 mM and 100 mM, and then computed Δ$ !"" , * = 100 mM). As such, one can omit the $ ($) -term and then connect Δ$ !"" ($) spectra with the total surface charge density,

Supporting Information Section 6). 24
The Im#Δ$ !"" ($) % spectra are shown in Fig The Raman G band data is shown in Fig. 1e, while the inferred ' + is shown in Fig. 1d. The ' + varies nearly linearly against the potentials, consistent with references, 41,43 but in contrast with This result indicates that the drastic variation of the charge density of the CaF2-supported graphene/water interface is caused by the change in the surface nature of CaF2 substrate itself.
Taking the ' + together with the ' &!' , we deduced the ' ,-. ! using Eq. (2). Fig. 1d shows that the The proton generated at the CaF2/graphene interface can readily penetrate through the singlelayer graphene to reach the bulk solution. [48][49][50] Remarkably, the potential of zero charge on the CaF2 substrate is observed slightly above -0.33 V (Fig. 1d), implying that at this very moderate potential, already substantial hydroxide formation occurs, sufficient to increase the pH from 5-6 to 9-10. The pH change occurs at a remarkably low current density below 1 μA/cm 2 (Fig. 2a). To examine this hypothesis, we measured the Im#$ !"" ($) % spectra under various pH conditions at the CaF2-supported graphene/water interface. The data is displayed in Fig. 2b. At bulk pH lower than 9.5, the spectra exhibit a negative H-bonded O-H band. When elevating the bulk pH above 9.5, the 2950-3550 cm -1 band changes the sign from negative to positive, and a sharp negative peak at around 3630 cm -1 appears, consistent with an isoelectric point between pH 9 and 10.
The obtained correspondence of the applied potential and local pH is shown in Fig. 2a, which is correlated according to the integrated peak intensity of the H-bonded O-H peak and the Ca-O-H peak (see the details in the Supporting Information Section 9). The inferred pH value saturates at increasingly low potential. We expect the pH to increase continuously with increasing negative potential, but below -0.63 V, the SFG signal saturates. We tentatively assign the observed saturation to the limited number of water molecules confined between the graphene and CaF2 as a reactant in chemical reaction (I), limiting that reaction. In any case, it is evident that applying a negative potential to the graphene induces the HER, elevating the local pH. The increase in local pH promotes the deprotonation of the water molecules confined in the CaF2/graphene interface.
This molecular picture is schematically depicted in Fig. 3a.
This mechanism can explain the sudden appearance of the Ca Finally, this process is reversible and reproducible (Fig. 3c). At positive potentials, proton transfer across the monolayer graphene causes fluoride ion dissociation, leaving a positively charged CaF2 surface. Applying a negative potential drives HER to occur. Protons at the interface are rapidly consumed, and the fluoride ion dissociation is suppressed, leaving OHions at the CaF2 surface, thereby discharging the CaF2 surface. More importantly, by reversing the applied potential from negative to positive, the charging of the CaF2 surface can again be triggered, although in a slower manner.

Conclusion
We employed surface-specific spectroscopy to study water at the CaF2-supported electrified graphene/water interface, and gain molecular-level insight into the chemical reaction and interfacial water molecules' arrangement near the electrified graphene. We found that the water at the graphene electrode surface. Finally, we note that this phenomenon is reversible and reproducible, and is general at the supported-graphene/water interface.

Supporting Information
Surface-specific vibrational spectroscopy of interfacial water reveals large pH change near graphene electrode at low current densities were immersed in a piranha solution for 15 minutes before use. After that, two 100 nm-thick gold strips were thermally evaporated onto the CaF2 and SiO2 windows with a shadow mask. The gold strips enable us to measure the resistance across the graphene electrode (typical resistance of the graphene monolayer is 0.5-1.5 kΩ at a size of 6×12 mm 2 ) and to manipulate electrochemical potentials between the graphene and the reference electrode. Besides, the gold strips also serve as the reference sample to generate a stable and precise reference phase.
The CVD-grown monolayer graphene was transferred onto the cleaned CaF2 and SiO2 windows by using the polymer-assisted wet transfer technique. 1 In brief, the copper foil was spincoated with CAB (30 mg/mL dissolved in ethyl acetate) at 1,000 rpm for 10 seconds, followed by 4,000 rpm for 60 seconds and then baked at 180 °C for 3 minutes. After cooling down to room temperature, the copper foil/CAB was placed into HCl/H2O2/H2O mixture solution (volume ratio, 1:1:10) for 60 seconds to remove the graphene layer grown on the backside of the copper foil.
After being rinsed with deionized water, the copper foil/CAB was then etched away in 0.1 M ammonium persulfate aqueous solution. Subsequently, the obtained CAB-graphene films were rinsed in deionized water several times to remove residual chemical species, and then were transferred onto CaF2 and SiO2 windows. The samples were dried for more than 12 hours at 110 ℃ in a vacuum (~1 mbar) to remove residual water. Finally, the CAB layer on graphene was dissolved in acetone. To ensure good electrical contact, two gold wires were bound to the two gold strips on the CaF2 and SiO2 window with conductive silver paste (TED PELLA, INC.).

Supporting Information 2 Electrochemical cell
Our electrochemical flowing liquid cell is schematically depicted in Fig. 1a.

Supporting Information 3 Raman measurement
The Raman spectra of graphene samples under various electrochemical potentials were recorded with a WITec confocal Raman spectrometer (alpha 300 R, ×10 objective, 600 grooves/mm grating, 5 mW) with a 532 nm excitation and 10 seconds integration. The G-band peak frequency was obtained by fitting it with Lorentzian curves after the background subtraction.

Supporting Information 4 SFG measurement
HD-SFG measurements were performed on a non-collinear beam geometry with a Ti:Sapphire regenerative amplifier laser system (Spitfire Ace, Spectra-Physics, centered at 800 nm, ∼40 fs pulse duration, 5 mJ pulse energy, 1 kHz repetition rate). A part of the output was directed to a grating-cylindrical lens pulse shaper to produce a narrowband visible pulse (10 μJ pulse energy, FWHM = ∼10 cm −1 ), while the other part was used to generate a broadband infrared (IR) pulse For the comparison of the spectra in this work, we assume the reflectivity for the electrolyte solution/graphene interfaces is insensitive to the type of electrolyte and concentration below 100 mM.
Note that the absolute phase of the gold thin film is not uniquely determined. To obtain the phase of the gold sample, we measured the O-H stretching Im/" !"" ($) 0 spectrum of the CaF2supported graphene/D2O interface via the normalization with HD-SFG interferogram of CaF2/gold.
As D2O does not have any vibrational response in this region, and its " !"" ($) response arises solely from the interface 3 , we can determine the phase of gold based on the fact that the Im/" !"" ($) 0 spectrum of the CaF2-supported graphene/D2O interface shows a flat zero line (Fig. S2).

Supporting Information 5 Wettability of monolayer graphene on CaF2
The Im #" eff is mainly related to the difference of " (?) terms at different concentrations. By dividing ∆" !"" with " (?) , we further obtained We compared the experimentally obtained left side of Eq. S3 and computed right side, and subsequently obtained the 2 =!& . Here, to compute the left side, we used " (?) spectrum obtained from the reference 7 to improve signal-to-noise ratio. Note that the spectral shape between the experimentally obtained Im>" (?) ? spectrum in this work and that reported in Ref. 7 perfectly matched (Fig. S5). Thus, this operation does not affect the results of the surface charge determinations. The obtained 2 =!& at various potentials are displayed in Fig. 1d.   Figure S6 shows the results we got at the OCP condition, which gives an estimation of 21.5 mC/m 2 net surface charge density. Since the charge density in graphene is nearly zero at the OCP condition, the 21.5 mC/m 2 surface charge mainly comes from the CaF2 substrate.  Fig. 1d.

Supporting Information 8 Hydrogen evolution reaction
As discussed in the main text, the CV curve in Fig. 2a shows an electrochemical reduction peak starting below ~-0.23 V. In this potential region, two reduction reactions may occur, HER  Fig. S7a. The two CV curves are the same, which suggests that the electrochemical reduction peak is due to HER.
Further, we measured CV curves under various pH conditions. We use SiO2-supported graphene rather than CaF2-supported graphene as the working electrode to avoid the detaching of graphene from the CaF2 surface under acid conditions. The data is shown in Fig. S7b. Under acidic conditions (pH=2), the CV curve shows first a significant proton reduction peak at around -1.1V and then a water reduction peak at lower than -1.5 V. The proton reduction peak becomes much weak under neutral conditions due to the limited number of protons in the solution, and the peak nearly vanishes under alkaline conditions (pH=12). These results illustrate that applying negative potentials on graphene triggers the HER, 14 and the HER is responsible for the local pH change. NaClO4 solution before and after argon-saturated treatment. The graphene electrode is supported on CaF2 substrate. (b) CV curves of graphene in contact with 10 mM NaClO4 solution at various pH conditions. The graphene electrode is supported on an SiO2 substrate. We set the scan rate to 50 mV/s.

Supporting Information 9 Determination of local pH value at various applied potentials
As discussed in the main text, the lineshapes of the spectra of the O-H stretching mode at various pH conditions resemble those obtained upon applying potentials. As both the H-bonded O-H peak and Ca-O-H peak in the Im/" !"" ($) 0 spectra will change when the surface charge density on the CaF2 substrate is altered by pH, one can determine the local pH value by comparing the variations of the peak area of either the H-bonded O-H peak or the Ca-O-H peak by pH change and by potential change.
Here, to correlate them, first, we use the Henderson-Hasselbalch equation 15  where Area +V= = −2.0 and Area +(5 = 5.2 are the minimum and maximum values of the peak area, respectively. The fit yields a pKa of 9.6 ( Fig. S8a). From this, one can calculate the corresponding local pH value at each potential once the peak area of the H-bonded O-H peak upon applying potential is known.
For potentials at -0.33 V--1.23 V, the peak area of the of Ca-O-H peak at 3630 cm -1 also provides a measurement of local pH. In the same way, we use Eq. S4 to describe the relationship between the pH and the peak area of the Ca-O-H peak. A reasonable fitting yield Area +V= = 0.0, Area +(5 = −0.3 and pKa = 9.9 (Fig. S8b). one can also calculate the corresponding local pH value with the known peak area of the Ca-O-H peak.
The calculated local pH value at each applied potential using the above two methods gives the same results as shown in Fig. S9c.