This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article Cite This: J. Phys. Chem. C 2019, 123, 24031−24038 pubs.acs.org/JPCC Surface-Specific Spectroscopy of Water at a Potentiostatically Controlled Supported Graphene Monolayer L. B. Dreier,†,‡ Z. Liu,†,# A. Narita,† M.-J. van Zadel,† K. Müllen,†,§ K.-J. Tielrooij,∥ E. H. G. Backus,†,⊥ and M. Bonn*,† † Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ‡ Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany § Institute of Physical Chemistry, Johannes Gutenberg-Universität Mainz, Duesbergweg 10−14, 55128 Mainz, Germany ∥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain ⊥ Department of Physical Chemistry, University of Vienna, Währinger Strasse 42, 1090 Vienna, Austria ABSTRACT: Knowledge of the structure of interfacial water molecules at electrified solid materials is the first step toward a better understanding of important processes at such surfaces, in, e.g., electrochemistry, atmospheric chemistry, and membrane biophysics. As graphene is an interesting material with multiple potential applications such as in transistors or sensors, we specifically investigate the graphene−water interface. We use sum-frequency generation spectroscopy to investigate the pH- and potential-dependence of the interfacial water structure in contact with a chemical vapor deposited (CVD) grown graphene surface. Our results show that the SFG signal from the interfacial water molecules at the graphene layer is dominated by the underlying substrate and that there are water molecules between the graphene and the (hydrophilic) supporting substrate. ■ INTRODUCTION Due to the importance of the graphene−water interactions in as the water molecules are aligned by the field arising from the charge at the interface.13 Since the surface charge changes various technological applications, it is of great interest to upon varying the pH or introducing electrolytes, the interfacial improve our understanding of the water structure at the water orientation is also altered by these effects.12,14 Both the graphene−water interface. For example, it has been shown that effect of pH and electrolyte concentration on the interfacial nanoporous graphene can be used for desalination of water1 water structure have already been thoroughly investigated.15,16 and in electricity generation devices.2 Furthermore, as the However, altering electrolyte concentration or pH modifies not electronic properties of graphene are changed by its interaction just the surface charge, but often also the chemistry of the with water, the resistance of graphene-based transistors and system. Graphene overcomes this limitation, as applying a sensors varies with a change in humidity.3 Despite substantial potential induces purely physical changes at the interface. previous research on graphene in contact with water during the Understanding changes in the structure of interfacial water last decades, many open questions remain. For instance, there as a function of surface potential at electrified interfaces is not is still no scientific consensus regarding the wettability of only fundamentally interesting but also highly technologically graphene, one of the most fundamental properties of the relevant for electrochemical, electrocatalytic, and biochemical graphene−water interface. The reported water contact angles applications.12 To study the water structure at the interface, for graphene range from 20° to 127°.4,5 These variations are sum-frequency generation (SFG) spectroscopy can be used. often attributed to chemical doping or substrate effects.6−8 SFG is a second-order nonlinear optical spectroscopy, which Given that materials with a water contact angle below (above) requires symmetry breaking to give a signal. As bulk water is 90° are defined as hydrophilic (hydrophobic),9 this implies inherently centrosymmetric, a vibrational signal from only the that it is debated whether graphene is hydrophilic or interfacial water molecules is obtained. At a charged surface, hydrophobic. the interfacial water molecules are aligned by the charge, Given its outstanding conductive nature, graphene further resulting in general in a higher order and thus an enhanced provides a unique opportunity. Graphene can be charged at SFG signal. To investigate the influence of an applied potential will and can thus be used to probe changes in the water on the interfacial water structure, the SFG signal of water in structure as a function of an applied external potential. contact with conductive materials has to be measured. Aqueous solutions are often in contact with charged, solid Recently, Campen and co-workers have reported the surfaces. Examples include water at charged mineral surfaces,10 in riverbeds, or in technologically relevant processes such as in Received: June 19, 2019 catalysis11 and electrochemistry.12 The surface charge of these Revised: September 5, 2019 solid surfaces greatly influences the interfacial water structure, Published: September 9, 2019 © 2019 American Chemical Society 24031 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article potential-dependent structure of free −OH groups of interfacial water at a gold surface.17 In their experiment, they direct the laser beams through a thin layer of water to investigate the interface they are interested in. Due to the absorption of IR light by the water molecules, their approach cannot easily be used to investigate the hydrogen-bonded water structure. The interactions of water with gold, ITO, and titanium have also been studied previously.18 However, the nonresonant contributions to the SFG signal, originating from electronic transitions in the respective solid material, were overwhelming. It was thus not possible to determine the SFG signal from water at the interface. Since graphene is inherently atomically thin,19 its nonresonant contributions are expected to be substantially reduced. Thus, it should be possible to observe the water vibrations at the water−graphene interface. As graphene is a highly conductive material, the charge carrier density can be changed by applying a voltage. By measuring SFG spectra of the water−graphene interface, while applying a voltage to the graphene layer, we should thus be able to monitor the effect a change in surface potential has on the interfacial water orientation. ■ METHODS Graphene Layer Preparation. Chemical vapor deposited (CVD) graphene was grown on copper (Cu) foil in a custom- made hot wall furnace with a sealed quartz tube. After annealing of the Cu foil surface at 1040 °C for 40 min under ultrapure hydrogen gas flow (100 sccm), methane (CH4) gas flow (25 sccm) was opened for 20 min as carbon source. Samples from commercial sources (Graphenea) were also used. The resulting graphene layer on the Cu foil was subsequently coated with a poly(methyl methacrylate) Figure 1. Representative (a) AFM image and (b) Raman spectrum of (PMMA) protecting layer. The Cu was etched away, and the a graphene layer used for the experiments. The lines in the AFM graphene under the PMMA film was transferred onto the image originate from the roughness of the CaF2 substrate, not the desired substrate, either CaF2 or SiO2, in water. Finally, the graphene. PMMA was removed by immersing it into hot acetone at 55 °C for 30 min, which was repeated for three times. In contrast to previous reports,20 no PMMA residue was observed with As the CVD method results in continuous graphene layers, SFG spectroscopy in the vibrational region of the CH stretch all experiments reported here were performed on this type of mode. However, it is well-known that it is difficult to entirely graphene. Several other methods, namely depositing electro- remove PMMA residues;21,22 there might thus still be PMMA chemically exfoliated graphene27 or graphene oxide (GO) via residue on the samples. Atomic force microscopy (AFM) the Langmuir−Blodgett technique as well as spin-coating the images show large areas of free graphene and some residue that GO dispersion, were also tried. However, none of them yielded seems to appear as bright protrusions covering up to a few high-quality, continuous, macroscopically conducting graphene percent of the surface (see Figure 1a). The clearly visible dark layers. stripes in the AFM image originate from the CaF2 substrate Design of the Spectro-Electrochemical Cell. A spectro- underneath the graphene layer. In addition to AFM, the electrochemical cell was designed for measuring SFG spectra transferred graphene layers were further characterized using while applying a potential. A schematic of the cell is shown in Raman spectroscopy and optical microscopy. Additionally, the Figure 2. sheet resistance of the graphene film, which typically had a The cell mainly consists of three polytetrafluoroethylene value of ∼1 kΩ/sq, was measured before using it for the (PTFE) parts, labeled “base”, “electrolyte chamber”, and experiments. A characteristic Raman spectrum of a graphene “clamp ring” in Figure 2a. The base has four small conical layer is shown in Figure 1b. Two spectral features appear in the holes in the raised center. Two are used as electrolyte inlet and Raman spectrum at ∼1585 and ∼2690 cm−1, corresponding to outlet and two are used to hold the counter and reference the G and 2D band, respectively. The symmetric shape and electrodes (CE and RE). PTFE tubes are pulled through the small full width at half-maximum of up to 40 cm−1 of the 2D holes. The conical shape of the holes deforms the PTFE tube signal indicates that the sample consists of 1 to 2 layers of to create a watertight seal between base and tube. For the graphene.23 The absence of a defect signal at ∼1350 cm−1 electrolyte in- and outlet, Bola tubes with an inner diameter shows that there is no significant amount of defects present in (ID) of 0.5 mm and an outer diameter (OD) of 1.6 mm were the monolayer.24 Furthermore, one can infer a carrier density used. Both the electrodes are inside Bola tubes (0.8 mm ID/ in the order of 1012 cm−2 from the G band position at ∼1585 1.6 mm OD) and are pulled through the holes in the base cm−1.25,26 together with the tubes. 24032 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article Figure 3. Schematic of the electrochemical measurement setup. WE, RE, and CE are the working, reference, and counter electrodes, respectively. and again with water. The reference electrode was wrapped around a platinum wire (0.5 mm diameter, 99.997%, Alfa Aesar) that was used as a connection. A gold wire (0.5 mm diameter, 99.95%, Alfa Aesar) was used as a counter electrode. After assembling all parts, the cell was filled with the electrolyte solution by pumping it in using the pressure of argon gas. To Figure 2. Schematic of the newly designed spectro-electrochemical achieve a bubble-free filling, the direction of the flow was cell (a) and enhanced image of the electrolyte chamber (b). WE, RE, reversed once or twice during the filling process. After the cell and CE are the working, reference, and counter electrodes, had been filled, the inlet and outlet tubes were connected and respectively. wrapped with Parafilm to achieve a watertight system and to avoid air bubbles appearing in the cell. In the first attempt, Ag/AgCl was used as a reference system. The electrolyte chamber is placed on the base. An O-ring However, in this configuration, we observed the deposition of (#1) is used to create a seal between the base and the Au and Ag on the graphene. Therefore, we changed the electrolyte chamber. The bottom of the electrolyte chamber reference system to a Pd/H2 electrode. For this a palladium has a diameter of 12 mm to create sufficient electrolyte volume wire (0.5 mm diameter, 99.95%, MaTeck) was loaded with and space for the counter- and reference electrodes. The top of hydrogen by putting it in a 0.1 M solution of perchloric acid the electrolyte chamber has a diameter of only 5.5 mm. A (Suprapur, 70%, Merck) and applying 5 V, using a gold wire as second O-ring (#2) is needed to create a seal between the the anode. The reaction was stopped after a couple of minutes, electrolyte chamber and the graphene layer (working as soon as the evolution of H2 was visible at the Pd cathode. electrode) on the window. A thin gold foil ring (99.99%, 0.1 The as-prepared Pd wire was wrapped around the Pt wire in mm thickness, Hauner Metallische Werkstoffe) is used as an the cell. The electrolyte used in this configuration was a 0.1 M electrical connection between the graphene working electrode potassium perchlorate (Suprapur, 99.999%, Merck) solution at and the potentiostat. The gold foil ring has an inner diameter pH 4, where the pH was adjusted using perchloric acid. larger than the outer diameter of O-ring #2 but still small Cyclic Voltammograms of Graphene. Cyclic voltammo- enough to overlap with the graphene layer and create a grams (CV) of two different graphene layers deposited on sufficient contact area between graphene and gold. Once the CaF2 substrates were acquired using a Pd/H2 reference system cell is pressed together, a soft silicone disk (1.75 mm thick, see to test the functionality of the cell and the layers. The open- Figure 2b) on top of the electrolyte chamber is used to press circuit potential (OCP) for these systems varied between 400 the gold foil ring onto the graphene layer while at the same and 760 mV depending on the sample and sample history. As time a seal is created between the electrolyte chamber and the can be seen upon comparing Figure 4a with Figure 4b, the graphene by O-ring #2. This way, we ensure the gold foil ring shape of the CVs varied between samples. is never in contact with the electrolyte and can be biased In the CVs shown in Figure 4, the current only varies slowly externally. The cell is pressed together with the clamp ring and with varying potential close to the OCP (around 400, 700, and stainless steel screws. Since PTFE is very soft, a stainless steel 500 mV in Figure 4, parts a, b, and c, respectively). These small ring with threaded holes is used. A schematic of the spectro- slopes in the center result in the CVs exhibiting a shape electrochemical measurement setup is shown in Figure 3. somewhat comparable to that of a rectangle. As the CV of a Experimental Procedure. All the parts of the cell and all capacitor has a rectangular shape,28 this is a good indication the glassware used to prepare the solutions were boiled in 40% that we are indeed contacting the graphene. The electronic nitric acid before usage. After cooling down, the cell parts were properties of supported CVD grown graphene depend on the rinsed with water, while the glassware for the sample CVD growth, as grain boundaries and possibly defect sites are preparation was boiled twice in water before being used. The introduced. Furthermore, the local electronic properties are H2O used for all rinsing steps as well as for boiling the influenced by the transfer process since it is almost impossible glassware and preparing the electrolyte solutions was deionized to entirely remove physisorbed PMMA residues. Trapped or with a Millipore unit (resistivity ≥18.2 MΩ cm). The substrate adsorbed H2O and O2 may further influence the electronic with the graphene on top was only rinsed with water, ethanol, state of transferred graphene.29 We quantified the Fermi level 24033 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article used in this study is Pd filled with H2. After the electrode is filled with hydrogen, it is rinsed with water and mounted into the cell. The cell is subsequently closed and filled with the electrolyte solution. Depending on how long the reference electrode is kept in air during these steps and how much solution is pumped through the cell until there are no air bubbles, the state of the electrode changes slightly, which influences the OCP. The OCP, as well as the shape of the cyclic voltammograms, is further influenced by the oxygen content of the solution. The reduction of O2 within the solution results in a tilt in the CVs. Slight variations in the oxygen content of the solution between the experiments might thus lead to different shapes of the measured cyclic voltammograms. Sample Preparation for Static SFG Experiments. The aqueous solution was sandwiched between two windows of 2 mm thickness. The top window was placed in the cell in such a way that the graphene layer was facing the solution. The graphene substrate and all parts of the cell were rinsed with water, ethanol (absolute, Emsure) and a second time with water before assembling the cell and filling it with the aqueous solution. The H2O used for rinsing as well as for the measurements was deionized with a Millipore unit (resistivity ≥18.2 MΩ cm). D2O (99.9%) was obtained from Euriso-top and used as received. The acidic and basic solutions were prepared by dissolving hydrochloric acid (≥37%, Sigma- Aldrich) and sodium hydroxide (98−100%, Sigma-Aldrich) in H2O, respectively. Sodium chloride (≥99.5%, Sigma- Aldrich) was baked at 650 °C for a couple of hours to remove organic impurities and dissolved in H2O at the desired concentration immediately after cooling down. SFG Experiments. The SFG experiments were performed on a setup using a Ti:sapphire regenerative amplifier (Solstice Ace, Spectra-Physics) generating 800 nm pulses with a duration of 40 fs and a repetition rate of 1 kHz. The broadband infrared pulses were generated in an optical parametric amplifier (TOPAS Prime, Spectra-Physics) with a noncollinear translation stage. The visible pulse was spectrally narrowed in a Fabry−Perot etalon (SLS Optics Ltd.), and the SFG signal was spectrally resolved and detected with a spectrograph (Acton Spectro Pro, SP-2300, Princeton Instru- ments) and an electron-multiplied charge couple device Figure 4. Cyclic voltammograms of graphene in 0.1 M KClO4 at pH 4 (emCCD) camera (ProEM 1600, Roper Scientific). All spectra with a Pd/H2 reference electrode for three different graphene layers were collected in ssp polarization. The signals from 100 nm on CaF2. All CVs were recorded in the spectro-electrochemical cell. gold-coated CaF2 and SiO2 windows, respectively, were used to The insets show representative Raman spectra of the respective normalize the spectra for the shape of the IR pulse. For the graphene layers. The position of the G band that reflects the graphene experiments, the height and tilt of the sample cell were carrier density is marked in all spectra. adjusted, to match those of the gold reference, using a HeNe laser that reflected from the sample surface, directed through a of the different samples using the G band in the Raman spectra pinhole and projected onto the wall. If not otherwise indicated, of all three samples.25,26 In all samples, the G band is situated the power of both the IR and VIS laser pulses was reduced to 2 between 1582 and 1585 cm−1, corresponding to a carrier μJ to avoid damaging the graphene layer. Both laser pulses density of (1−9) × 1011 cm−2. The variations in the position of were weakly focused onto the sample at an angle of incidence the G band of the Raman spectra shown in the insets of Figure of 33° (IR) and 37° (VIS), respectively. The spot sizes at the 4 do not represent significant differences between the samples, sample surface were estimated to have a diameter of several as we observed similar fluctuations within each of the graphene hundreds of micrometers. Due to the low laser power, the layers. It is not completely apparent, however, how the signal had to be acquired for at least 20 min for the signal-to- different local environments in the different samples may affect noise ratio to be sufficient. The signal-to-noise ratio could not the correlation between the OCP and the Fermi level. In be improved further, as the samples gradually change with time addition to the described sample-to-sample variations that are (see also below), especially during the electrochemical inherently unavoidable, the differences in OCPs for the experiment. As such, it is not feasible to further increase the different samples might also be induced by the reference acquisition time, without averaging over effectively different system used in these experiments. The reference electrode samples. During the potential-controlled measurements, at 24034 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article least one cyclic voltammogram (CV) was acquired before and has been attributed by Ohto et al.6 to the presence of a after each SFG measurement. substrate in the experiments. ■ RESULTS AND DISCUSSION Static SFG Experiments. To study the hydrophobicity of Another interesting property of graphene is its proposed wetting transparency. It seems like the wetting properties of numerous materials are not significantly influenced by the graphene, we investigate the free OH SFG signal at the presence of a layer of graphene on top of that material.7,34 This graphene−water interface. The water−air interface exhibits a suggests that graphene is at least partially transmitting significant spectral feature at 3700 cm−1 which originates from interactions between the underlying substrate and the water non-hydrogen-bonded groups that are dangling in air. The molecules. We thus investigate the substrate-dependent SFG interface of water with hydrophobic materials, such as an water response at a graphene monolayer. Figure 6 shows the octadecyltrichlorosilane (OTS) layer, has been shown to exhibit the same spectral feature.30 The spectral feature of the dangling OH bonds can thus be used as an indicator for a hydrophobic material. Such a feature has recently been reported in an SFG simulation study, for the graphene− water interface.6 Figure 5 shows an SFG spectrum of the Figure 5. SFG spectra of a graphene−D2O interface and a CaF2−D2O interface at pD 12. The CaF2−D2O spectrum is offset for clarity. Note the absence of a free OD resonance for the graphene−D2O system, whereas the O−D stretch vibration of Ca−OD groups is clearly visible for the CaF2−D2O interface. graphene−D2O interface in the free-OD frequency region. In this spectrum, we do not observe a dangling OD signal, which is expected around 2750 cm−1.31 The spectrum of the graphene−D2O interface is compared with a spectrum from the CaF2−D2O interface at pD = 12. At pD = 12, the CaF2 surface exhibits a Ca−OD spectral feature that appears roughly 40 cm−1 lower than the free OD at the water−air interface.14,32 Figure 6. SFG spectra of the graphene−water interface where the The observation of this peak assures that the free OD could graphene had been deposited on a CaF2 (a) and a SiO2 (b) substrate. have been monitored despite the low laser powers. The spectra of the respective substrate−water interface without The lack of a free OH signal at the graphene surface has also graphene layer are also shown for comparison. The gray spectra in been reported in an experimental SFG study, where the both panels are the nonresonant spectra of the D2O−graphene graphene was deposited on a sapphire substrate.33 There are interface in the OH vibration region. (c) pH-dependent SFG water two possible explanations for the absence of the free OD signal at the graphene−water and CaF2−water interface. signal. One is that the graphene surface is not hydrophobic, and thus its interface with water does not display a free OD SFG spectra of a CVD graphene−water (H2O) interface, feature. The second possible explanation is that water where the graphene layer had been deposited on two different molecules could be present on both sides of the graphene substrates, namely CaF2 and SiO2. The spectra of the water− (see schematic inset in Figure 5this scenario is discussed in graphene interface are compared with the spectra of the more detail below). Please note that this schematic is just a respective bare substrate-water interface. This comparison cartoon to show the presence of water on both sides of the clearly shows that the shape of the SFG water response is not graphene layer and the water molecules at the graphene layer dominated by the graphene layer but by the underlying are not necessarily strictly ordered in the way indicated here. substrate. The gray SFG spectra in the panels a and b of Figure The signal from the water molecules sandwiched between the 6 show the SFG signal from the graphene−D2O interface in substrate and the graphene layer would then counteract the the O−H stretch region. The fact that there is no signal for the signal from the water molecules on the side of the graphene graphene-D2O interface at OH stretch frequencies indicates layer in contact with bulk water. In line with the second that the signal from the graphene−H2O interface originates possible explanation, the presence of a free OH at the graphene from interfacial water molecules and not from a nonresonant water interface in simulations and its absence in experiments, signal from graphene or the substrate. The spectral shape at 24035 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article this interface is thus not dominated by nonresonant both the IR and VIS laser beam. As this signal extends beyond contributions from the conductive layer, which had been the frequency of the IR pulse, it cannot originate from any reported previously for other solid conductive materials.18 nonresonant SFG response. Thus, the offset might arise from As shown below, the pH dependence of the water signal is two-photon fluorescence (IR + VIS). However, as the intensity another strong indicator that the SFG signal at the graphene− of the offset depends nonlinearly on the power of the IR pulse water interface is dominated by the underlying substrate. As (see inset in Figure 7a), the process seems to be even more explained in the introduction, the intensity of the SFG water complicated. In the process of analyzing the spectra, this offset signal depends on the number of oriented interfacial water is accounted for by moving the background up until it matches molecules and the extent of their orientation. This, in turn, the signal intensity at the low- and high-frequency sides. Thus, depends on the charge of the surface. A higher surface charge even though the water SFG signal at the graphene surface induces a higher order in the interfacial water molecules and seems to be dominated by the underlying substrate, there is therefore, a higher SFG signal. The water SFG signals at pH 3, some contribution from the graphene layer itself, indicating neutral pH, and pH 11 are shown in Figure 6c for a graphene that the graphene is indeed present at the measurement spot. monolayer on CaF2 and a bare CaF2 surface, respectively. As All the static SFG results indicate that we are indeed able to expected from previous studies in literature, the water SFG detect a water signal at a graphene monolayer−water interface. signal at CaF2 is largest for a pH 3 solution.14,32 The water However, the water signal seems to be dominated by the SFG spectra of the graphene−water interface appear to be very underlying substrates, despite the presence of graphene. similar to the ones of the CaF2-water interface, independent of We have shown that our CVD graphene layers are the pH of the solution. Please note that even though the continuous and we can contact them in our spectro- graphene water signals shown in Figure 6 all appear to be electrochemical cell to do electrochemical experiments. Since larger than the corresponding signals from the bare substrate− the nonresonant signal of the graphene layer does not seem to water interface, this is not generally the case. be dominating the signal, we can also apply a potential to the Even though the graphene−water spectra do not show layer, and examine the effect of the applied potential on the marked differences to the substrate−water spectra, it is evident interfacial water molecules. from the raw spectra that the signal originates from a point Potential-Dependent SFG Experiments. SFG spectra of where graphene is present. The raw SFG spectra of a the graphene−water interface at the open circuit potential graphene−water and CaF2−water interface together with a (OCP) and at an applied voltage of 1.2 V for three identically background spectrum from the respective interface are shown prepared graphene layers on CaF2 are shown in Figure 8. The in Figure 7. The background spectra are acquired by blocking samples used for obtaining the potential-dependent spectra in Figure 7. Raw data showing an SFG signal and background (acquired with blocked IR beam) for a CaF2/graphene (a) and a bare CaF2 (b) surface in contact with water. The inset in panel a shows the IR power dependence of the offset between the signal and the background on the sides of the spectra. Figure 8. Water SFG spectra of the graphene−water (0.1 M KClO4, the infrared (but not the visible) pulse. For the CaF2−water pH 4) interface, with graphene on a CaF2 substrate, at the open circuit potential (OCP), and at an applied voltage of 1.2 V, for three interface, the SFG spectrum and the background spectrum different samples. The OCP corresponds to a voltage of 410 mV, 760 (Figure 7b) are superimposed at the low- and high-frequency mV, and 510 mV for the sample shown in parts a, b, and c, side. In contrast, for the graphene−water interface, there is an respectively. Panel c also shows a spectrum acquired at 1.2 V where offset between the signal spectrum and the background the H2O had been exchanged for D2O. The yellow spectra called spectrum. This seems to indicate that graphene exhibits “OCP 2” in all three panels are spectra acquired at the OCP after the some unique frequency-independent signal originating from sample had been exposed to 1.2 V. 24036 DOI: 10.1021/acs.jpcc.9b05844 J. Phys. Chem. C 2019, 123, 24031−24038 The Journal of Physical Chemistry C Article Figure 8, parts a, b, and c, correspond to those for which the CVs are shown in Figure 4, parts a, b, and c. The spectra ■ CONCLUSIONS In conclusion, we have built a spectro-electrochemical cell and labeled “OCP 2” in all three panels of Figure 8 were acquired have shown that we can apply a potential to a conductive after the sample had been exposed to 1.2 V, to check if the graphene layer and acquire SFG spectra simultaneously. potential induced changes are reversible. Upon measuring the Although potential induced changes in the SFG spectra are first sample, an additional signal at 2900 cm−1 appeared upon observed, sample variations make it difficult to draw the application of 1.2 V, which disappeared again upon conclusions about potential-induced changes to the interfacial removing the applied voltage. The appearance of this water structure at these surfaces. Moreover, our data indicate additional signal was reversible and reproducible within the the presence of water on both sides of the graphene and that sample. That is to say, upon applying and removing the the substrate dominantly influences the water orientation at potential several times, the signal always appeared and the surface. The interface is thus much more complex than just disappeared. However, a second sample did not show any graphene−water, since water also intercalates in between the changes in the SFG signal upon changing the potential (see substrate and graphene. Figure 8b). The spectra acquired from a third sample (Figure In the future, it would be interesting to change the reference 8c) showed a somewhat similar trend as the spectra in Figure electrode and electrolyte once again to see whether it is 8a. There was also an additional signal appearing at 2900 cm−1. possible to find a system in which the graphene layers are However, for this sample, the whole signal increased as well. stable. However, it might be more promising to try different Furthermore, a similar change in signal was observed for that thin layer materials that might be more stable and more easily sample when the H2O was exchanged for D2O (green produced than graphene. ■ spectrum in Figure 8c). This suggests that, in this case, we were mainly inducing changes to the nonresonant SFG signal. AUTHOR INFORMATION The magnitude of the nonresonant signal is substantially Corresponding Author smaller than that for much thicker (10−100 nm) gold or ITO *(M.B.) E-mail: bonn@mpip-mainz.mpg.de. Telephone: films studied previously.18,35 (06131) 379 536. The fact that there is only a small change in the water signal upon changing the surface potential quite drastically is ORCID surprising, as there are some IR studies where the change in A. Narita: 0000-0002-3625-522X the water signal upon changing the potential of a gold K. Müllen: 0000-0001-6630-8786 electrode is substantial.36−38 Furthermore, the shape of the K.-J. Tielrooij: 0000-0002-0055-6231 CVs changed upon exposing the layer to a certain potential for E. H. G. Backus: 0000-0002-6202-0280 a longer period. This suggests that we were changing the layer M. Bonn: 0000-0001-6851-8453 upon exposing it to the electrolyte or upon applying a Present Address potential. In addition to the changes in CV shape, the # Université de Strasbourg and CNRS, ISIS, 8 allée Gaspard experiments also induced an optically visible change in the Monge, 67000 Strasbourg, France. graphene layers. The samples appeared more turbid in the Notes center after the experiments. Thus, the samples were not fully The authors declare no competing financial interest. ■ stable upon treating them electrochemically. The conditions in our experiment are similar to those used for electrochemically ACKNOWLEDGMENTS exfoliating graphite into graphene,27 a process during which the surface of the graphite anode is corroded at grain The authors thank Helma Burg for the AFM images, Ulmas boundaries and edges. This might well play a role in Zhumaev for his help in designing the spectroelectrochemical deteriorating the graphene layers within the course of our cell, Sheng Yang for providing us with various graphene experiments. samples and Yuki Nagata, Jonas Pfisterer, and Massoud The experimental observation that the SFG signal change Baghernejad for fruitful discussions. L.B.D is a recipient of a upon applying a potential seems to be negligibly small can be position funded by the Deutsche Forschungsgemeinschaft explained by the presence of water molecules between the through the Excellence Initiative by the Graduate School graphene layer and the underlying substrate. There is always a Materials Science in Mainz (GSC 266). This work is funded by layer of water molecules present at hydrophilic surfaces under a DFG grant (BA 5008/3) and an ERC Starting Grant ambient conditions.39,40 As our CVD graphene samples are (336679). ICN2 is supported by the Severo Ochoa program transferred onto the CaF2 and SiO2 substrates under ambient from Spanish MINECO (Grant No. SEV-2017-0706). K.-J.T. conditions, it is not surprising that there would be water acknowledges financial support through the MAINZ Visiting Professorship. ■ molecules underneath the graphene. 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