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Cite this: Chem. Commun., 2014, 50, 15537 Received 9th September 2014, Accepted 24th October 2014 DOI: 10.1039/c4cc07093c
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Potential-dependent hydration structures at aqueous solution/graphite interfaces by electrochemical frequency modulation atomic force microscopy† Toru Utsunomiya,a Yasuyuki Yokota,a Toshiaki Enokib and Ken-ichi Fukui*a
www.rsc.org/chemcomm
Potential-dependent solvation structures of aqueous electrolyte– graphite interfaces were studied using electrochemical frequency modulation atomic force microscopy. Oscillatory modulations on the force curves reversibly changed with the applied potential on the graphite electrode, and also strongly depended on the anion species in electrolyte solutions.
Exploring the molecular-scale structures at electrolyte/electrode interfaces is a key topic in interfacial electrochemistry. In particular, the detailed structure and dynamics of water molecules and coadsorbed ions in direct contact with the electrified electrode surface is essential for the further understanding of electrocatalytic and other interfacial reactions, and has been extensively studied throughout the history of electrochemistry. Several classical models for water at the electrified interface1,2 suggested that three possible water species populated the electrode surface: flip-up and flip-down water monomer species and the dimers which can be regarded as a manifestation of hydrogen bonding interactions.3 Various improvements and modifications of the models had been reported for decades,4,5 but many properties of water had remained unexplained at the molecular level.6 Over the past decades, the results of novel experimental methods have begun to unveil the molecular-scale pictures at electrolyte/electrode interfaces. A major contribution has come from in situ structure sensitive techniques, in particular, surface X-ray scattering (SXS)7–9 and infrared spectroscopy.10–13 These results showed the layered liquid structures and potential-dependent orientation of water molecules under electrochemical conditions. But these methods provide the in-plane averaged information at a
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. E-mail: [email protected]; Fax: +81-6-6850-6235; Tel: +81-6-6850-6235 b Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan † Electronic supplementary information (ESI) available: CVs and EC-FM-AFM topographies of HOPG in each electrolyte, and the force curves converted from Fig. 1 and 2. See DOI: 10.1039/c4cc07093c
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the whole working electrode surfaces and are not sufficient to illustrate how the hydration networks change and affect the structuring of water molecules in each interfacial layer. Atomic force microscopy (AFM) has also been used to investigate the liquid structures over the liquid/solid interfaces.14 Latest developments of frequency modulation (FM-) AFM with low-noise optical deflection sensors15 have enabled us to perform sub-nanometer imaging even in aqueous solutions16 and in viscous ionic liquids,17,18 and also to determine the tip–surface force with a precision of better than 10 pN in liquids.16 It is also reported that force distributions, derived from the FM-AFM, correspond to interfacial water density distributions, in other words, the hydration structure, which is specific to the surface site19–21 and probably to the local irregularity of the surface structure. The hydration structure at electrolyte/ electrode interfaces has been one of the most attractive subjects in electrochemistry since it plays an important role in constructing the electric double layer. This study aims to address the hydration structures on the highly oriented pyrolytic graphite (HOPG) basal plane in aqueous solutions under potential control. Our experimental strategy is based on electrochemical (EC-) FM-AFM22,23 to characterize the structure of the electrolyte/HOPG interfaces. In particular, we employed force spectroscopy, force curves, as an experimental approach to identify the solution structures at various electrode potentials. We report experimental data obtained in HClO4 and H2SO4 solutions, to address the influence of the nature of the anion on the water structures adjacent to the HOPG surface. The selection of perchlorate and sulfate anions for this comparison is due to the different hydration properties of these anions.24 Sulfate anions form strong hydrogen bonds with water molecules, with a local structure similar to the pure water hydrogen bond network. In contrast, water molecules binding to perchlorate anions behave quite differently from bulk water. The strength of the hydrogen bonds formed between water molecules and perchlorate anions is much smaller than in the case of sulfate anions. Concentrated HNO3 (special grade, Kishida), concentrated H2SO4 (special grade, Wako), acetone (special grade, Wako), H2SO4 (ultrapure, Kanto Kagaku), and HClO4 (ultra-pure, Kanto Kagaku) were
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purchased and used without further purification. Water for aqueous solutions and cleaning the cells was prepared using a Millipore system. All solutions were prepared using ultrapure acid and Milli-Q water. HOPG was purchased from SPI supplies (SPI-2 grade). EC-FM-AFM measurements were carried out using JSPM-5200 (modified for the electrochemical environment, JEOL). The HOPG electrode was mounted into a PEEK electrochemical cell with an Au/AuOx reference electrode and a Pt wire counter electrode. The potentials of the HOPG electrode (Es) and the cantilever tip (Et) were controlled using a bipotentiostat (AFCBP1, Pine) for EC-FM-AFM measurements. Silicon cantilevers backside coated with gold (PPP-NCH-AuD, Nanosensors) were used. The nominal spring constant, resonance frequency, and a Q-factor of the resonance peak were 40 N m1, 130–160 kHz, and 8 in the aqueous solution, respectively. The absolute cantilever deflection was calibrated using the theoretical amplitude of the thermal Brownian motion of the cantilever. Measurements were performed at room temperature. All potentials in this paper are quoted with respect to the Au/AuOx electrode (1.0 V vs. Ag/AgCl (3 M NaCl), BAS). The typical cyclic voltammograms (CVs) and topographies at some electrode potentials in the two electrolytes are shown in the ESI† (Fig. S1 and S2). The force curve is measured as follows. A Si cantilever, which gives high spatial resolution images, is fixed close to the HOPG surface showing a preset Df value. Subsequently, the AFM feedback loop is switched off, and the tip is retracted about 2 nm. Then the oscillating cantilever is scanned vertically from the solution side to the surface for the preset distance and Df is monitored as a function of the vertical (relative) coordinate. The FM-AFM feedback is enabled after every single approaching and retracting cycle in order to allow stabilization of the system for a few seconds before the next open-feedback cycle is recorded. The Es was limited to the region, where the flat HOPG terrace structures remained in the time scale for the series of measurements. The etch pit formation at the basal plane, which has been reported in the literature25 after anodic potential cycles, was frequently observed in the HClO4 solution at 0.6 V, as shown in Fig. S3 (ESI†), but not at 0.4 V (Fig. S1(d) and (d 0 ), ESI†). Etch pits were scarcely observed at 0.6 V in the H2SO4 solution during the experiments (Fig. S1(h) and (h 0 ), ESI†). Fig. 1 shows selected force spectra of an HOPG basal plane in contact with 0.1 M HClO4. A frequency shift versus distance curve at the cathodic potential (Es = 0.9 V) is shown in Fig. 1a with the tip scanned from the solution to the surface. The amplitude of the tip vibration (Ap–p) was constant while the tip scanned. The positive frequency shift represents repulsive interaction with the tip. The resulting frequency shift versus distance curve (Fig. 1a) shows a decaying oscillatory profile indicative of structural hydration forces with two clearly visible valleys. These corrugations of the force versus distance curve reflect the molecular scale solvation at the electrolyte/graphite interface. The period of the oscillation was about 0.4 nm as shown in Fig. 1a. Such oscillatory profiles have also been observed by using AFM14 or surface force apparatus26 at other aqueous solution/solid interfaces without potential control. Due to the agreement between the measured oscillation period and the expected thickness of a water layer (0.2–0.4 nm), the oscillatory
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Fig. 1 (a)–(d) Frequency shift versus distance between a cantilever tip and an HOPG electrode surface in 0.1 M HClO4, showing decaying oscillatory force profiles indicative of structural hydration forces (Ap–p = 0.25 nm, Et = 0.9 V) at each sample potential. The blue curve in (a), which was measured at Es = 0.9 V after applying Es = 0.4 V, is shifted by 500 Hz for illustration. Red broken lines indicate the valleys of force profiles.
profile has been considered to reflect the layered distribution of the interfacial water (hydration layers).8,19 The hydration layer spacing on HOPG obtained here also agrees with a recent publication of Suzuki et al. using FM-AFM without potential control.27 Frequency shift-distance curves at several electrode potentials are shown in Fig. 1b–d. At more anodic potential, the intensity of the oscillations became smaller than that in Fig. 1a. Furthermore, in Fig. 1d, the number of apparent valleys increased to three, and the period of oscillations was reduced to about 0.3 nm. These features indicate that the structural hydration force, the structures of water molecules near the interface, changed with the electrode potential. It should be noted that the amplitude of the cantilever tip vibration was constant at 0.25 nm, and the potential of the tip was regulated using a bipotentiostat at 0.9 V, so the difference among Fig. 1 was due to the difference of the electrode potential. It should also be noted that the force profile measured at 0.9 V after measuring Fig. 1d (0.4 V), which is shown as a blue curve in Fig. 1a, had identical features, especially in the number of apparent valleys and the measured oscillation period. The reversible changes indicate that the solution structures near the HOPG basal plane change with the electrode potential. At the anodic potential, the concentration of ClO4 anions near the electrode surface increases. Therefore, the structure of the water layer may be affected by ClO4. The ClO4 anion is hydrophobic and is classified as a ‘‘structure breaker’’, which means that water molecules next to the anions are believed to be highly disordered and thus the hydrogen bonding network is highly broken down.24 This structure breaking property is suggested by the thermodynamic behavior. The entropy change due to the effects of the ClO4 anion on the structure of water was reported to be 38 J K1 mol1.24 Furthermore, the water behavior at the interface between a gold electrode and the HClO4 solution, as was observed by vibrational spectroscopy,
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Fig. 2 (a)–(d) Frequency shift versus distance between a cantilever tip and an HOPG electrode surface in 0.1 M H2SO4, showing decaying oscillatory force profiles indicative of structural hydration forces (Ap–p = 0.24 nm, Et = 0.9 V) at each sample potential. Red broken lines indicate the valleys of force profiles.
was also explained by the structure breaking property.28 From Fig. 1 and the property of ClO4, we have concluded that the ClO4 anion reduces the stability of water layers on the HOPG at the anodic potential. Fig. 2 shows selected force spectra of HOPG basal planes in contact with 0.1 M H2SO4. The resulting frequency shift versus distance curve at 0.9 V (Fig. 2a) shows a decaying oscillatory profile indicative of structural hydration forces with two clearly visible valleys. The period of the oscillation was about 0.4 nm as shown in Fig. 2a. These features in 0.1 M H2SO4 were consistent with the results in 0.1 M HClO4 (Fig. 1a). Interestingly, at more anodic potential, the intensity of the oscillations became larger than that shown in Fig. 2a. This trend against the potential change was opposite to that in HClO4, suggesting a different stability of the hydration layers in both electrolytes. At the most anodic potential in Fig. 2d, the number of apparent valleys increased to four, and the period of oscillations was reduced to about 0.3 nm. This is similar to the case in HClO4. A recent report also indicated that the measured hydration force spacing was not altered in different electrolyte solutions (different ions) because the dominant factor to determine the measurable layer-spacing is the size of the water molecules rather than coexisting hydrated ionic species.29 Fig. 2 demonstrates that applying the anodic potential increases the peak intensity of the structural hydration force and the number of apparent hydration layers. Positively charged surfaces increase the structural hydration magnitude (increase the stability) as ordered water layers adjacent to the HOPG surface in contrast to the HClO4 solution. The SO42 anion is hydrophilic and is classified as a ‘‘structure maker’’, which means that water molecules next to the anions are believed to be ordered and thus the hydrogen bonding network is formed.24 The entropy change due to the effects of the SO42 anion on the structure of water is 69 J K1 mol1.24 It should be noted that the HSO4 anion, which can be dominant in bulk solution, shows a similar entropy change (44 J K1 mol1),24 so the conclusion from
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anion properties does not change. Furthermore, the water molecules at the interface between a gold electrode and the H2SO4 solution were highly hydrogen bonded.12 From Fig. 2 and the property of SO42, we have concluded that the SO42 anion increases the stability of water layers on the HOPG at anodic potential. Our EC-FM-AFM experiments demonstrated the changes of hydration structures on an HOPG basal plane, and indicated that the potential-dependent hydration structures are a general feature. In order to quantify the frequency shift profiles (Fig. 1 and 2), the conversion of the measured frequency shift to force versus distance was performed by the method of Sader and Jarvis,30 and the force versus distance curves are presented in the ESI† (Fig. S4 and S5). Additionally, the magnitude of the primary hydration peak adjacent to HOPG for each potential is plotted in Fig. 3a and b. The primary hydration peak was measured from the secondary minimum to the primary maximum (see Fig. S4 and S5, ESI†). A similar analysis has been reported by Jarvis et al.29 The correlation between the primary hydration peak and the value of the valleys in force spectroscopies is different between in HClO4 and in H2SO4. We believe that the intensity of the primary hydration peak is correlated to an entropic contribution, because this value provides the estimate of the easiness to penetrate the tip to the nearest water layer. Anion-dependent results indicate that the hydrogen bonded properties at the nearest water layer are modulated by the anions, and similar results had been obtained using surface enhanced IR spectroscopy at a gold electrode surface.12,31 The adsorption of hydrophobic anions, such as perchlorate anions, breaks hydrogen bonds in the ice-like water layer, yielding an additional sharp band for a non-hydrogen bonded OH moiety at around 3600 cm1. In the H2SO4 solution, such a band is not observed, suggesting that the sulfate anion is adsorbed on the
Fig. 3 A plot of the primary hydration peak versus electrode potential in 0.1 M HClO4 (a) and in 0.1 M H2SO4 (b) from several force curves at each potential. (c) Possible schematic presentations of potential-dependent interface structures in each electrolyte.
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electrode surface by forming strong hydrogen bonds with surrounding water molecules. Similar interpretations might be applied for our systems. The structural hydration force is detected due to modulations of the probability density of the water molecules directly beneath the tip due to interface-induced orderings.29 The adsorption of anions (both ClO4 and SO42) may break the ice-like water,12,31 so the decrease of the layer spacing at the anodic potential may originate from this breaking of the ice-like water at the HOPG surface. Conceptual models of the aqueous solution/graphite interfaces, as described above, are shown in Fig. 3c. In situ IRAS measurements on an HOPG surface will elucidate the details, but the results focusing on the water molecules near the HOPG surface have not been reported, to the best of our knowledge. The amount of anions near the HOPG surface at anodic potentials is much smaller than that of water molecules. However, the EC-FM-AFM detect the average tip–surface interaction over the time scale of the measurement and allows for molecules to freely diffuse into and out of the tip–sample gap during the measurement.29 Therefore, we consider that the relatively increased possibility of the presence of anions near the HOPG surface could modulate the stability of the water layers adjacent to the HOPG, as derived from force curve measurements. The number of apparent valleys in force curves, which reflects the modulated density distribution of water molecules, increased with the electrode potential, as shown in Fig. 1 and 2. This indicates that the interface affects the structure of liquids more effectively to longer distance at the anodic potential. For screening the increased positive charge of the electrode surface, the water density distribution was modulated to longer distance. The SXS measurements also indicated more layers of water density distribution on the Ag(111) electrode at the anodic potential,8 but the SXS results cannot evaluate the stability of the water layers, which is reflected in the magnitude of the primary hydration peak in our study. Therefore, we consider that our approach sheds new light not only on the number, but also on the stability of the layered structures at the electrified interfaces. Complete interpretation is beyond the scope of this study and remains a topic of continued experimental and theoretical interests. Here we simply demonstrated that the electrode potential and the anion in the electrolyte induce the changes in the layered water structures on the HOPG basal plane surface. These results and methods could contribute to unveiling the structures of water molecules at the electrified interfaces, which is the fundamental of electrochemistry. In conclusion, we presented the direct observations of structural hydration forces as a function of the electrode potential for different anions on the HOPG basal plane. As the electrode potential increased in the HClO4 solution, the magnitude of the primary hydration peak decreases while the number of apparent valleys increases. In contrast both the magnitude of the primary hydration peak and the number of apparent valleys increased in the H2SO4 solution. By comparing
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these two systems, the correlations between hydration layers and the anion structure making/breaking properties were suggested. Further studies to understand the role of the electrode potential and ions, and their effects on hydration forces are necessary to unveiling the functions of electrolyte/electrode interfaces. This work was partially supported by Grant-in-Aid for Scientific Research No. 20001006 and 12J02584 from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Government of Japan through its ‘‘Funding Program for Next Generation World-Leading Researchers’’ (GR071). T.U. expresses his special thanks for the research Fellow of the Japan Society for the Promotion of Science.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
B. Damaskin and A. Frumkin, Electrochim. Acta, 1974, 19, 173–176. R. Parsons, J. Electroanal. Chem. Interfacial Electrochem., 1975, 59, 229–237. J. O. Bockris and M. A. Habib, Electrochim. Acta, 1977, 22, 41–46. W. Fawcett, S. Levine, R. M. DeNobriga and A. C. McDonald, J. Electroanal. Chem. Interfacial Electrochem., 1980, 111, 163–180. X. Gao and H. S. White, J. Electroanal. Chem., 1995, 389, 13–19. J. O. Bockris, A. K. N. Reddy and M. Gamboa-Aldeco, Modern Electrochemistry 2A, Kluwer Academic Publishers, New York, 2000. B. M. Ocko, G. M. Watson and J. Wang, J. Phys. Chem., 1994, 98, 897–906. M. F. Toney, J. N. Howard, J. Richer, G. L. Borges, J. G. Gordon, O. R. Melroy, D. G. Wiesler, D. Yee and L. B. Sorensen, Nature, 1994, 368, 444–446. T. Kondo, J. Morita, K. Hanaoka, S. Takakusagi, K. Tamura, M. Takahasi, J. Mizuki and K. Uosaki, J. Phys. Chem. C, 2007, 111, 13197–13204. M. A. Habib and J. O. Bockris, Langmuir, 1986, 2, 388–392. M. Osawa, Bull. Chem. Soc. Jpn., 1997, 70, 2861–2880. K. Ataka and M. Osawa, Langmuir, 1998, 14, 951–959. T. Wandlowski, K. Ataka, S. Pronkin and D. Diesing, Electrochim. Acta, 2004, 49, 1233–1247. ¨ffer and P. Hansma, Phys. Rev. B, 1995, 52, J. Cleveland, T. Scha 8692–8696. T. Fukuma, M. Kimura, K. Kobayashi, K. Matsushige and H. Yamada, Rev. Sci. Instrum., 2005, 76, 053704. T. Fukuma, K. Kobayashi, K. Matsushige and H. Yamada, Appl. Phys. Lett., 2005, 87, 034101. Y. Yokota, T. Harada and K. Fukui, Chem. Commun., 2010, 46, 8627–8629. Y. Yokota, H. Hara, T. Harada, A. Imanishi, T. Uemura, J. Takeya and K. Fukui, Chem. Commun., 2013, 49, 10596–10598. T. Fukuma, Sci. Technol. Adv. Mater., 2010, 11, 033003. K. Kimura, S. Ido, N. Oyabu, K. Kobayashi, Y. Hirata, T. Imai and H. Yamada, J. Chem. Phys., 2010, 132, 194705. T. Hiasa, K. Kimura and H. Onishi, Phys. Chem. Chem. Phys., 2012, 14, 8419–8424. K. Umeda and K. Fukui, Langmuir, 2010, 26, 9104–9110. K. Umeda and K. Fukui, J. Vac. Sci. Technol., B, 2010, 28, C4D40. Y. Marcus, Ion solvation, Wiley-Interscience, Chichester, 1985. H.-S. Choo, T. Kinumoto, M. Nose, K. Miyazaki, T. Abe and Z. Ogumi, J. Power Sources, 2008, 185, 740–746. J. N. Israelachvili and R. M. Pashley, Nature, 1983, 306, 249–250. K. Suzuki, N. Oyabu, K. Kobayashi, K. Matsushige and H. Yamada, Appl. Phys. Express, 2011, 4, 125102. F. Kitamura, N. Nanbu, T. Ohsaka and K. Tokuda, J. Electroanal. Chem., 1998, 452, 241–249. J. I. Kilpatrick, S.-H. Loh and S. P. Jarvis, J. Am. Chem. Soc., 2013, 135, 2628–2634. J. E. Sader and S. P. Jarvis, Appl. Phys. Lett., 2004, 84, 1801. K. Ataka, T. Yotsuyanagi and M. Osawa, J. Phys. Chem., 1996, 100, 10664–10672.
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Cite this: Chem. Commun., 2014, 50, 15537 Received 9th September 2014, Accepted 24th October 2014 DOI: 10.1039/c4cc07093c
View Article Online View Journal | View Issue
Potential-dependent hydration structures at aqueous solution/graphite interfaces by electrochemical frequency modulation atomic force microscopy† Toru Utsunomiya,a Yasuyuki Yokota,a Toshiaki Enokib and Ken-ichi Fukui*a
www.rsc.org/chemcomm
Potential-dependent solvation structures of aqueous electrolyte– graphite interfaces were studied using electrochemical frequency modulation atomic force microscopy. Oscillatory modulations on the force curves reversibly changed with the applied potential on the graphite electrode, and also strongly depended on the anion species in electrolyte solutions.
Exploring the molecular-scale structures at electrolyte/electrode interfaces is a key topic in interfacial electrochemistry. In particular, the detailed structure and dynamics of water molecules and coadsorbed ions in direct contact with the electrified electrode surface is essential for the further understanding of electrocatalytic and other interfacial reactions, and has been extensively studied throughout the history of electrochemistry. Several classical models for water at the electrified interface1,2 suggested that three possible water species populated the electrode surface: flip-up and flip-down water monomer species and the dimers which can be regarded as a manifestation of hydrogen bonding interactions.3 Various improvements and modifications of the models had been reported for decades,4,5 but many properties of water had remained unexplained at the molecular level.6 Over the past decades, the results of novel experimental methods have begun to unveil the molecular-scale pictures at electrolyte/electrode interfaces. A major contribution has come from in situ structure sensitive techniques, in particular, surface X-ray scattering (SXS)7–9 and infrared spectroscopy.10–13 These results showed the layered liquid structures and potential-dependent orientation of water molecules under electrochemical conditions. But these methods provide the in-plane averaged information at a
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. E-mail: [email protected]; Fax: +81-6-6850-6235; Tel: +81-6-6850-6235 b Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan † Electronic supplementary information (ESI) available: CVs and EC-FM-AFM topographies of HOPG in each electrolyte, and the force curves converted from Fig. 1 and 2. See DOI: 10.1039/c4cc07093c
This journal is © The Royal Society of Chemistry 2014
the whole working electrode surfaces and are not sufficient to illustrate how the hydration networks change and affect the structuring of water molecules in each interfacial layer. Atomic force microscopy (AFM) has also been used to investigate the liquid structures over the liquid/solid interfaces.14 Latest developments of frequency modulation (FM-) AFM with low-noise optical deflection sensors15 have enabled us to perform sub-nanometer imaging even in aqueous solutions16 and in viscous ionic liquids,17,18 and also to determine the tip–surface force with a precision of better than 10 pN in liquids.16 It is also reported that force distributions, derived from the FM-AFM, correspond to interfacial water density distributions, in other words, the hydration structure, which is specific to the surface site19–21 and probably to the local irregularity of the surface structure. The hydration structure at electrolyte/ electrode interfaces has been one of the most attractive subjects in electrochemistry since it plays an important role in constructing the electric double layer. This study aims to address the hydration structures on the highly oriented pyrolytic graphite (HOPG) basal plane in aqueous solutions under potential control. Our experimental strategy is based on electrochemical (EC-) FM-AFM22,23 to characterize the structure of the electrolyte/HOPG interfaces. In particular, we employed force spectroscopy, force curves, as an experimental approach to identify the solution structures at various electrode potentials. We report experimental data obtained in HClO4 and H2SO4 solutions, to address the influence of the nature of the anion on the water structures adjacent to the HOPG surface. The selection of perchlorate and sulfate anions for this comparison is due to the different hydration properties of these anions.24 Sulfate anions form strong hydrogen bonds with water molecules, with a local structure similar to the pure water hydrogen bond network. In contrast, water molecules binding to perchlorate anions behave quite differently from bulk water. The strength of the hydrogen bonds formed between water molecules and perchlorate anions is much smaller than in the case of sulfate anions. Concentrated HNO3 (special grade, Kishida), concentrated H2SO4 (special grade, Wako), acetone (special grade, Wako), H2SO4 (ultrapure, Kanto Kagaku), and HClO4 (ultra-pure, Kanto Kagaku) were
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purchased and used without further purification. Water for aqueous solutions and cleaning the cells was prepared using a Millipore system. All solutions were prepared using ultrapure acid and Milli-Q water. HOPG was purchased from SPI supplies (SPI-2 grade). EC-FM-AFM measurements were carried out using JSPM-5200 (modified for the electrochemical environment, JEOL). The HOPG electrode was mounted into a PEEK electrochemical cell with an Au/AuOx reference electrode and a Pt wire counter electrode. The potentials of the HOPG electrode (Es) and the cantilever tip (Et) were controlled using a bipotentiostat (AFCBP1, Pine) for EC-FM-AFM measurements. Silicon cantilevers backside coated with gold (PPP-NCH-AuD, Nanosensors) were used. The nominal spring constant, resonance frequency, and a Q-factor of the resonance peak were 40 N m1, 130–160 kHz, and 8 in the aqueous solution, respectively. The absolute cantilever deflection was calibrated using the theoretical amplitude of the thermal Brownian motion of the cantilever. Measurements were performed at room temperature. All potentials in this paper are quoted with respect to the Au/AuOx electrode (1.0 V vs. Ag/AgCl (3 M NaCl), BAS). The typical cyclic voltammograms (CVs) and topographies at some electrode potentials in the two electrolytes are shown in the ESI† (Fig. S1 and S2). The force curve is measured as follows. A Si cantilever, which gives high spatial resolution images, is fixed close to the HOPG surface showing a preset Df value. Subsequently, the AFM feedback loop is switched off, and the tip is retracted about 2 nm. Then the oscillating cantilever is scanned vertically from the solution side to the surface for the preset distance and Df is monitored as a function of the vertical (relative) coordinate. The FM-AFM feedback is enabled after every single approaching and retracting cycle in order to allow stabilization of the system for a few seconds before the next open-feedback cycle is recorded. The Es was limited to the region, where the flat HOPG terrace structures remained in the time scale for the series of measurements. The etch pit formation at the basal plane, which has been reported in the literature25 after anodic potential cycles, was frequently observed in the HClO4 solution at 0.6 V, as shown in Fig. S3 (ESI†), but not at 0.4 V (Fig. S1(d) and (d 0 ), ESI†). Etch pits were scarcely observed at 0.6 V in the H2SO4 solution during the experiments (Fig. S1(h) and (h 0 ), ESI†). Fig. 1 shows selected force spectra of an HOPG basal plane in contact with 0.1 M HClO4. A frequency shift versus distance curve at the cathodic potential (Es = 0.9 V) is shown in Fig. 1a with the tip scanned from the solution to the surface. The amplitude of the tip vibration (Ap–p) was constant while the tip scanned. The positive frequency shift represents repulsive interaction with the tip. The resulting frequency shift versus distance curve (Fig. 1a) shows a decaying oscillatory profile indicative of structural hydration forces with two clearly visible valleys. These corrugations of the force versus distance curve reflect the molecular scale solvation at the electrolyte/graphite interface. The period of the oscillation was about 0.4 nm as shown in Fig. 1a. Such oscillatory profiles have also been observed by using AFM14 or surface force apparatus26 at other aqueous solution/solid interfaces without potential control. Due to the agreement between the measured oscillation period and the expected thickness of a water layer (0.2–0.4 nm), the oscillatory
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Fig. 1 (a)–(d) Frequency shift versus distance between a cantilever tip and an HOPG electrode surface in 0.1 M HClO4, showing decaying oscillatory force profiles indicative of structural hydration forces (Ap–p = 0.25 nm, Et = 0.9 V) at each sample potential. The blue curve in (a), which was measured at Es = 0.9 V after applying Es = 0.4 V, is shifted by 500 Hz for illustration. Red broken lines indicate the valleys of force profiles.
profile has been considered to reflect the layered distribution of the interfacial water (hydration layers).8,19 The hydration layer spacing on HOPG obtained here also agrees with a recent publication of Suzuki et al. using FM-AFM without potential control.27 Frequency shift-distance curves at several electrode potentials are shown in Fig. 1b–d. At more anodic potential, the intensity of the oscillations became smaller than that in Fig. 1a. Furthermore, in Fig. 1d, the number of apparent valleys increased to three, and the period of oscillations was reduced to about 0.3 nm. These features indicate that the structural hydration force, the structures of water molecules near the interface, changed with the electrode potential. It should be noted that the amplitude of the cantilever tip vibration was constant at 0.25 nm, and the potential of the tip was regulated using a bipotentiostat at 0.9 V, so the difference among Fig. 1 was due to the difference of the electrode potential. It should also be noted that the force profile measured at 0.9 V after measuring Fig. 1d (0.4 V), which is shown as a blue curve in Fig. 1a, had identical features, especially in the number of apparent valleys and the measured oscillation period. The reversible changes indicate that the solution structures near the HOPG basal plane change with the electrode potential. At the anodic potential, the concentration of ClO4 anions near the electrode surface increases. Therefore, the structure of the water layer may be affected by ClO4. The ClO4 anion is hydrophobic and is classified as a ‘‘structure breaker’’, which means that water molecules next to the anions are believed to be highly disordered and thus the hydrogen bonding network is highly broken down.24 This structure breaking property is suggested by the thermodynamic behavior. The entropy change due to the effects of the ClO4 anion on the structure of water was reported to be 38 J K1 mol1.24 Furthermore, the water behavior at the interface between a gold electrode and the HClO4 solution, as was observed by vibrational spectroscopy,
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Fig. 2 (a)–(d) Frequency shift versus distance between a cantilever tip and an HOPG electrode surface in 0.1 M H2SO4, showing decaying oscillatory force profiles indicative of structural hydration forces (Ap–p = 0.24 nm, Et = 0.9 V) at each sample potential. Red broken lines indicate the valleys of force profiles.
was also explained by the structure breaking property.28 From Fig. 1 and the property of ClO4, we have concluded that the ClO4 anion reduces the stability of water layers on the HOPG at the anodic potential. Fig. 2 shows selected force spectra of HOPG basal planes in contact with 0.1 M H2SO4. The resulting frequency shift versus distance curve at 0.9 V (Fig. 2a) shows a decaying oscillatory profile indicative of structural hydration forces with two clearly visible valleys. The period of the oscillation was about 0.4 nm as shown in Fig. 2a. These features in 0.1 M H2SO4 were consistent with the results in 0.1 M HClO4 (Fig. 1a). Interestingly, at more anodic potential, the intensity of the oscillations became larger than that shown in Fig. 2a. This trend against the potential change was opposite to that in HClO4, suggesting a different stability of the hydration layers in both electrolytes. At the most anodic potential in Fig. 2d, the number of apparent valleys increased to four, and the period of oscillations was reduced to about 0.3 nm. This is similar to the case in HClO4. A recent report also indicated that the measured hydration force spacing was not altered in different electrolyte solutions (different ions) because the dominant factor to determine the measurable layer-spacing is the size of the water molecules rather than coexisting hydrated ionic species.29 Fig. 2 demonstrates that applying the anodic potential increases the peak intensity of the structural hydration force and the number of apparent hydration layers. Positively charged surfaces increase the structural hydration magnitude (increase the stability) as ordered water layers adjacent to the HOPG surface in contrast to the HClO4 solution. The SO42 anion is hydrophilic and is classified as a ‘‘structure maker’’, which means that water molecules next to the anions are believed to be ordered and thus the hydrogen bonding network is formed.24 The entropy change due to the effects of the SO42 anion on the structure of water is 69 J K1 mol1.24 It should be noted that the HSO4 anion, which can be dominant in bulk solution, shows a similar entropy change (44 J K1 mol1),24 so the conclusion from
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anion properties does not change. Furthermore, the water molecules at the interface between a gold electrode and the H2SO4 solution were highly hydrogen bonded.12 From Fig. 2 and the property of SO42, we have concluded that the SO42 anion increases the stability of water layers on the HOPG at anodic potential. Our EC-FM-AFM experiments demonstrated the changes of hydration structures on an HOPG basal plane, and indicated that the potential-dependent hydration structures are a general feature. In order to quantify the frequency shift profiles (Fig. 1 and 2), the conversion of the measured frequency shift to force versus distance was performed by the method of Sader and Jarvis,30 and the force versus distance curves are presented in the ESI† (Fig. S4 and S5). Additionally, the magnitude of the primary hydration peak adjacent to HOPG for each potential is plotted in Fig. 3a and b. The primary hydration peak was measured from the secondary minimum to the primary maximum (see Fig. S4 and S5, ESI†). A similar analysis has been reported by Jarvis et al.29 The correlation between the primary hydration peak and the value of the valleys in force spectroscopies is different between in HClO4 and in H2SO4. We believe that the intensity of the primary hydration peak is correlated to an entropic contribution, because this value provides the estimate of the easiness to penetrate the tip to the nearest water layer. Anion-dependent results indicate that the hydrogen bonded properties at the nearest water layer are modulated by the anions, and similar results had been obtained using surface enhanced IR spectroscopy at a gold electrode surface.12,31 The adsorption of hydrophobic anions, such as perchlorate anions, breaks hydrogen bonds in the ice-like water layer, yielding an additional sharp band for a non-hydrogen bonded OH moiety at around 3600 cm1. In the H2SO4 solution, such a band is not observed, suggesting that the sulfate anion is adsorbed on the
Fig. 3 A plot of the primary hydration peak versus electrode potential in 0.1 M HClO4 (a) and in 0.1 M H2SO4 (b) from several force curves at each potential. (c) Possible schematic presentations of potential-dependent interface structures in each electrolyte.
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electrode surface by forming strong hydrogen bonds with surrounding water molecules. Similar interpretations might be applied for our systems. The structural hydration force is detected due to modulations of the probability density of the water molecules directly beneath the tip due to interface-induced orderings.29 The adsorption of anions (both ClO4 and SO42) may break the ice-like water,12,31 so the decrease of the layer spacing at the anodic potential may originate from this breaking of the ice-like water at the HOPG surface. Conceptual models of the aqueous solution/graphite interfaces, as described above, are shown in Fig. 3c. In situ IRAS measurements on an HOPG surface will elucidate the details, but the results focusing on the water molecules near the HOPG surface have not been reported, to the best of our knowledge. The amount of anions near the HOPG surface at anodic potentials is much smaller than that of water molecules. However, the EC-FM-AFM detect the average tip–surface interaction over the time scale of the measurement and allows for molecules to freely diffuse into and out of the tip–sample gap during the measurement.29 Therefore, we consider that the relatively increased possibility of the presence of anions near the HOPG surface could modulate the stability of the water layers adjacent to the HOPG, as derived from force curve measurements. The number of apparent valleys in force curves, which reflects the modulated density distribution of water molecules, increased with the electrode potential, as shown in Fig. 1 and 2. This indicates that the interface affects the structure of liquids more effectively to longer distance at the anodic potential. For screening the increased positive charge of the electrode surface, the water density distribution was modulated to longer distance. The SXS measurements also indicated more layers of water density distribution on the Ag(111) electrode at the anodic potential,8 but the SXS results cannot evaluate the stability of the water layers, which is reflected in the magnitude of the primary hydration peak in our study. Therefore, we consider that our approach sheds new light not only on the number, but also on the stability of the layered structures at the electrified interfaces. Complete interpretation is beyond the scope of this study and remains a topic of continued experimental and theoretical interests. Here we simply demonstrated that the electrode potential and the anion in the electrolyte induce the changes in the layered water structures on the HOPG basal plane surface. These results and methods could contribute to unveiling the structures of water molecules at the electrified interfaces, which is the fundamental of electrochemistry. In conclusion, we presented the direct observations of structural hydration forces as a function of the electrode potential for different anions on the HOPG basal plane. As the electrode potential increased in the HClO4 solution, the magnitude of the primary hydration peak decreases while the number of apparent valleys increases. In contrast both the magnitude of the primary hydration peak and the number of apparent valleys increased in the H2SO4 solution. By comparing
15540 | Chem. Commun., 2014, 50, 15537--15540
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these two systems, the correlations between hydration layers and the anion structure making/breaking properties were suggested. Further studies to understand the role of the electrode potential and ions, and their effects on hydration forces are necessary to unveiling the functions of electrolyte/electrode interfaces. This work was partially supported by Grant-in-Aid for Scientific Research No. 20001006 and 12J02584 from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Government of Japan through its ‘‘Funding Program for Next Generation World-Leading Researchers’’ (GR071). T.U. expresses his special thanks for the research Fellow of the Japan Society for the Promotion of Science.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
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