Hydrogen Nanobubbles at Roughness Regulated Surfaces; Why does

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C: Physical Processes in Nanomaterials and Nanostructures

Hydrogen Nanobubbles at Roughness Regulated Surfaces; Why does the Standard Hydrogen Electrode Need a Platinized-Platinum Electrode? Kentaro Kashiwagi, Tamon Hattori, Yudai Samejima, Naritaka Kobayashi, and Seiichiro Nakabayashi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11648 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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The Journal of Physical Chemistry

Hydrogen Nanobubbles at Roughness Regulated Surfaces; Why does the Standard Hydrogen Electrode Need a Platinized-Platinum Electrode?

Kentarou Kashiwagi,a Tamon Hattori,a Yudai Samejima,a Naritaka Kobayashib and Seiichiro Nakabayashi*a,b a

Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Shimo-

okubo 255, Sakura-ku, Saitama 338-8570, Japan b

Division of Strategic Research and Development, Graduate School of Science and Engineering,

Saitama University, Shimo-okubo 255, Sakura-ku, Saitama 338-8570, Japan

ABSTRACT Hydrogen nanobubbles at the surfaces of platinum electrodes are investigated by atomic force microscopy and electrochemical methods. The existence of the bubbles at Pt (111) electrode is revealed by observing the images indicating the repetition of the bubble formation and dissolution, which is synchronized with the electrode potential cycling. The degree of the roughness at the surface of the single crystalline electrode is systematically modified by the oxidation reduction cycles (ORC), and the electroplating of platinum atoms provides the ultimately roughened surface. The nanobubble formation is favored at the polycrystalline electrode with the medium roughened surface (3x103 times ORC). Not only at the atomically flat Pt (111) but also the platinized platinum electrodes, however, the formation of the bubbles is much suppressed. The electrochemical hydrogen reaction is important for the energy standard in thermodynamics, since !G o is defined to be zero for 2H+ + 2e- ! H2 at SHE. Although the preparation of the platinized platinum electrode is much simpler than that of Pt (111), the both of them are concluded to be well designed for minimizing the possible potential fluctuation caused by the surface nanobubbles. These observations are harmonized with the conjecture by the molecular dynamics predicting the nanobubble formation is favorable at the surface with moderate roughness rather than at the surface with low and high roughness.

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1.! INTRODUCTION The nanobubbles at solid and liquid interface have been the subject of numerous studies.1,2 The physicochemical origin of their long lifetime is vital for the scientific consensus.1,3 Despite the fundamental principle of the stability is still unclear, a lot of the experiments reveal the surface nanobubbles are ubiquitous phenomena.1,4 A few protocols have been established for their formation; 1 the spontaneous formation at immersion, the solvent exchange method, the temperature controlling method, and the electrochemical nanobubble formation. All of these methods increase the degree of supersaturation of gas at the liquid and solid interfaces. At the same time, it is also reported that potential artifacts in the nanobubble studies are organic contaminants and blisters, which can be misidentified to the nanobubbles solely based on AFM measurement.5 Among the protocols, the electrochemical methods are contaminant free because the concentration of gases near the surface can be controlled simply by the electrode potential.6,7 The detection of surface contamination is also possible by the electrochemical method, which is highly sensitive to sub-monolayer level.8 White et al . have examined a single nanobubble formed on the top of a Pt micro-electrode, whose radius was ~100 nm.9,10 Despite that the modern electrochemistry has been studied much at the atomically defined single crystalline electrode,11-13 the nanobubbles have been less examined at these well-defined surfaces. The electrochemical hydrogen reaction is strongly dependent on the atomic arrangements at the upper most layer of the single crystalline platinum electrodes.11.13Ì The hydrogen reaction has been also extensively studied at rough platinum surfaces.14,15 IUPAC recommended to use the highest roughness of platinum (the platinized platinum) as the standard hydrogen electrode (SHE).6,16-18 The electrochemical reaction, 2H+ +2e- ! H2 at SHE is the energy standard in thermodynamics.16,19 In considering the protocols for the nanobubble formation, SHE is exactly followed the procedure of the spontaneous formation at immersion to form hydrogen nanobubbles at the electrode surface.1,16 Because of the typical radius curvature of the nanobubble, e.g. 10~100mn, the bubble formation might be sensitive to nanometer-deep surface morphology, and the majority of nanobubbles prefer to be formed in the grooves on the surface.20

Recently, Liu and Zhang have studied the effects of the surface roughness on the formation

of the nanobubble by molecular dynamics.21 At hydrophobic surface, the degree of the surface roughness affects the nanobubble stability. At the surface with moderate roughness, the nanobubble formation is favorable rather than at the surface with low and high roughness.21 Pristine platinum surface is hydrophilic,22 however, the existence of the hydrogen nanobubbles needs to be investigated not only at the platinum micro-electrode9,10 but also at the single and polycrystalline electrodes with larger surface area.


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Peremans and Tadjeddine found very different resonance in the sum-frequency generation (SFG, 1770 cm-1) at overpotential region where hydrogen gas evolution was proceeding.23-25 They attributed this resonance peak to an unknown intermediate in the hydrogen evolution reaction, which might be a footprint of the hydrogen nanobubbles.6,24 Thus, the hydrogen nanobubbles at the electrodes are also important from the view point of the physical electrochemistry. In this article, we examine the electrochemically controlled hydrogen nanobubbles at Pt (111) and roughness-controlled platinum electrodes immersed in 0.1M sulfuric acid aqueous solution.

2.! EXPERIMENTAL SECTION 2.1. Sample Preparation. The electrodes Pt (111) used were made by Clavilier method.26-30 Polycrystalline platinum electrode was made by the potential cycling (electrode potential from -0.25 V to 1.20 V vs. SCE with the potential scanning rate 200 mV/s for 4.4x 104 s) at the single crystal contacted with 0.1 M sulfuric acid aqueous solution. The possible contamination at the electrode surface was removed in advance by the flame-quenching method. The annealed platinum was rapidly quenched in degassed ultra-pure water. The platinized platinum electrode was made by galvanostatic electrodeposition (I = 30 mA/cm2) of platinum atoms at the platinum single crystal ball in aqueous solution containing 5M hydrochloric acid, 0.1M chloroplatinic acid and 10-4 M lead acetate. The identical procedure without addition of lead acetate was used to check the possible artifact by trace amount of lead. 2.2 AFM Measurement. We used our custom-built electrochemical atomic force microscopy (AFM), which was operated with tapping mode by using a commercially available AFM controller (ARC2, Oxford Instruments). The AFM was combined with a potentiostat (HZ-7000, Hokuto Denko) to regulate potential of the Pt (111) electrode (working electrode: WE). Both reference and counter electrodes (RE and CE) were a Pt wire and a Pt mesh. We used a Si cantilever whose backside was coated with Au for photothermal exaction (HQ-300-Au, Asylum Research). ItÕs typical resonance frequency in liquid and spring constant were 150 kHz and 42 N/m, respectively. 2.3 Electrochemical Measurement. HydrogenÌ nanobubbles were electrochemically formed and detected by using a potentiostat (HZ-7000, Hokuto Denko).

All the electrochemical measurements

were conducted in aqueous solution containing 0.1M sulfuric acid. The electrolyte solution was degassed by vacuum with magnetic agitation. The surface area of the electrodes A was obtained by following the methods.26 We used 240 and 210 µC/cm2 for Pt (111) and the polycrystalline platinum electrode including the platinized one, respectively.



In order to form hydrogen nanobubble, the electrode potential was kept -0.2V vs. SCE for t(s). After hydrogen evolution for the duration of t, the total amount of surface trapped hydrogen atoms (under potential deposited hydrogen atoms, UPD H) and hydrogen molecules in nanobubbles were detected by the total oxidation charge of the hydrogen atoms and the molecules. The potential was triangularly scanned after the hydrogen evolution during t. Q1+(t) at the first anodic scan consists of the UPD H and also the hydrogen molecules in the nanobubbles. On the second potential scan with t = 0 while negligible amount of hydrogen gas was evolved, the charge obtained at the second anodic scan (Q2+) only consisted of oxidation charge of UPD H. Then, "Q(t) = Q1+ (t) - Q2+ (t=0) is ascribed to the charge for oxidation of the hydrogen molecules in the nanobubbles.6 "Q (t) is given by "Q (t) = Q1+(t) - Q2+(t = 0)

----------- (1) 12 5&

= #$%&' ()* + %,' () - .*/ 0 13 4 +

67

-----(2)

+

where i1 and i2 are the blue and the red anodic current in Figure S1, respectively. ( dE/dt ) is the speed of the potential scan i.e., the slope of E (t), ( dE/dt ) = 100 mV/s. A collection efficiency of hydrogen molecules in the nanobubbles (!) is depending on t and given as 88888888888888888888888888888888888889()* - :;()*?@A * ------(3) 3

;B)=>?@A C - #J EFGHID% 5 ()*D 6)

-------(4)

where i- (t) is the time course of the cathodic current flowing at -0.2V from t = 0 to 1000s (tfinal). 3.! RESULTS The electrochemical responses of Pt (111) electrode in oxygen purged sulfuric acid (0.1 M) aqueous solution were shown in Figure 1. The cyclic voltammogram (CV) of Pt (111) is depicted in Figure 1a. The symmetric (butterfly) shape of the CV from -0.20 to 0.55 V vs. SCE was identical to that as reported.11-13 The peak appeared from -0.20 to 0.05 V vs. SCE was attributed to the electrochemical deposition and dissolution of UPD H.11-13 The total charge of the UPD H gives the surface area (A) of the electrode by 240 µC/cm2.13,26 The current from 0.05 to 0.20 V vs. SCE corresponded to the adsorption of bisulfate anion and the sharp current peak at 0.18 V was due to the reconstruction of the adsorption layer.11,12 The anodic current appeared from 1.0 V vs. SCE (red line in Figure 1b) was due to the formation of three-dimensional platinum oxide, where the surface atomic layers were reassembled. On the reverse potential scan, the cathodic current peak around 0.5 V (blue line in Figure 1b) indicated


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The hydrogen nanobubbles at the platinum electrode perturb the electrochemical equilibrium by the contribution of the three phases boundary and the big Laplace pressure. The formation and dissolution of the hydrogen nanobubbles decreases in the potential stability as the reference electrode as shown in Figure 8. Without hydrogen bubbling, Figure 8a shows the variation of the cyclic voltammograms obtained at every 2x102 cycles through the repetition of ORC. The oxidation charge of the hydrogen nanobubbles ("Q) was obtained after holding E= -0.2 V for 100 s. The roughness, R is defined R= A/Ao, where Ao is the surface area of Pt (111). In Figure 8b, "Q was plotted with respect to R, which indicates the initial rises (R1.3).

In 0.1M sulfuric acid aqueous solution

under continuous bubbling of 1 atm hydrogen gas, the open circuit potential (!V) of the polycrystalline platinum electrodes with R were shown in Figure 8c with respect to the platinized platinum electrode. !V starts to deviate from 0V at R=1.22, which corresponds to ORC> 2x103. The potential difference starts at the surface (see Figure 2) where the cauliflower like surface appears by the coalescence of the islands. At the cauliflower like surface, the images of the nanobubbles were not obtained by AFM measurement.

But the saturation in "Q (Figure 8b) suggests it is the high-density hydrogen

nanobubbles that is the origin of !V. Although the absolute value of !V is small ~ 1mV, the positive shift in !V implies the cathodic exchange current increase and/or the decrease in the anodic one. In order to maintain the stationary states of the nanobubbles, the loss of hydrogen molecule as the outflux through the nanobubble/electrolyte interface needs to be compensated by the influx of hydrogen molecule at the three phases boundary consisting of Pt/gas/solution interface. These dynamical flux balance phenomenologically harmonized with the increase in the cathodic exchange current.


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4.! DISCUSSION 4.1. Possible origin of hydrogen nanobubbles at platinum electrode. The hydrogen nanobubble in Figure 3 is not in spherical shape, though the shape reported at hydrophobic surface is commonly spherical. The pancake like shape obtained suggests the nanobubble might be more likely solid like hydrogen hydrate clathrate.

The nanobubbles (height; 1~1.5 nm, width; 25~40 nm) in Figure

3 mostly locate in the inner Helmholtz plane at Pt(111) electrode (E = -0.2V).

In the inner Helmholtz

plane, there exist a hydrogen-bound water network, which is attached to the electrode surface with noncovalent interactions.38-40 Hydrogen molecules are formed in layered structure of the 2-dimensional hydration sheets.49,50 The entropy loss to form the hydrogen hydrate clathrate is smaller at the electrode surface than in the bulk water, because the hydration water layers is pre-existing at the surface to wrap the hydrogen molecules. 41,42

Then, nucleation of gas hydrate clathrate is advanced at the interface.

In the bulk, the hydrogen molecules are stabilized in the cage of clathrate hydride at 2.2x103 atm at room temperature.44 At the surface, however, gas clathrate can exist at much lower pressure of two or three orders of magnitude smaller than that in the bulk.41-48 Kaschiev and Firoozabadi demonstrated that the hydrate nucleation preferentially occurred on the solid and liquid interface.43 The big Laplace pressure 150 atm in the bubble with the small radius curvature9 suggests the hydrogen hydride is one of the candidates as the chemical substances in the bubble. The unidentified resonance peak (1770 cm-1) in SFG by Tadjeddine23-25 was plausibly correlated to these hydrogen nanobubbles, since both of them appeared at the hydrogen overpotential region. Tadjeddine tentatively assigned this peak to an unknown intermediate of the hydrogen evolution.Ì The SFG peak at the frequency (1770 cm-1) appeared independently on the electrode surface, Pt (100), (110), (111) and also polycrystalline platinum, and also that the variation of the electrode potential in the over potential induced no Stalk shift in the frequency, which commonly appeared for the surface adsorbed species in the electric double layer.23-25 The SFG peak might be assigned to the hydrogen molecules in the water cage i.e., the nanobubbles / nano-pancakes in Figure 3 and 4. The nanobubble is not a reaction intermediate in the hydrogen evolution reaction because of very slow kinetics for the formation of the bubbles (~100s in Figure 6c and 7). But, the hydrogen nanobubbles is plausibly spectator, which inhibits the transport of the reactant (hydronium ion in the bulk) to the electrode surface. Conway proposed OPD H (over-potentially deposited hydrogen) that cover platinum surface in the hydrogen evolution potential region.51 They regarded OPD H is a weakly adsorbed state of hydrogen. Unfortunately, relatively little is known about the true nature of ODP H. At this stage, we cannot identify the relationship between the hydrogen nanobubble and OPD H.


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4.2.

Electrode potential shift caused by hydrogen nanobubbles.Ì ElectrochemicalÌ potential

(E) of hydrogen electrode is given as

E ="

2.303RT RT pH " log e PH 2 ----------(5) F 2F

where R,T and F are gas constant, temperature and Faraday constant, respectively. If the electrochemical equilibrium were homogeneous under conditionsÌ (pH=1 and PH2=1atm), E would be zero, which is identical to the definition of SHE. However, if it is heterogeneous (ascribed to the existence of hydrogenÌ nanobubbles), E differs from zero, since Laplace pressure makes the internal pressure of hydrogen in nanobubble much larger than 1 atom. Judging from the shape of nanobubbles in Figure 4c, PH2 is roughly estimated to 150 atm. If this value was simply substituted to eq.(5), E would be -61mV, which is three orders of magnitude larger than the observed potential difference in Figure 8. Moreover, the direction of the potential shift is positive in Figure 8, nevertheless the direction estimated by eq. 5 is negative. In the heterogeneous conditions, Nernst equation (5) is not valid, since the equation is applicable only for the homogeneous electrochemical equilibrium. The hydrogen nanobubbles can exchange electrons between the electrode only at the circumference i.e., the three phase (solid/liquid/gas) boundaries. Based on this, White presented dynamic equilibrium model to explain the stability of a single H2 nanobubble at micro-platinum electrode.9 At the equilibrium condition, the loss of hydrogen molecule as the outflux through the nanobubble/electrolyte interface needs to be compensated by the influx of hydrogen molecule at the three phases boundary. As shown in Figure 5, !(t) monotonically decreases, but in order to keep the steady state of the bubbles, small residual current for the hydrogen evolution is necessary to compensate the H2 outflux through the gas/liquid interface. At the equilibrium potential of platinum electrode with H2 nanobubbles, electron transfer for anodic and cathodic directions are balanced and the overall current should be zero. White has estimated the cathodic current at the three phases boundary of a single H2 nanobubble.

The equilibrium potential of

platinum electrode with H2 nanobubbles needs the total length of the bubble circumference and the electron transfer rate on it. At this stage, the direction of the potential shift in Figure 8 can be explains qualitatively by the dynamic equilibrium model. Since the hydrogen nanobubble is the spectator, the anodic current (H2¾> 2H+ + 2e- ) flows only at the pristine platinum surface, but the cathodic current (2H+ + 2e- ¾> H2 ) flows not only at the pristine surface but also on the three phases boundary. Then, the cathodic current is larger than anodic one at platinum electrode with H2 nanobubbles. This increase



in the cathodic exchanging current shifts the equilibrium potential toward the anodic (positive) direction as shown in Figure 8.

5.! CONCLUSIONS The electrochemical reaction at SHE is the energy standard in thermodynamics. Since the redox potential of half-cell cannot be measured in isolation, it has to be measured relative to the potential of other half-cell, which results in a thermodynamic ladder of the relative redox potentials. This ladder is anchored to SHE.16,19 The IUPAC recommendation to use the platinized platinum electrode for SHE is reasonable as decreasing the nanobubble effect as small as Pt (111). Although the preparation of the platinized platinum electrode is much simpler than that of Pt (111), the both of them are concluded to have the well-designed surfaces for minimizing the possible fluctuation caused by the surface nanobubbles.


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ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author *E-mail (S.N): [email protected]

ORCID Seiichiro Nakabayashi: 0000-0001-7427-0407 Naritaka Kobayashi: 0000-0003-0419-7974

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was partly supported by a Grant-in -Aid for Scientific Research (18H01806, 17H02734, 18K19052) of JSPS.

Supporting Information Available Comparison of CVs with and without nanobubble in Figure S1. Comparison of CVs between Pt (111) and platinized platinum in Figure S2.



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Hydrogen Nanobubbles at Roughness Regulated Surfaces; Why does

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