Force Spectroscopy Revealed a High-Gas-Density State near the

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Force spectroscopy revealed a high gas density state near the graphite substrate inside surface nanobubbles Shuo Wang, Limin Zhou, Xingya Wang, Chunlei Wang, Yaming Dong, Yi Zhang, Yongxiang Gao, Lijuan Zhang, and Jun Hu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03383 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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Schematic for AFM force spectroscopic setup for probing the gas density inside nanobubbles

ACS Paragon Plus Environment

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Figure 2. Morphology and force curves obtained in different conditions.


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Figure 3. Estimation of gas density inside nanobubbles using the adhesion data.


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MD simulation results of nitrogen nanobubble formation on the patterned surface.


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Force spectroscopy revealed a high gas density state near the graphite substrate inside surface nanobubbles Shuo Wanga,b,c#, Limin Zhoua,d#, Xingya Wanga,f, Chunlei Wanga, Yaming Dongd , Yi Zhanga, Yongxiang Gaob, Lijuan Zhang*e and Jun Hu**a aKey

Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China bInstitute

cKey

for Advanced Study, Shenzhen University, Shenzhen, 518060, China

Laboratory of Optoelectronic Devices and System of Ministry of Education and

Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China dUniversity

of Chinese Academy of Sciences, Beijing 100049, China

eShanghai fShanghai #Shuo

Normal University, Shanghai 200234, China

Synchrotron Radiation Facility, Shanghai 201204, China

Wang and Limin Zhou contributed equally to this work. *Correspondence to *[[email protected]] **[[email protected]]

Abstract The absorption of gas molecules at hydrophobic surfaces may have a special state and play an important role in many processes in interfacial physics, which has been rarely considered in previous theory.

In this paper, force spectroscopic experiments were performed by a nano-sized

AFM probe penetrated into individual surface nanobubbles and contacted with a HOPG substrate. The results showed that the adhesion force at the gas/solid interface was much smaller than that in


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air measured with the same AFM probe. The adhesion data was further analyzed by the van der Waals force theory, and the result implied that the gas density near the substrate inside the surface nanobubbles was about three orders of magnitude higher than that under the Standard Pressure and Temperature (STP). Our MD simulation indicated that the gas layers near the substrate exhibited a high density state inside the surface nanobubbles. This high density state may provide new insight to the understanding of the abnormal stability and contact angle of nanobubbles on hydrophobic surfaces, and have significant impact on their applications.

Introduction Surface nanobubbles are nanoscopic gas bubbles formed at the liquid-solid interface. They are fundamentally important in interfacial science, and has often been associated with various phenomena occurred at the solid-liquid interface, such as boundary slip11,

12

and rupture of the

wetting film13. Furthermore, they have many potential applications, including surface cleaning14, 15, 16, 17,

mineral floating18 and hydrogen storage19. Surface nanobubbles were first proposed to

exist to explain the steps observed in the measured forces between hydrophobic surfaces in water1. Later, they were directly observed by atomic force microscopy (AFM) on a few different substrates2,

3, 4.

Their existence has also been confirmed by degassing

5, 6,

and many other

techniques7, 8, 9, 10.

Nanobubbles are strange and quite different from the conventional bubbles. The morphology of nanobubbles has been shown to be a spherical cap with a curvature radius in the range of 100-1000 nm, measured by AFM. Putting into consideration the Laplace pressure arising from the curved interface and the resulting increase in gas solubility, these nanobubbles should dissolve in air-saturated solution rapidly (less than tens milliseconds) according to the classical diffusion theory20. However, it has been observed in experiments that nanobubbles are stable for hours21. Besides, the contact angle of nanobubbles generally ranges from 150° to 170° on hydrophobic surfaces5, which is much larger than that of macroscopic bubbles on the same substrates. To explain

these

two

exotic

properties,

several

theories


have

been

proposed,

e.g.,

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dynamic-equilibrium22, 23, contamination layer24, and high density inside nanobubbles25. Recently, Zhang and Lohse pointed that the Laplace pressure balanced by gas oversaturation in the liquid, and pinning of the contact line is a necessary condition to achieve the stability and large contact angle26, 27. However, some problems remain, e.g., the lifetime of surface nanobubbles in degassed water was found to be much longer than that predicted by theory28.

Another mystery is the gas density inside nanobubbles. By applying the state equation of ideal gas, the gas density inside nanobubbles can be calculated as:

 / 0  2 / RP0  1 , where

 / 0 is gas density inside nanobubbles normalized by gas density at ambient pressure,  is the surface tension of water, R is curvature radius of bubble, P0 is the ambient pressure. For a nanobubble with a 100 nm curvature radius, the density

 / 0 is about 15. The high density

state of nanobubbles has also been revealed in many molecular dynamical simulations29,

30, 31.

Experimental verification of the existence of high density state of gas inside nanobubbles is important since they may lead to many potential applications, e.g., high gas density nanobubbles could largely enhance the efficiency of oxygen delivery and hydrogen storage, enable some extreme chemical reactions to happen under normal conditions, and have unique physiological effects, etc.

However, the experimental determination of the gas density inside nanobubbles is very difficult because nanobubbles distribute heterogeneous on surfaces with low coverage, and most spectroscopic based techniques do not have the nanoscale resolution to collect signal from an individual nanobubble. Only preliminary results have been obtained. The first measurement of gas density inside surface nanobubbles came from Zhang et al. By imaging CO2 nanobubbles on hydrophobic surface using infrared spectrum with an attenuated total internal reflection configuration (ATR-IR)9, they could observe the fine rotational structural characteristic of CO2 molecules in the gaseous state inside nanobubbles. The density of gas was estimated to be 44±16 mol/m3 (1.2±0.4 kg/m3) from absorbance, which was in the range of density of ideal gas at standard temperature and pressure (STP). Note that the FTIR technique does not allow one to image and identify nanobubbles with high resolution and the estimated density was an ensemble


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average of all bubbles absorbed on the interface. Recently, Schlesinger et al. have imaged the fine structure of condensed gas layer on hydrophobic surface in water with high resolution FM-AFM32, and they obtained a density about 300 times larger than nitrogen at STP. Synchrotron based scanning transmission soft X-ray microscopy (STXM) may be a potential technique to reveal the inner density of a single nanobubble since it has both the nanoscale spatial resolution and the chemical sensitivity. Recently, Zhang et al. has reported a new water structure inside nanobubbles by using STXM, but they could not determine the gas density because of the low signal/noise ratio.33

Force spectroscopy has been widely used in the field of nanobubble research. For example, Yang et al. studied the friction reduction behavior of an AFM probe sliding through nanobubbles on HOPG.34 In order to estimate the gas density inside surface nanobubbles, here we propose to perform standard force spectroscopy experiments by using AFM. When a nanoprobe penetrates into a nanobubble and contacts with the substrate, the van der Waals interaction between the probe and the substrate can be described as35,

Fadhesion  

AR 6D2

(1)

Where R is probe radius, D is separation between probe and substrate, A is the Hamaker constant determined by the dielectric properties of the medium between the probe and substrate, which is directly related to the density of the medium. However, due to the finite spring constant of the cantilever used in the force spectroscopic experiments, the force curve becomes discontinued when the gradient of the force is larger than the stiffness of the cantilever, which makes it very difficult to measure the attractive force accurately at very close distance. Herein, we propose a method by comparing the adhesion force captured in air and inside the nanobubble, to extract the density information inside the nanobubble. This method may not be accurate in the low gas density regime, but if there is a high density inside the nanobubble, we expect that an obvious difference between the adhesion forces would be observed.

In this paper, we measured the adhesion forces of the AFM tip contacted with HOPG substrate


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inside the surface nanobubbles. By comparing them with those obtained in air with the same tip on the same HOPG substrate, we found that the adhesion force inside the nanobubbles was much smaller than expected. This abnormal adhesion was further analysis by the Lifshitz theory of van der Waals force, and the gas density near the substrate inside nanobubbles was estimated to be approximately three orders of magnitude larger than that of air at STP. Our MD simulation indicated that there might exist a new condensed phase for gas molecules inside the surface nanobubbles .

Figure 1. Schematic for AFM force spectroscopic setup for probing the gas density inside nanobubbles. A nanosized probe penetrated into the surface nanobubble and contacted with substrate, as the probe pull off from the substrate, the adhesion force was measured. The adhesion was also measured in air in a similar setup with the same probe and substrate.

These two forces

were compared to provide the gas density inside nanobubbles.

Experimental Section Materials and apparatus Millipore water was obtained from a USF-ELGA Maxima water purification system. Ethanol (>99%, GR) was purchased from Sinopharm Chemical Reagent Co., Ltd. Decane solution was


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prepared by mixing ethanol and Decane (99% Sigma-Aldrich) in a volume ratio of 1000:1. Hightly Ordered Pyrolytic Graphite (HOPG, 1.2×1.2 cm2, ZYH grade, Bruker) was used as the substrate freshly cleaved before use. The force spectroscopy data were acquired on a Bruker multimode 8 AFM equipped with a Nanoscope 5 controller. Silicon Nitride probes (DNP-10, Bruker) with nominal spring constant 0.35 N/m were used. The spring constant of the cantilevers was calibrated using a built-in thermal noise method. The cantilevers were treated with Plasma Cleaner (Harrick Plasma, Plasma Cleaner PDC-32G) for 1 minute beforehand.

Force measurements First, the force curve obtained in air with temperature and humidity controlled by a homemade glove box. A piezo stage was used to adjust the separation between the probe and the HOPG substrate. The force was measured by a cantilever connected to the probe.

To measure the adhesion force inside surface nanobubbles, the same probe was used in both experiments. Nanobbubles were prepared on the HOPG surface by an ethanol/water exchange method using two glass syringes topped with metal needles. First, a drop of 400

 L ethanol was

placed on the substrate. Two syringes were then used to simultaneously introduce in and pump out liquid volume of 400

 L water for exchange. The process was repeated three times before the

sample was transported to the AFM stage. It should be noted that all fluids were handled with glass syringes, and no polymeric materials were involved during the whole process, including the O ring and the silicon tubing. After imaged the stable nanobubbles using PeakForce Tapping, the AFM was switched to the force volume mode. The force curve data was obtained from complete force curve cycles at frequencies of 5~10 Hz scanned across the nanobubbles.

In the control experiment, we first measured the adhesion in pure decane on HOPG. Then decane nanodroplets were produced by a similar solvent exchange method as described in refs36, imaged with PeakForce Tapping, and the force data were acquired using the same procedure described above. These data were presented in the Supporting Information.


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Exclusion of contamination It is well known that the force curve is dependent on the geometry of the probe and contaminants absorbed on the surface. Therefore, the force spectroscopy data have to be analyzed with care for quantitative statements. During our experiments, the shape of the force curve was very sensitive to the contamination. If some high molecular weight chains adsorbed on the probe or the substrate, the force curves showed a broad minimum as it was pulled off from the substrate. In this case, the adhesion could no longer represent the van der Waals interaction between the probe and the sample, and these force data were discarded.

To further confirm the observed nanobubbles were not nanodroplets from some insoluble organic liquids, we checked the force curves on the bubbles in the nonlinear and hysteresis region, which has been used to distinguish nanobubbles from nanodroplets in some recent work37, 38. In addition, when degassed ethanol was used for solvent exchange, no nanobubbles were formed. We also compared our force data with those in literatures, and found that they were in good agreement (Table 2). All these confirmed that the observed nanobubbles were not nanodroplets.

Simulation Methodology To simulate the nucleation process of surface nanobubbles, we created a patterned substrate (9.86 nm×9.96 nm) of three layers of atoms using hexagonal lattice. The top two layers consisted of two kinds of solid particles. One is carbon atom, and the other is a modified Lennard-Jones particle with a stronger interaction with water molecules. The carbon atoms formed a circular region with a diameter of 8.02 nm in the center of the first two layers, as shown in Figure 4a. This heterogeneous structure could provide a pinning force on the three phase line during the formation of the nanobubble, which was important for nanobubble nucleation and stabilization39. The bottom layer was built with only modified Lennard-Jones particles. The gaps between two neighboring


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layers are 0.34 nm. The plane was placed in the center of the box and fixed during the simulation. The Lennard-Jones parameters were given in Table 1.

All the MD simulations were carried out using the open source software Gromacs (2018 version)40. Periodic boundary conditions were applied to all directions. In the initial configuration, a total of 1000 nitrogen molecules were randomly placed in the box (11nm×11nm×11nm). Then the system was solvated with 34400 water molecules. The whole system was first equilibrated at NVT ensemble for 100ps with Berendsen thermostat to reach the target temperature (300K). Then, a 20ns NPT equilibration was performed with the temperature coupling Nosé-Hoover approach and the Parrinello-Rahman pressure coupling method to produce correct canonical ensemble. It should be noted that in the simulation the Parrinello-Rahman pressure coupling was performed in the isotropic case. The box vector only changes in z direction during the NPT equilibration. The 20ns NPT equilibration was enough for our system to reach equilibrium as discussed in the supporting information. After this step, another 2ns NPT ensemble was produced and the data was collected every 2ps for further analysis. A cut-off of 1 nm was employed to calculate the van der Waals interactions and short-range electrostatic interactions. The SPC/E water model was used. The particle-mesh Ewald method with a real-space cut-off of 1nm was used to treat long-range electrostatic interactions.


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Table 1. The Lennard-Jones parameters used in the simulations. The LJ parameters between different atoms were calculated by taking the geometric average. Atom types

𝜎(Å)

ε(kJ/mol)

Sp2 Carbon (yellow)

3.28

0.121

Hydrophilic atom (cyan)

3.28

0.628

Nitrogen

3.25

0.711

Oxygen (in water)

3.17

0.651

The same parameters were widely used in the literature for MD simulations. Since the results are highly dependent on parameters used in the simulation, we verified that these force parameters could reproduce some of the observed material properties, e.g., the phase diagram of liquid nitrogen, the Henry constant of nitrogen in water, the contact angle of water droplet on graphite and the isotherm of nitrogen on graphite41.


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Results and discussion

Figure 2. Morphology and force curves obtained in different conditions. (a) Force-separation curves captured on the air/HOPG interface (at humidity 99%; left) and the water/HOPG interface (right) with the same probe. For clarity, the force-separation curve for the water/HOPG interface has been shifted to the right by 30 nm. It should be noted that the pull-off force in air was much larger that in water. (b) Force curve measured on the central region of one nanobubble (label # in the inset). The probe snapped into contact with the bubble at separation about 40 nm, after which the three phase line slipped on the probe, and snapped into contact with substrate. As the motion was reversed, the probe first snapped off contact with the substrate, and then snapped off contact with the bubble at a separation of 75 nm. The magnitude of the adhesion is 0.7 nN, as indicated by the two arrows. The influence of the three phase line at the probe was subtracted.


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First, we compared the force curve obtained in water and in air. A typical force curve on HOPG surface obtained in air (99% humidity) was shown in Figure 2a (left curve). The interaction force was plotted against the separation between the probe and the substrate. The measurement started with the sample far away and the cantilever was in its rest position. As the sample was moved toward the probe, the cantilever snapped into contact with the sample once the gradient of the attractive force exceeded the cantilever spring constant. The force increased linearly after the snap-in, until reached its maximum. When the probe was withdrawn from the surface, the probe jumped off contact with the substrate at 4.6 nN. The same experimental procedure was used in water, and the characteristic of the force curve was similar with that in air. However, the adhesion force (2 nN) in water was much smaller.

The typical force curve captured on one nanobubble was shown in Figure 2b. The probe snapped into contact with the bubble surface at a separation of about 40 nm. Then the three phase line slipped along the probe as the probe penetrated into the bubble. The force response was nonlinear in this region. A second snapped in occurred as the probe approached to the substrate. As the motion is reversed, the probe first jumped off contact with the substrate. During the retraction motion, there was a large hysteresis in this region. Finally, the probe jumped off contact with the bubble at a separation of about 75 nm. It should be noted that the large hysteresis in this region was caused by the contact angle hysteresis of the three phase line on the probe. After the probe was pulled off abruptly from the substrate, the contact angle jumped to a new value.

In order to measure the adhesion force inside the surface nanobubbles, the influence of the three phase line as the probe penetrated into the nanobubbles must be subtracted. As the probe contacted with the substrate, the three phase line at the probe exerted an upward capillary force, which could be estimated before the probe snapped into contact with the substrate. Thus, we recorded the difference as the adhesion force in the force curve just before the probe snapped into contact and jumped off contact, as indicated by the two arrows in Figure 2b. The distance between the snap-in point and the substrate was about 2 nm, leading to an error about 20 pN during the procedure, which was negligible comparing to the magnitude of the adhesion force (average 1.2 nN). It should be noted that after the subtraction, the adhesion force inside nanobubble was surprisingly


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low as compared with that in air (in this case, 0.7 nN inside bubble and 4.6 nN in air), as shown in Figure 2b. The statistics was provided in Figure 3a.

Figure 3. Estimation of the gas density inside nanobubbles using the adhesion data. (a) Histograms of adhesion inside nanobubbles. (b) The gas density of nanobubbles as a function of the normalized adhesion inside nanobubble, according to the model in the main text. The measured adhesion fell in the grey region (1.0±0.3)×103, (1.7±0.3)×103 kg/m3, which indicated the gas density was about 3 orders of magnitude larger than ambient air.

To confirm this large difference in adhesion behavior was not experimental artifact, we performed a control experiment by using decane nanodroplets. The adhesion in decane nanodroplets was


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approximately the same with that in pure decane, as expected. The adhesion inside nanobubbles was close to that in decane for the same probe. However, the behavior of the force curve on nanobubbles was quite different from that on decane nanodroplets (see Supporting Information for details). We also checked the available force data in the literature since 2006, and found that all the experiments showed a very low adhesion inside surface nanobubbles, as shown in Table 2. Although different probes and substrates were used in those studies, the adhesion forces were generally below 1 nN in most experiments. The reason for rather large adhesion (3.5 nN) by Walczyk et al might be due to the use of a hydrophobic probe. Overall, experimental results with different probes and substrates were consistent with ours.


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Table 2. Available literature data on the adhesion inside nanobubbles measured by AFM. All experiments have showed a very low adhesion inside surface nanobubbles and most of them were well below 1 nN. The reason for rather high adhesion (3.5 nN) of Walczyk et al might be due to the use of a hydrophobic probe. The probe radius were obtained from the products’ webpages.

Ref.

Probe

(tip

radius)

Adhesion force inside bubble (nN)

/substrate materials Zhang

et al. 20065

SiN probe (20 nm) on OTS

0.1 nN

silicon Walcyzk et al. 201442 Walcyzk

et

al42.2014

SiN probe (20 nm) on HOPG

0.2 nN

Hydrophobic probe (20 nm)

3.5 nN

(hydrophobic probe)

on HOPG

An et al. 201543


0.3 nN

An et al. 201638


0.7 nN

Wang et al. 201644

SiN probe (15 nm) on PS

0 nN

For a hydrophobic surface, the interaction is dominated by the van der Waals force and the capillary force between the probe and substrate was negligible. We measured the force with varied humidity and found the adhesion was almost constant, as shown in Figure s1. According to the Lifshitz theory of van der Waals force, the non-retarded attractive force between a sphere and a plate surface is described by Equation (1), which is valid for D

Force Spectroscopy Revealed a High-Gas-Density State near the

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