Infrared Spectral and Dynamical Properties of Water Confined in

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Infrared Spectral and Dynamical Properties of Water Confined in Nanobubbles at Hybrid Interfaces of Diamond and Graphene: A Molecular Dynamics Study Abhijit Kayal and Amalendu Chandra* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, India 208016 S Supporting Information *

ABSTRACT: In this work, we have studied the behavior of water molecules confined in nanobubbles at the hybrid interfaces of graphene and diamond surfaces at varying temperatures. We have performed molecular dynamics simulations to investigate different spectral and dynamical properties of the confined water. The confined water molecules are characterized through calculations of their vibrational spectral properties. The spectral features are found to change significantly with variation of temperature and density of the nanobubbles. The calculated vibrational spectral results are found to be in reasonably good agreement with available experiments. Furthermore, we have looked at the dynamical properties of water molecules in the graphene nanobubbles. The current results reveal the presence of strong heterogeneity in the dynamical properties in nanobubbles at supercritical temperature. Water molecules that are confined in small nanobubbles at high density are found to possess very slow relaxation time scales because of stronger hydrogen bonding and spatial constraints. These water molecules can be considered as essentially irrotational water molecules. In other cases, water molecules confined in nanobubbles at lower density at supercritical temperature are found to show very fast relaxation time scales as the thermal energy dominates the dynamics of water molecules in these cases.

1. INTRODUCTION Graphene, an allotrope of carbon, is a two-dimensional singleatom-thick layer of carbon atoms bonded in a hexagonal lattice. The two-dimensional flat graphene surface is the thinnest elastic membrane found in nature until date. This flat surface can be strain-engineered to form ripples or bubbles due to its softness. Experimentally, first rippled graphene surface was observed by Meyer et al.1 By transmission electron microscopy, they revealed that suspended graphene sheets are not flat but possess intrinsic microscopic roughening. The ripples in a graphene surface strongly influence its electronic properties. The strained surface makes the flat lying 2p orbitals distorted, which in turn, creates midgap states between the conduction and valence bands and enhances the charge transfer processes. Formation of bubbles has been commonly found at the silicon oxide/graphene interfaces in large flakes.2 Although it is still not well understood how these graphene nanobubbles are formed, it has been proposed that air or hydrocarbon residuals are trapped between the graphene and substrate surfaces.3 Loh and co-workers4 showed that lattice mismatch between the graphene and diamond surfaces at high temperature can create graphene nanobubbles that can trap water molecules at high density. At high temperature, covalent bond formation between the graphene and diamond surfaces leads to nano pockets where small clusters of water molecules can be trapped inside. A recent study also looked at the polymerization of buckministerfullerene in pressurized nanobubbles which is symmetrically forbidden under ambient conditions.5 These © XXXX American Chemical Society

bubbles or nanocavities can be used for trapping different kinds of molecules. Stolyarova et al.2 created graphene bubbles at the graphene/SiO2 interface by using exfoliated graphene mounted on a SiO2/Si substrate and subjected to HF/H2O irradiation. Their experiments showed that not only these bubbles are capable of containing gas molecules but also help in effective mass transport at the graphene/SiO2 interface. In addition to the studies mentioned above, there have been a number of other experimental6−11 and theoretical12−30 studies on the structure and dynamics of water in nanoconfined environments over the last few decades. However, a majority of the existing studies have considered water molecules inside carbon nanotubes, graphene sheets, or biological pores. There have been very few studies on the structure and dynamics of water confined in nanopockets between hybrid solid surfaces. Recently, Loh and co-workers4 carried out an elegant study of the graphene−diamond hybrid interfaces at high temperature and pressure and observed the formation of graphene nanobubbles. The lattice mismatch between the two surfaces induced periodic rippling in graphene sheet and, at higher temperatures, covalent bond formation took place between the hydrogen terminated diamond and graphene surfaces. Atomistic simulations based on reactive force field also showed the existence of covalent bond formation between graphene and Received: July 13, 2017 Revised: October 5, 2017

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DOI: 10.1021/acs.jpcc.7b06911 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Snapshot of a representative system from the simulation. The system is for 298 K.

hydrogen terminated diamond surfaces at 1100 K.4 The formation of interfacial bonding facilitated the growth of high density nanobubbles in the diamond−graphene interfacial region. Infrared spectroscopic studies on these graphene nanobubbles showed that the residual water from graphene was trapped inside the newly formed bubbles at the interfaces. The impermeability of the graphene forced the water molecules to remain trapped inside these bubbles. Infrared studies revealed the emergence of five major peaks at supercritical conditions (673 and 773 K) which were named as P1, P2, P3, P4, and P5 and their characterization was as follows: (1) P1 at ∼3650 cm−1 is the free OH frequency at the bare diamond surface. (2) P2 at ∼3510 cm−1 is the OH frequency of supercritical water. (3) P3 at ∼3220 cm−1 for water molecules condensed inside the bubbles. The red-shifted frequency means relatively weaker OH bonds due to enhanced hydrogen bonding. (4) P4 and P5 at 2800−3000 cm−1 represent the clusters of water molecules that are trapped in small regions at high density and forced to form strong hydrogen bonds. The P2, P4, and P5 peaks appeared at supercritical conditions, and these peaks showed the highly inhomogeneous nature of the trapped water molecules. In these graphene nanobubbles, some water molecules were highly condensed inside a very small confined region and some water molecules were reported to be just at the flat part of the graphene surface without forming clusters of high density.4 In the current work, we have studied the behavior of water molecules confined in nanobubbles formed by graphene and

diamond surfaces at varying temperatures. We performed molecular dynamics simulations to investigate the different spectral, structural and dynamical properties of trapped water molecules. The nature of these confined water molecules is first characterized by calculating their vibrational spectroscopic properties. Subsequently, we have looked at the dynamical properties of trapped water molecules inside the nanobubbles. Specifically, we have investigated the orientational relaxation of dipole and O−H bond vectors and also the hydrogen bond dynamics of water molecules confined in nanobubbles. The organization of the rest of the paper is as follows. In section 2, we have described the details of models and simulation methodologies that are used in this work. In subsection 2.1, we have described the mathematical modeling of graphene nanobubbles which is followed by modeling of vibrational spectra of OH modes of water in subsection 2.2. Subsection 2.3 deals with the simulations methodologies. Next, in section 3, we have presented the distributions of OH frequencies for different systems and compared them with experimental spectral results. Section 4 deals with dynamical properties such as OH and dipole relaxation of confined water molecules calculated from different time correlation functions. In section 5, we have discussed the results of hydrogen bond dynamics. Finally, in section 6, we have included a brief summary of the main results of this work and our conclusions. B


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2. METHODS 2.1. Modeling of Graphene Nanobubbles. In Figure 1, we have shown a typical graphene nanobubble which has a Gaussian shape. Because of the elastic nature of graphene, these graphene nanobubbles are formed with different shapes and sizes in experimental conditions.3 In the current simulations, the atomic coordinates of a nanobubble are generated from a Gaussian distribution of z coordinates of the carbon atoms as given by the following equation

In the smaller cavities, we put an excess number of water molecules to create a high density of water in these cavities. The system details are given below: (1) T = 298 K, height = 15 Å, diameter = 64 Å, and number of water molecules = 345. (2) T = 373 K, height = 10 Å, diameter = 36 Å, and number of water molecules = 120. (3) T = 573 K, height = 5−8 Å, diameter = 15−20 Å, and number of water molecules = 22−40. (4) T = 673 K, height = 5−8 Å, diameter = 10−15 Å, and number of water molecules = 12−20. (5) T = 773 K, height =4−8 Å, diameter =10−15 Å, number of water molecules =12−20. In case of systems 3, 4, and 5, we have considered more than five different graphene bubbles with varying number of water molecules so as to have varying density of water molecules trapped inside nanobubbles of different sizes. Further details of these bubbles of systems 3, 4, and 5 and the simulation set up are given in the Supporting Information (Tables S1−S3). We have considering these multiple bubbles of varying sizes and density to mimic the experimental conditions at higher temperatures. The equations of motion were integrated using the leapfrog algorithm with a time step of 1 fs. The equilibration for each system was done in the NVT ensemble using the Nosè−Hoover thermostat37,38 for 5 ns and then the data collection for the production run was done for 2 ns. The diamond and graphene atoms were kept fixed at their initial positions during the simulations. However, in order to see the effects of flexibility of the graphene nanobubbles, we have also performed additional simulations of some chosen smaller nanobubbles where the carbon atoms of the bubble surfaces were allowed to move. The parameters of these flexible nanobubbles were taken from ref 39. The simulations were performed at three different temperatures for the same waterfilled nanobubbles using both flexible and fixed models of the C−C bonds. Further details of simulations and results of these flexible nanobubbles are presented in the Supporting Information. We have also performed a simulation of bulk water with 894 SPC/E water molecules in a cubic box of edge length of 30 Å.

z(x , y) = h × exp{−[(x − x0)2 + (y − y0 )2 ]/(2λ)2 } (1)

where (x0,y0) represent the x and y coordinates of the center of the bubble. The height of the bubble is denoted by h and the lateral dimension is denoted by λ. The typical height of graphene nanobubbles is around 20 Å and diameter is about 50−300 Å in experimental conditions.4 2.2. Spectroscopic Modeling. In order to describe the deformation of OH stretch potential by external forces, we have followed the model introduced by Oxtoby.31,32 The frequency shift of an OH bond is calculated from the mass weighted intermolecular force (F1) projected along the O−H bond vector as ⎛ F⃗ F⃗ ⎞ F1 = μ⎜ O − H ⎟ rOH ⃗ mH ⎠ ⎝ mO

(2)

where μ is the reduced mass, F⃗O and F⃗H denote the total forces on the O and H atoms, respectively, of a particular O−H bond and rO⃗ H is the unit vector along the O−H bond. Using perturbation theory, one can relate the frequency shift with the mass weighted force (F1) as33 δω(t ) =

3f 2

μ ω03

F1(t )

(3)

where ω0 is the unperturbed gas phase frequency and f is the third order expansion term of the potential energy surface. This third order expansion term f can be obtained by assuming a Morse potential for the O−H stretch and can be connected to the anharmonic shift Δ using perturbation theory and eq 3 can then be rewritten as32 δω(t ) =

3 2ω0

Δ F1(t ) ℏμ

3. VIBRATIONAL FREQUENCIES OF O−H STRETCHING MODES We have characterized different types of water molecules in the nanobubbles by their vibrational spectral features. The O−H stretching modes are known to be highly sensitive to their local environments. The probability distributions of O−H frequencies of water molecules in the bulk and confined system 1 are shown in Figure 2. As expected, the bulk water shows a broad distribution from 3200 to 3500 cm−1. System 1 shows clearly two peaks corresponding to distributions of two different types of water. The broad distribution from 3200 to 3500 cm−1 (Figure 2b) arises from hydrogen bonded water molecules that are trapped inside confined regions and we call the corresponding peak as P3 by following the notations used in ref.4 We will follow the notations of ref.4 for discussions of other peaks also which are presented below. The other peak of system 1, around 3700 cm−1, called P1, arises from the free OH modes of water that are located near the surfaces. For system 2 which is at 373 K, another peak emerges at 3300 cm−1 which is due to enhanced condensation of water molecules in the confined regions. At the higher temperature of 673 K, the new

(4) −1

We have taken Δ = 240 cm according to the literature value.33 2.3. Simulation Details. All the simulations were performed by using the GROMACS 4.5.6 simulation package.34 Initially, the numbers of water molecules inside the bubbles were determined by performing individual simulations with graphene bubbles in contact with water bath but without the diamond surface at the bottom of the bubbles. In our simulations, we have used SPC/E35 model for water and we have used the parameters of sp2 carbon atoms of the Amber96 force field36 for the graphene carbon atoms. The bubbles were gradually filled up by water molecules after starting of the simulations. Then, we attached the diamond surface at the bottom of the graphene sheets to trap water molecules inside the bubbles. We have considered graphene bubbles of different lengths and diameters in the current study. To mimic the experimental conditions at higher temperatures, we have considered a set of Gaussian shaped cavities of varying sizes. C


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experimental work,4 the origin of these lower frequency spectral features can be attributed to the clustering of water molecules with enhanced hydrogen bonding due to spatial constraints. In the current simulations, we observe a similar phenomenon for our model systems. Water molecules which are trapped inside the nanobubbles of high water density show the maximally shifted OH stretching frequencies. A similar type of spectral feature is also observed at the higher supercritical temperature of 773 K.

4. DIPOLE AND O−H ROTATIONAL DYNAMICS In this section, we present the results of dipole and O−H rotational relaxation times of water molecules in different regions of the confined systems. The rotational relaxation time scales are calculated from the time correlation function Cαl (t), which is defined as Clα(t ) =

⟨Pl(e α(t) ·e α(0))⟩ ⟨Pl(e α(0) ·e α(0))⟩

(5)

where Pl is the Legendre polynomial of order l and eα is a unit vector pointing toward the molecular dipole or along an O−H bond of a water molecule. We have calculated the second order (l = 2) orientational time correlation functions from simulation trajectories. It is to be noted that the experimentally measured rotational anisotropy is directly related to the second-rank rotational function C2(t). We have divided the water molecules into four categories (P1, P2, P3, and P4) according to the spectral signatures of water molecules inside the graphene nanobubbles as described in the previous section. Since at higher temperatures, the time scales of rotational relaxation are quite similar, we have only compared the two extreme cases of system 1 and system 5. In Figure 4, we have shown the dipole and OH rotational correlation functions for system 1 and bulk which are at 298 K. The dipole relaxation of water molecules that contribute to the P1 category shows slower relaxation than P3 and bulk water molecules. This is due to the fact that the

Figure 2. Probability distributions of O−H frequencies of (a) bulk water and (b) system 1 at 298 K.

P2 peak arises at 3510 cm−1. The results of OH frequency distributions for systems 2−5 are presented in Figure 3. At the supercritical condition, the weak OH bonds are responsible for the origin of the P2 peak. However, at this temperature, an additional peak also appears around 3000 cm−1 which corresponds to the highly condensed water molecules that are confined in smaller cavities with high density of water. It may me noted that for the supercritical systems, the term “condensed” is used to mean a state of compressed water of high density in a small nanobubble. As described in the

Figure 3. Distributions of O−H vibrational frequencies of different systems at different temperatures. The spectra, as given by the distributions, were fitted with the minimum number of Gaussian functions. D


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Figure 5. Time dependence of the second rank OH and dipole orientational time correlation functions for different regions of System 5(773 K).

Figure 4. Time dependence of the second rank dipole and OH orientational time correlation functions for bulk water and System 1 (298 K).

Table 2. Integrated Time Scales of Dipole and O−H Rotational Relaxation for System 5 and Bulka

water molecules of the P1 category are mostly at the surface of the graphene and diamond sheets and their orientational relaxation takes place at a slower rate than the bulk water molecules due to their interactions with the surfaces. The integrated relaxation times are included in Table 1. The Table 1. Integrated Time Scales of Dipole and O−H Rotational Relaxation for System 1 and Bulka

a

regions

τ2μ

τ2OH

P1 P3 bulk

3.75 2.96 1.54

3.84 2.43 1.16

a

regions

τ2μ

τ2OH

P1 P2 P3 P4 bulk

0.03 0.03 10.56 7.17 0.04

0.03 0.03 11.01 8.12 0.03

Time scales are given in units of ps.

these water molecules are rotationally trapped and they can be termed as essentially “irrotational” water molecules. It is interesting that the current study of the dynamics of waterfilled nanobubbles at graphene−diamond hybrid interfaces reveals the existence of rotationally bound water molecules even at a supercritical temperature. It may be noted that we have calculated the rotational relaxation times as the integrals of the respective rotational time correlation functions. Strictly speaking, the integral of the time correlation function would give the relaxation time if the decay of the correlation function is exponential. The decay of rotational correlations of water molecules of the current systems is, however, not exponential as can be seen from the semilog plots of Figure S1 of the Supporting Information. The nonexponential decay is, in fact, more prominent for the confined systems as can be seen from this figure. Thus, although such integrated relaxation times are widely used in the literature, they are meaningful only for semiquantitative comparison between different systems. We note that water molecules in the nanobubbles have been divided into four categories (P1, P2, P3, and P4) based on their vibrational spectral features. Out of these four categories, only P1 and P3 appear for confined water at room temperature while the features of P2 and P4 appear only at high temperatures. In particular, P4 becomes more prominent only for high density of confined water at supercritical temperatures. While calculating the dynamics of water of these different categories, only the initial time was considered for assigning a correlation function to a given type of water. Clearly, the water might change its category at a later time but would still contribute to the same time correlation function of its initial category. Thus, the discussion of distinct dynamics of different categories of water would be meaningful if the survival time of water of a given category is long enough compared to the typical time scale of

Time scales are given in units of ps.

integrated relaxation times for the bulk water for τ2μ and τ2OH are found to be 1.54 and 1.16 ps, respectively. We have also calculated the rotational relaxation times for bulk water at higher temperatures for comparison. However, due to the effects of confinement, the rotational relaxation of water molecules of system 1 occurs at a slower rate. In the case of system 2, we see the emergence of the P4 peak, and the rotational relaxation is also found to slow down further than P1 and P3 categories (results are not shown here). For system 5 at 773 K, the highly inhomogeneous environment gives both very fast as well as very slow components of orientational relaxation. The rotational correlations are shown in Figure 5. Here, water molecules of the P1 and P2 categories, which are at the supercritical conditions and have a lower density of water molecules, show very fast decay for both the dipole and OH rotational correlations. The water molecules of P3 and P4 categories, which contain highly condensed water molecules in small cavities, show a very slow rotational relaxation. The integrated time scales are given in Table 2. It is found that the water molecules, which contribute to the P1 and P2 types of system 5, have their rotational relaxation times in the subpicosecond domain whereas water molecules contributing to the P3 and P4 types have significantly longer relaxation time scales. For the P1 and P2 types, the density of water molecules is less and the high thermal kinetic energy plays the dominant role in the relaxation. However, for the P3 and P4 types, the confinement effect at high density is more dominant even though the temperature is very high. The slow rotational nature of water molecules of the P3 and P4 categories means that E


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may be noted that for t = 0, h(0)=H(0), and the normalization of the correlation function of eq 6 takes the same form as defined in some of the earlier work.43,44 Clearly, SHB(t) describes the probability that a water pair, which was hydrogen bonded at time t = 0, remains continuously bonded up to time t. The time integral of SHB(t) gives the average hydrogen bond lifetime. Following earlier work,40−49 we have used a set of geometric criteria to define a hydrogen bond. We have, however, considered only the distance cut-offs of 2.5 Å for the oxygen−hydrogen distance and 3.5 Å for the oxygen−oxygen distance. We note in this context that although similar geometric criteria of hydrogen bonds have been used in many earlier studies40−44,47,48 for calculations of hydrogen bond lifetimes and are believed to be reasonable, it has also been shown that such an approach may not provide very accurate short-time intermittent breaking of hydrogen bonds and one may require a combination of both geometric and energetic criteria for better description of such dynamics.50,51 Indeed, an accurate description of the fast intermittent breaking and reformation of a hydrogen bond remains a difficult problem.49−51 It may also be noted that since the decay of the hydrogen bond time correlation is not always strictly singleexponential, the integrated relaxation time of hydrogen bond time correlation function may not provide an accurate relaxation time, but rather it gives an effective relaxation time which can be used for semiquantitative comparison of different systems. The integrated relaxation times of hydrogen bond time correlation functions for different systems are included in Table 3. The hydrogen bond time correlation functions for bulk water

the dynamical correlation under consideration. For this purpose, we have calculated the survival probability of a water molecule for being in the same category from t = 0 until time t and the results are shown in Figures S2 and S3 of the Supporting Information for systems 1 and 2 (i.e., at 298 and 373 K). The average survival times, as obtained by the integration of the respective correlation functions, are found to be 18.5 and 9.2 ps for P1 and P3 molecules, which are longer than the typical rotational times scales of these molecules given in Table 1 at 298 K. For the higher temperature (system 2 at 343 K), the survival times of P1, P2, and P3 molecules are calculated to be 18.9, 2.35, and 2.96 ps. Although the survival times of P2 and P3 water are now shorter due to higher temperature, these time scales are still longer than typical rotational and hydrogen bond time scales of these water molecules at 343 K. For systems 3−5 at very high and supercritical temperatures, there are multiple bubbles of varying water density with no exchange of water between the bubbles due to impermeability of the graphene surfaces. Typically, P4 is found only in the cavities with high water density while other types are found in the cavities of relatively lower or normal densities. Not all types of water are found in a given cavity, and since there is no exchange of water between the cavities, calculations of exchange dynamics and survival times would not be very meaningful for these systems. For example, a water which was initially found to be of P4 category in a smaller bubble at high density, remained so for essentially the entire duration of the simulation. In other words, the dynamical heterogeneity of these high temperature systems arises not from a single bubble but from multiple bubbles of varying size and density. The results discussed until now are for the water-filled nanobubbles with fixed carbon atoms. As described in section 2, we have also carried out additional simulations of some chosen smaller nanobubbles as the preliminary test cases where the C− C bonds were allowed to vibrate. The same systems were also simulated with carbon atoms kept fixed at their mean positions. The results of these additional systems are presented in Figure S4 and the corresponding time scales are included in Table S4 of the Supporting Information for both the flexible and fixed models of the chosen nanobubbles. It is found that for these chosen nanobubbles, the rotational dynamics remains very similar for the two models at a given temperature. It may be noted that since the density of water in these nanobubbles is not very high, the slow rotational relaxation which was found in the high density bubbles at supercritical temperatures, is absent in these latter systems.

Table 3. Integrated Time Scales of Continuous Hydrogen Bond Time Correlation Functionsa

a

regions

system 1 (298 K)

system 5 (773 K)

P1 P2 P3 P4 bulk

3.7 1.96

0.28 0.12 4.22 4.91 0.2

1.5

Time scales are expressed in units of ps.

5. HYDROGEN BOND DYNAMICS In this section, we have discussed the hydrogen bond dynamics of confined water molecules inside nanobubbles. The hydrogen bond dynamics is studied by using the so-called population time correlation function approach. 40−48 We define a continuous hydrogen bond time correlation function, SHB(t), as follows SHB(t ) =

⟨h(0)H(t )⟩ (⟨h2(0)⟩⟨H2(0)⟩)1/2

(6)

where, H(t) = 1, if an initially water−water hydrogen bond remains intact continuously from time 0 to t, otherwise H(t) = 0. The population variable h(t) = 1 if a tagged pair of water molecules is hydrogen bonded at time t, and zero otherwise. It

Figure 6. Time dependence of continuous hydrogen bond time correlation functions for (a) different regions of system 1 and bulk at 298 K and (b) different regions of system 4 at 773 K. F



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ACKNOWLEDGMENTS Financial support through a J. C. Bose Fellowship to A.C. from the Science and Engineering Research Board, a statutory body of the Department of Science and Technology, and Council of Scientific and Industrial Research (through a Junior/Senior Research Fellowship to A.K), Government of India, is gratefully acknowledged. The calculations were done at the High Performance Computing Facility at Computer Center, IIT Kanpur.

and system 1 are shown in Figure 6a. The integrated time scales reveal that water molecules belonging to the P2 category have a longer relaxation time than bulk and P1 molecules. Again, interesting results are found for system 5 (Figure 6b). The condensed water molecules of the P3 and P4 categories are found to decay slower than the P1 and P2 types. High thermal kinetic energy leads to faster breaking and reformation of hydrogen bonds for water molecules of P1 and P2 types. The slow decay of SHB(t) for water molecules of the P3 and P4 categories is likely due to strong hydrogen bonds that the water molecules are forced to form in these highly condensed regions inside smaller nanobubbles with spatial constraints and also slow rotational movement of these water molecules due to high density and confinement.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06911. Simulation setup, orientational relaxation of water molecules in nanobubbles, exchange of water molecules between different categories, and simulations of flexible nanobubbles, (PDF)

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REFERENCES

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6. CONCLUSIONS In this work, we have performed molecular dynamics simulations of water confined in nanobubbles of varying sizes at the hybrid interfaces of graphene and diamond sheets at varying temperatures. We have characterized the water molecules in terms of their vibrational spectral features and our findings are found to be similar to the experimental results.4 At the room temperature of 298 K, the vibrational spectra of water in nanobubbles show two distinct features: one corresponds to the hydrogen-bonded water and the other originates from the dangling modes of water. For nanobubbles at higher temperatures and containing water at higher density, the vibrational features of water molecules are found to show highly inhomogeneous character. Highly condensed water molecules in smaller nanobubbles are found to have significantly red-shifted OH frequencies arising from the formation of strong hydrogen bonds. The dynamical features of water molecules in such nanobubbles also reveal strong heterogeneity. Water molecules that are confined inside small nanobubbles at high density of water are characterized by very slow rotational relaxation, whereas in the low density bubbles, water molecules are found to undergo very fast rotational relaxation due to high thermal energy. Results are also presented for the dynamics of hydrogen bonds in these nanobubbles. We believe that our study would be helpful in the dynamical characterization of nanoconfined water molecules at different hybrid interfaces and also at extreme thermodynamic conditions.

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AUTHOR INFORMATION

Corresponding Author

*(A.C.) E-mail: [email protected]. Telephone: +91 512 2597241. ORCID

Amalendu Chandra: 0000-0003-1223-8326 Notes

The authors declare no competing financial interest. G


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