Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Thick Two-Dimensional Water Film Confined between the Atomically Thin Mica Nanosheet and Hydrophilic Substrate Cong Wei, Weihao Zhao, Xiaotong Shi, Chengjie Pei, Pei Wei, Jindong Zhang, and Hai Li* Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China
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S Supporting Information *
ABSTRACT: The interesting properties of water molecules confined in a two-dimensional (2D) environment have aroused great attention. However, the study of 2D-confined water at the hydrophilic−hydrophilic interface is largely unexplored due to the lack of appropriate system. In this work, the behavior of water molecules confined between an atomically thin mica nanosheet and a hydrophilic SiO2/Si substrate was investigated using an atomic force microscope in detail at ambient conditions. The confined water molecules aggregated as droplets when the relative humidity (RH) of the environment was 11%. A large-area 2D water film with a uniform thickness of ∼2 nm was observed when the mica flake was incubated at 33% RH for 1 h before being mechanically exfoliated on a SiO2/Si substrate. Interestingly, the water film showed ordered edges with a predominant angle of 120°, which was the same with the lattice orientation of the mica nanosheet on top of it. The water film showed a fluidic behavior at the early stage and reached a stable state after 48 h under ambient conditions. The surface properties of the upper mica nanosheet and the underlying substrate played a crucial role in manipulating the behavior of confined water molecules. When the surface of the upper mica nanosheet was modified by Na+, Ni2+, and aminopropyltriethoxysilane (APS), only some small water droplets were observed instead of a water film. The surface of the underlying SiO2/Si substrate was functionalized by hydrophilic APS and hydrophobic octadecyltrimethoxysiliane (OTS). The small water droplets were imaged on a hydrophobic OTS-SiO2/Si substrate, while the water film with regular edges was maintained on a hydrophilic APS-SiO2/Si substrate. Our results might provide an alternative molecular view for investigating structures and properties of confined water molecules in 2D environments.
1. INTRODUCTION
With the rapid advances of two-dimensional (2D) materials, the constrained space between 2D materials and underlying substrates has been regarded as a model system to reveal the ubiquitous thermodynamics and structures of confined water and other molecules.7,9,22−29 It has been reported that fluidic organic molecules were transformed into a solid-like structure when they were sandwiched between the graphene and the substrate.22,23 The monolayer water film with a regular shape was visualized on a hydrophilic surface by using graphene as a cover,7,24,25 which largely increased the friction and screened the charge transfer.25,26 Confined water molecules between graphene and boron nitride nanosheets exhibited a quite low
Confined water in a nanoscale environment usually shows an intriguing behavior in comparison with the bulk water, which is fundamentally important in diverse chemical and biological processes1−7 including nanofluidic transport,8 catalysis,9,10 dewetting/rewetting,11,12 self-assembling,13,14 and enzymatic reaction.1,15,16 For instance, the confined water meniscus between hydrophilic surfaces shows an ultrahigh viscosity, which is seven orders of magnitude higher in comparison with that of the bulk water.17−20 The supercooled water could not be frozen in a nanoporous structure due to the confinement effect.21 It has been reported that the confined water is believed to enhance the migration behavior of cancer cells and plays a crucial role in determining the protein structure and stability in crowded environment of a living cell.2−6 © XXXX American Chemical Society
Received: December 20, 2018 Revised: March 15, 2019 Published: March 25, 2019 A
DOI: 10.1021/acs.langmuir.8b04232 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Scheme 1. Schematic Illustration for the Formation and Characterization of Water Filma
a (a) Mechanical exfoliation of the mica flake from the bulk crystal. (b) Incubating exfoliated mica on the Scotch tape in a sealed box with controlled humidity at ambient conditions. (c) Depositing the incubated mica nanosheet onto the hydrophilic SiO2/Si substrate by mechanical exfoliation. (d) Characterization of water film confined between mica and SiO2/Si substrate by AFM.
dielectric constant of ∼2 compared with that of the bulk water (ε = 80).27 The monolayer water adlayer confined between the single-layer (1 L) MoS2 and mica substrate significantly decreased the photoluminescence (PL) intensity of 1 L MoS2 by 30−50 times.28 In addition, ultrathin water film confined between graphene and mica can affect the electronic properties of the upper graphene through hole doping.7 The confined water between graphene and sapphire made the G and 2D peaks of graphene blue-shifted.29 Aforementioned results indicate that the nanoenvironment formed by the 2D material and supporting substrate offers a promising opportunity to explore the intriguing properties of confined water molecules from the view of molecular scale. The confined water film is adjacent to the bottom surface of the upper 2D material and the top surface of underlying substrate. Thus, the behavior of confined water molecules should be highly dependent on the surface properties of the 2D material and substrate. According to the surface hydrophilicity/hydrophobicity, the interfaces of 2D material and substrate can be classified as hydrophilic−hydrophilic, hydrophilic−hydrophobic, and hydrophobic−hydrophobic interfaces. Confined water molecules between hydrophilic−hydrophobic and hydrophobic−hydrophobic interfaces have been extensively investigated by both theoretical simulations and experimental techniques.7,24−26,30−38 Molecular dynamics (MD) simulations demonstrated that ordered 2D water structures were found both in hydrophilic−hydrophobic and hydrophobic−hydrophobic sheets.32,36,38−41 The introduction of hydrophilic sheet can enhance the stability of ordered 2D water at a higher temperature compared to that confined in hydrophobic−hydrophobic sheets.39 Since water molecules have stronger affinity to a hydrophilic surface than to a hydrophobic one, it is desirable to reveal the properties of water molecules confined between the hydrophilic−hydrophilic interface. However, the behavior of water molecules confined between hydrophilic−hydrophilic interfaces has been investigated primarily by theoretical simulation and indirect experimental technique.17,41−46 For instance, the MD simulation indicated that confined water between hydrophilic−
hydrophilic surfaces can enhance the thermal conductivity.46 Direct observation of water molecules through experimental work is almost unexplored due to the lack of a suitable model system. In this work, the hydrophilic−hydrophilic interface was realized by mechanically exfoliating mica nanosheets on a hydrophilic SiO2/Si substrate. By controlling the environment humidity and incubation time, the water molecules confined in such a hydrophilic−hydrophilic interface transformed from small droplets to large-area 2D films with various sizes and configurations. The evolution and stability of 2D confined water films within hydrophilic−hydrophilic interface were systematically investigated using an optical microscope (OM) and atomic force microscope (AFM). After being trapped in the hydrophilic−hydrophilic interface, water molecules were formed into a uniform film with a height of ∼1.9 nm and showed ordered edges with a predominant angle of ∼120°, which showed the same orientation to that of the mica lattice.47−49 Importantly, we found that the formation of water film was highly sensitive to the crystalline structure of the mica surface. Small water droplets, instead of an ordered water film, were observed when the bottom surface of mica nanosheet was functionalized by either metal ions or organosilane, indicating that the mica lattice should template the morphology of 2D water film. Moreover, the water film showed a fluidic behavior in the early stage and became stable after 48 h at ambient conditions. Thereafter, it remained stable even at a high temperature of 300 °C. Furthermore, the confined water molecules can be exchanged with other molecules in an external environment. Our results illustrated the formation and dynamic evolution of confined water molecules sandwiched between a hydrophilic−hydrophilic interface.
2. RESULTS AND DISCUSSION The experimental process is illustrated in Scheme 1. Thin mica flakes were first exfoliated from the bulk crystal onto the Scotch tape (Scheme 1a) and then incubated in a sealed box with controlled humidity at room temperature for a certain time (Scheme 1b). It is known that the mica surface is B
DOI: 10.1021/acs.langmuir.8b04232 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Optical microscopy image of the mica nanosheet containing confined water film. (b) AFM image of the mica nanosheet and water film marked by red box shown in (a). (c) Magnified AFM image of the water film marked by the blue box shown in (b). (d,e) Atomic lattice images of mica nanosheets on (d) SiO2/Si substrate and (e) water film. (f) Angle distribution of water film edges.
water film is flat enough and does not influence the mica lattice. Furthermore, as indicated by the arrows shown in Figure 1c−e, the water film showed regular edges with an angle of 120°, which is in good agreement with the lattice orientation of the mica nanosheet. As shown in Figure 1f, the confined thick water film showed regular edges with angles ranging from 30 to 160°, which favorably centered around 120° (Figure 1f), further implying the orientation correlation between the water film and mica nanosheet. It has been reported that the coverage and thickness of water film on the hydrophilic surface are relevant to the relative humidity of the environment.50,51 In order to investigate the effect of environment humidity on the water film, thin mica flakes adhered on the Scotch tape were incubated in a sealed box at various RH of 11, 33, 57, 75, and 98% for 1, 5, 10, 30, and 60 min, respectively. Water droplets with height in the range of 1−10 nm were observed under low humidity and short incubation time, while uniform water films with a regular shape and a thickness of 1.9 nm were observed at a higher humidity and longer incubation time. As shown in Figure 2a, the water molecules were aggregated as small droplets with a height of 8.5 nm when the mica flakes were incubated at 57% RH for 1 min. Larger droplets with a height decreased to 6.74 nm were trapped between the mica nanosheet and SiO2/Si substrate as the incubation time was increased to 5 min (Figure 2b). After being incubated at 57% RH for 10 min, the edges of mica nanosheet were fulfilled by a continuous water film with a height of 1.83 nm and island-like structures with a height of 3.76 nm (Figure 2c). Interestingly, the porous water film with a thickness of 1.90 nm was observed after being incubated for 30 min at 57% RH (Figure 2d), which was further extended to a large-area uniform water film after 60 min of incubation at the same condition, as shown in Figure 2e. No significant difference was found for the water film when the incubation time was extended to 48 h (Figure 2f).
hydrophilic and water molecules can wet it easily to form a thin water film.11,50−53 Subsequently, they were taken out from the sealed box and brought into contact with a freshly cleaned SiO2/Si substrate (Scheme 1c), and atomically thin mica nanosheets were left on a hydrophilic substrate. Thus, the thin water film on the mica surface was confined between the mica nanosheet and SiO2/Si substrate. Finally, OM and AFM were utilized to characterize the mica nanosheet and confined water film (Scheme 1d). As shown in Figure 1a, the confined water film between the mica nanosheet and hydrophilic SiO2/Si substrate with size up to several tens of micrometers can be clearly observed by the OM at ambient conditions with a relative humidity (RH) of 57%. As shown in the AFM image, a uniform large-area water film with many depressions underlying the mica nanosheet was observed (Figure 1b), indicating the 2D structure of the water film. The thicknesses of the mica nanosheet and water film are 24.4 and 1.88 nm, respectively, as indicated by the AFM measurement (Figure S1 in the Supporting Information). As shown in Figure 1c, some edges of the water film also showed a regular angle of 120°, as indicated by the light blue arrows marked on the water film edges, implying that the orientation of the water film could be correlated with the lattice orientation of the mica nanosheet. It has been reported that the monolayer water film grown on a mica substrate at ambient conditions showed an epitaxial relationship with the mica lattice orientation, indicating that the structure of ultrathin water film should be templated by a mica surface.48 Similar phenomena have also been reported on the hexagonal structure of water layer confined between graphene and mica.26,47 In order to investigate the correlation between the mica nanosheet and confined thick water film shown in Figure 1c, the atomic lattice orientation of mica nanosheet was imaged by AFM (Figure 1d,e). Figure 1d,e shows the atomic lattice images of the bare mica and mica on a water film. There is no obvious difference between them, indicating that the C
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aforementioned conditions might be the reason for the formation of water film with a thickness of ∼2 nm but not a monolayer. Our results are consistent with the previous studies.50−52,54 It is well known that water can be adsorbed on the mica surface to form thin films with various thicknesses at ambient conditions. By using a phase-modulated ellipsometer, the water film with a thickness of ∼2 nm was measured when the RH approached to 100%.50,51 Five layers of water were observed at saturated RH by infrared spectroscopy at room temperature,52 which should have a thickness of ∼1.85 nm if one layer of the water is 0.37 nm thick. However, direct observation of the water film with a thickness of ∼2 nm at ambient conditions has not been reported in an experimental work. Therefore, the controlled formation and observation of water film with a thickness of ∼2 nm in this work should be of great importance for the fundamental research on confined water at ambient conditions. In order to quantitatively describe the correlation between the amount of confined water film and incubation time at different environment humidities, the normalized volume (NV) of confined water droplets or films to the unit area of the corresponding mica nanosheet was defined by eq 1 N V = (V / S ) × 1 μ m 2
(1)
Here, NV is the normalized volume of confined water, V is the volume of water droplets or films, and S is the area of corresponding mica nanosheet on top of the confined water. The detailed calculation for V and S was described in Figures S2−S4 in the Supporting Information. The NV as a function of environment humidity and incubation time is plotted in Figure 2g. It is obvious that NV is exponentially increased with increased humidity and incubation time and reached saturation after 30 min of incubation. That is, the amount of confined water is positively correlated with the environment humidity and incubation time. Meanwhile, the detailed AFM measurements at various conditions are correspondingly presented in Figures S5−S8 in the Supporting Information. As shown in Figure 2g and Figure S5, the confined water molecules always aggregate as droplets when mica flakes were incubated at 11% RH even for 60 min, indicating that the water film cannot be formed at the hydrophilic mica surface under low environment humidity. As shown in Figure S9 in the Supporting Information, spherical water droplets with a height of ∼3.2 nm were clearly observed. In this case, the NV was slightly increased from 0.03 × 106 to 0.09 × 106 nm3 as the incubation time was increased from 1 to 60 min (Figure 2g). When the environment humidity was increased to 33%, water droplets were observed when the incubation time was less than 30 min (Figure S6). The network-like structure of confined water with a thickness of 1.82 nm was observed when the incubation time was increased to 30 min (Figure S6d). Moreover, a uniform 2D water film with a regular depression was observed when the incubation time was further increased to 60 min (Figure S6e), which has a thickness of 1.89 nm. At 33% RH, the NV was increased from 0.06 × 106 to 0.98 × 106 nm3 as the incubation time was increased from 1 to 60 min. The NV was greatly increased from 0.08 × 106 to 1.07 × 106 nm3 when the environment humidity was increased to 57%, indicating more water molecules were quickly formed as films at this condition. The NV was rapidly increased when the mica flakes were further incubated at 75 and 98% RH (Figure 2g and Figures S7,8 in the Supporting
Figure 2. (a−f) AFM images of mica nanosheets deposited on SiO2/ Si substrate before being incubated at 57% RH for 1, 5, 10, 30, and 60 min and 48 h. Schemes below each AFM image show the schematic illustration of the confined water from front view. (g) Normalized volume of the confined water as a function of incubation time at various humidities of 11, 33, 57, 75, and 98% RH.
Previously, a statistical monolayer of water was thought to be formed at 50−75% RH or at higher humidity at room temperature.51,52 By using a scanning polarization force microscope, monolayer water islands were observed on the mica surface at 40% RH and room temperature.48 Monolayer water films with a thickness of 0.37 nm were confined between graphene nanosheets and mica or SiO2/Si substrate.7,24 Moreover, three layers of water film (∼1 nm thickness) were encapsulated between the graphene nanosheet and mica substrate,25 indicating the layer-by-layer formation of water film at ambient conditions. In these reports, water films with thicknesses ranging from 0.37 to ∼1 nm were confined between the hydrophobic graphene and hydrophilic mica or SiO2/Si substrate. In addition, both the graphene nanosheet and mica substrate were not incubated at controlled humidity or ambient conditions for an enough time. It has been reported that water molecules prefer to be adsorbed on the hydrophilic surface rather than the hydrophobic surface. In our work, both the mica nanosheet and SiO2/Si substrate are hydrophilic. Thus, more water molecules were confined between the hydrophilic−hydrophilic interface compared with those between the hydrophobic−hydrophilic interface. Meanwhile, mica flakes were incubated in a sealed box at controlled humidity for a certain time to adsorb enough water molecules to form a thick water film. A water film with a thickness of ∼2 nm was adsorbed on the mica surface at a near-saturating humidity by measuring the force−distance curve with atomic force microscopy.54 Thereafter, the adsorbed water film was encapsulated between the mica nanosheet and SiO2/Si substrate during mechanical exfoliation. We think that the D
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at 57% RH. The confined water molecules diffused quite slowly at ambient conditions (temperature is 22.0 ± 0.1 °C and RH is 57%). After 2 h, the morphology of the water film was slightly varied, as shown in Figure 3b. As indicated by the red arrows shown in Figure 3a,b, two neighboring holes were merged after 2 h. Meanwhile, the thickness of water film was kept unchanged. As shown in Figure 3c, the confined water molecules were further diffused slowly and the merged hole was enlarged after 4 h, implying the viscous behavior of confined water molecules. In addition, the edge of water film gradually shrank and some holes were disappeared after 4 and 24 h, as indicated by the blue arrows shown in Figure 3b−d. The morphology of water film was almost unchanged after 48 and 72 h, as shown in Figure 3e,f. The relationship between the storing time under ambient conditions and NV of water film is presented in Figure 3g. The NV decreased quickly from 1.32 down to 0.78 × 106 nm3 during the first 24 h. It remained stable from 48 to 168 h, indicating that the water film acted as a viscous fluid during 0 to 24 h and almost equilibrated after 48 h. As the area of the water film decreased with an increasing incubation time under ambient conditions, the height of the water film was maintained (see corresponding AFM height profiles shown in Figure 3), further indicating the fluidity of confined water (Figure S10 in the Supporting Information). It has been reported that the monolayer water adlayer confined between the hydrophobic−hydrophilic interface was quite sensitive to the environment−temperature variation, while the monolayer water adlayer confined between the hydrophilic−hydrophilic interface was stable at the same condition.38 We then investigated the stability of the thick water film confined between the hydrophilic−hydrophilic interface as the environment, temperature, and humidity varied. After the water film was equilibrated at ambient conditions, it was annealed at 200 °C for 1 h. As shown in Figure 4a−d, the morphology and height of the water film were kept unchanged before and after high-temperature annealing. The water film underlying the mica nanosheet was further annealed at 300 °C for another 1 h, and it still remained unchanged (Figure S11a−c in the Supporting Information). This phenomenon is in good agreement with the behavior of the monolayer water adlayer confined between the hydrophilic−hydrophilic MoS2/mica interface.38 The high stability of the thick water film could have arisen from the strong affinity between water molecules and hydrophilic surface.11,50−53 On the other hand, the strong adhesion between the mica nanosheet and hydrophilic SiO2/Si substrate might also prevent the permeation of confined water molecules at high temperatures. However, the stabilized water film was almost completely diffused out of the mica cover after it was incubated at 100% RH for only 20 min (Figure 4e−h). As shown in Figure 4e,g, the thicknesses of the stabilized water film and the mica nanosheet were 1.86 and 15.2 nm, respectively. After being incubated at 100% RH for 20 min, there were only some droplets under the mica nanosheet (Figure 4f,h). Meanwhile, the thickness of the mica nanosheet was still 15.2 nm (Figure 4h), indicating that there should be no water film underlying it. This phenomenon could be explained as follows: the hydrophilic SiO2/Si substrate and the mica surface should be fully covered by liquid water film at 100% RH.50,51 In this case, the outside water molecules on the substrate could easily permeate into the hydrophilic−hydrophilic interface between the SiO2/Si substrate and mica nanosheet due to the capillary interaction, which connect
Information). As shown in Figure 2g and Figures S7c and S8c in the Supporting Information, 10 min incubation time was enough to obtain a large-area water film in an environment with high humidity. The NV was increased by an order of magnitude as the incubation time just increased from 1 to 5 min at 75% RH. The NV was almost saturated after 10 min incubation at 75 and 98% RH, indicating that the environment humidity played an important role in determining the size of 2D confined water film at this hydrophilic−hydrophilic interface. Moreover, we found that the NV could be well fitted by the following equation y=
a−b 1+
e
( xc )
+d
where, x and y represent incubation time and NV, respectively. In addition, a, b, c, d, and e are the different constants at controlled humidity. In order to study the stability of 2D confined water film between mica and SiO2/Si substrate at ambient conditions, AFM was used to monitor the evolution of water film by scanning the same region. As shown in Figure 3a, a porous water film with a thickness of 1.81 nm was observed just after being taken out from the sealed box after 30 min of incubation
Figure 3. (a−f) AFM images of the water film under mica nanosheet stored at ambient conditions for 0, 2, 4, 24, 48, and 72 h. (g) Normalized volume of the confined water film as a function of storage time at ambient conditions. E
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Figure 4. (a−b) AFM images of the water film (a) before and (b) after being annealed at 200 °C for 1 h. (c−d) Height profiles of the region marked by red and blue boxes shown in (a) and (b). (e−f) AFM images of the water film (e) before and (f) after being incubated in an environment with 100% RH for 20 min. (g−h) Height profiles of the region marked by red and blue boxes shown in (e) and (f).
changed from a network structure to a large-area film (Figures 5e,f). The AC value of the water/ethanol film was largely increased to 75% in comparison with 32% of the pure water film (Figure 5i), indicating that more ethanol molecules were permeated into the confined space than acetone molecules at this condition. Adlayers of small organic molecules (tetrahydrofuran and cyclohexane) confined between the graphene nanosheet and substrate have been imaged by atomic force microscopy (AFM).22 Previously, we have also found that acetone and ethanol molecules can form adlayers with thicknesses varying from 0.3 to more than 3 nm when they were confined between a few-layer MoS2 nanosheet and SiO2/Si substrate.55 Therefore, it is possible for acetone and ethanol molecules to form layered structure in the confined environment between mica nanosheet and SiO2/Si substrate. In this work, the water molecules could be exchanged with the ethanol molecules when the thick water film confined between the mica nanosheet and SiO2/Si substrate was exposed to an ethanol vapor. Similar phenomena have been observed when the confined water monolayer between the graphene nanosheet and mica substrate was exposed to an ethanol vapor.34,56,57 The intercalated acetone and ethanol molecules could have strong affinity to the surrounding water molecules and might be remained even after the evaporation process. Thus, the films shown in Figure 5b,f should be the mixtures of water and acetone/ethanol. The formation of water film seems irrelevant to the thickness of the mica nanosheet on top of it. As shown in Figure S12 in the Supporting Information, mica nanosheets with thicknesses of 1.5, 10, and 22 nm were imaged by AFM. After, they were incubated at 57% RH for 60 min. A large-area and uniform 2D water film with a height of 1.9 nm was observed under all mica nanosheets. It is well known that the surface hydrophilicity/hydrophobicity can regulate the behavior of water molecules. In our work, the hydrophilic−hydrophilic interface consists of two components, the bottom surface of the mica nanosheet and the top surface of the substrate. In order to reveal the influence of the surface property on the formation of the confined water film, the bottom surface of the mica nanosheet and the top surface of the substrate were functionalized. First, the mica
together with the inside confined water molecules. When the sample was exposed to ambient conditions, the water molecules on the SiO2/Si substrate should have evaporated quickly. Since the confined water molecules in the hydrophilic−hydrophilic interface were connected with the outside water molecules on the SiO2/Si substrate, they could be dragged out during evaporation. Consequently, there was no water film formed, but some water droplets were confined between the SiO2/Si substrate and mica nanosheet. Besides water, volatile organic molecules such as acetone and ethanol have also been reported to be trapped between the hydrophobic−hydrophilic and hydrophilic−hydrophilic interfaces.22,23,34,55 In order to investigate the influence of organic molecules on the stability of the water film, the stabilized water films under the mica nanosheets were incubated in two sealed boxes containing acetone and ethanol at room temperature for 1 h. As shown in Figure 5a,c, the thickness of the water film confined between the mica and substrate was measured as 1.88 nm. Meanwhile, 37% of the area of the mica nanosheet was occupied by the water film (Figure 5i). In order to quantitatively describe the correlation between the amount of confined water film and incubation time in an organic environment, the area ratio of the confined water film to the corresponding mica nanosheet was defined as area coverage (AC), which is described by eq 2 AC = Sw /Sm
(2)
where Sw represents the area of the confined water and Sm represents the area of mica nanosheet on top of the confined water. After being incubated in an acetone environment for 1 h, an enlarged film with a thickness of 1.88 nm was observed (Figure 5b,d), which is same to that of the water film. However, the AC value of the water film had been increased from 37 to 52%, indicating that the acetone molecules could be intercalated into the confined system. In addition, the thickness of the mica nanosheet was kept unchanged before and after being incubated in an acetone environment (Figure 5c,d). When the water film covered by mica nanosheet was incubated in an ethanol environment at room temperature for 1 h, the situation was quite different. Although the film thickness was remained before and after incubation (Figure 5g,h), the morphology was F
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Figure 6. (a) Schematic illustration of the functionalization of mica nanosheet. (b−d) AFM images of mica nanosheets whose bottom surfaces were functionalized by (b) Na+, (c) Ni2+, and (d) APS. The functionalized mica nanosheets were incubated at an environment with 57% RH for 1 h before being deposited on a SiO2/Si substrate.
octadecyltrimethoxysiliane (OTS) and APS (Figure 7a). As shown in Figure 7b, there was no water film but some small
Figure 7. (a) Schematic illustration of the functionalization of SiO2/Si substrate. (b−c) AFM images of mechanically exfoliated mica nanosheets on (b) OTS-SiO2/Si and (c) APS-SiO2/Si. Mica nanosheets were incubated at an environment with 57% RH for 1 h before exfoliation.
Figure 5. (a−b) AFM images of the water film before and after being incubated in an acetone environment for 1 h. (c−d) Height profiles of the region marked by red and blue boxes shown in (a) and (b). (e−f) AFM images of the water film before and after being incubated in an ethanol environment for 1 h. (g−h) Height profiles of the region marked by red and blue boxes shown in (e) and (f). (i) Coverage plot of water films before and after being incubated in an acetone and ethanol environment.
water droplets with a height of ∼1 nm were observed between the OTS-SiO2/Si substrate and mica surface. Interestingly, a large-area uniform 2D water film with a thickness of 1.96 nm was still observed between the mica and APS-SiO2/Si substrate (Figure 7c). The OTS-SiO2/Si substrate has a water contact angle of 108° while that of the APS-SiO2/Si substrate is 49° (Table S1 in the Supporting Information). Therefore, the OTS-SiO2/Si substrate is hydrophobic and the APS-SiO2/Si substrate is a bit hydrophilic. Thus, thick water film can be formed at hydrophilic−hydrophilic interface rather than at hydrophilic−hydrophobic interface, indicating that surface hydrophilicity also played an important role in forming a 2D thick water film in a confined environment.
surface was functionalized by Na+, Ni2+, and aminopropyltriethoxysilane (APS) before being incubated in a sealed box at 57% RH for 1 h (Figure 6a). As shown in Figure 6b−d, no water film was formed and only water droplets with various heights were observed within the confined space, implying that the crystalline structure of the mica surface played a key role in determining the formation of the confined water film. Subsequently, the influence of the substrate functionalization was investigated. The SiO2/Si substrate was functionalized by G
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microscope (AFM) were used to locate and measure the confined water film between the mica and substrates at ambient conditions. Tapping mode AFM images were obtained using a Dimension ICON (Bruker, USA). Silicon cantilevers with a normal resonance frequency of 300 kHz and a spring constant of 40 N/m (Tap300Al-G, BudgetSensors) were used. All images were captured with a scan rate at 1−2 Hz and with a 256 × 256 pixel resolution. The water contact angle was measured by using KRÜ SS DSA1005.
3. CONCLUSIONS In summary, a 2D water film with a thickness of ∼1.9 nm confined between a hydrophilic−hydrophilic interface has been systematically investigated at ambient conditions. The morphology of the water film was highly dependent on the environment humidity at ambient conditions. Only water droplets were observed at 11% RH no matter how long the incubation time is. When the humidity was increased to 33% RH, a uniform large-area water film with a thickness of ∼1.9 nm was obtained after 1 h incubation. The water film showed ordered edges, which adopted the same predominant orientation with the lattice orientation of the mica nanosheet on top of it. The confined water film showed a similar behavior to the viscous fluid from 0 to 24 h after its formation. It equilibrated after 48 h at ambient conditions and then became stable even under a high temperature of 300 °C. However, volatile organic molecules can be intercalated into the confined space to form a mixture with confined water molecules, which altered the morphology instead of the thickness of the water film. We also found that the surface properties of the upper mica nanosheet and the underlying substrate showed a great influence on the formation of the confined water film. No water film was observed, but water droplets were observed when the mica surface was functionalized by metal ions or organosilane. The hydrophilic functionalization of the substrate surface showed no effect on the formation of the water film. No water film was observed after the substrate surface was functionalized by hydrophobic organosilane. Our results might provide an alternative view for investigating structures and properties of confined water molecules in 2D environments.
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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.langmuir.8b04232.
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In situ observation of the evolution of confined water molecules at 11, 33, 75, and 98% RH by AFM, measurement of the coverage of mica flake by confined water adlayer, AFM images of confined water films before and after being annealed at 200 and 300 °C for 1 h, respectively, contact angles of different substrates (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hai Li: 0000-0002-9659-1153 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 21571101), the Natural Science Foundation of Jiangsu Province in China (grant no. BK20161543), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (grant no. 15KJB430016).
4. EXPERIMENTAL SECTION 4.1. Incubation of Mica Flakes in a Controlled Environment. Mica flakes were first exfoliated from a muscovite crystal by using the Scotch tape at ambient conditions with a temperature of 22 °C and a relative humidity of 30−40%. The Scotch tape with mica flakes was put into the sealed quartz box containing saturated LiCl, MgCl2, NaBr, NaCl, and K2SO4 solutions, which generated controlled relative humidities (RH) of 11, 33, 57, 75, and 98%.58 After a certain time, the tape with wetted mica flakes was taken out and immediately brought into contact with freshly cleaned 90 nm-SiO2/Si substrate (Bonda Pte Ltd.). Then, the tape was pressed down to let the tape and substrate contact well. After a while, the tape was gently peeled off from one end of the substrate until complete detachment from the substrate. Thereafter, few-layer and thick mica flakes were deposited on the substrate. 4.2. Surface Functionalization of Mica and SiO2/Si Substrate. 4.2.1. Metal Ion Functionalization of Mica. NaCl or NiCl2 solution (200 μL) with a concentration of 0.1 M was deposited on the Scotch tape with freshly exfoliated mica flakes for 4 min. Thereafter, the solution was removed by N2, and as-modified mica flakes were incubated in a sealed quartz box with controlled humidity for a certain time. 4.2.2. APS Functionalization of Mica and SiO2/Si Substrate. One hundred microliters of 1% APS aqueous solution was dropped on the mica flake or SiO2/Si substrate for 4 min. The mica flake or SiO2/Si substrate was then washed with deionized (DI) water for three times and dried with N2. 4.2.3. OTS Functionalization of SiO2/Si Substrate. Freshly cleaned SiO2/Si substrate was immersed into anhydrous toluene containing 3% OTS (v/v) at 60 °C for 12 h. After that, the SiO2/Si substrate was rinsed with ethanol and DI water for three times and then dried with N2. 4.3. OM, AFM, and Water Contact Angle Characterizations. An optical microscope (Axio Scope A1, Zeiss) and atomic force
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REFERENCES
(1) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74−108. (2) Stroka, K. M.; Jiang, H.; Chen, S.-H.; Tong, Z.; Wirtz, D.; Sun, S. X.; Konstantopoulos, K. Water Permeation Drives Tumor Cell Migration in Confined Microenvironments. Cell 2014, 157, 611−623. (3) Liu, Y.-J.; Le Berre, M.; Lautenschlaeger, F.; Maiuri, P.; CallanJones, A.; Heuzé, M.; Takaki, T.; Voituriez, R.; Piel, M. Confinement and Low Adhesion Induce Fast Amoeboid Migration of Slow Mesenchymal Cells. Cell 2015, 160, 659−672. (4) Eggers, D. K.; Valentine, J. S. Molecular Confinement Influences Protein Structure and Enhances Thermal Protein Stability. Protein Sci. 2001, 10, 250−261. (5) Eggers, D. K.; Valentine, J. S. Crowding and Hydration Effects on Protein Conformation: A Study with Sol-Gel Encapsulated Proteins. J. Mol. Biol. 2001, 314, 911−922. (6) Vaitheeswaran, S.; Thirumalai, D. Interactions between Amino Acid Side Chains in Cylindrical Hydrophobic Nanopores with Applications to Peptide Stability. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 17636−17641. (7) Lee, D.; Ahn, G.; Ryu, S. Two-Dimensional Water Diffusion at a Graphene-Silica Interface. J. Am. Chem. Soc. 2014, 136, 6634−6642. (8) Gao, J.; Feng, Y.; Guo, W.; Jiang, L. Nanofluidics in TwoDimensional Layered Materials: Inspirations from Nature. Chem. Soc. Rev. 2017, 46, 5400−5424. H
DOI: 10.1021/acs.langmuir.8b04232 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Spectra in Graphene-on-Sapphire Systems. J. Phys. Chem. C 2012, 116, 10084−10089. (30) Kastelowitz, N.; Molinero, V. Ice−Liquid Oscillations in Nanoconfined Water. ACS Nano 2018, 12, 8234−8239. (31) Zhao, W. H.; Wang, L.; Bai, J.; Yuan, L. F.; Yang, J.; Zeng, X. C. Highly Confined Water: Two-Dimensional Ice, Amorphous Ice, and Clathrate Hydrates. Acc. Chem. Res. 2014, 47, 2505−2513. (32) Pestana, L. R.; Felberg, L. E.; Head-Gordon, T. Coexistence of Multilayered Phases of Confined Water: The Importance of Flexible Confining Surfaces. ACS Nano 2018, 12, 448−454. (33) Zhu, Y.; Wang, F.; Bai, J.; Zeng, X. C.; Wu, H. Compression Limit of Two-Dimensional Water Constrained in Graphene Nanocapillaries. ACS Nano 2015, 9, 12197−12204. (34) Bampoulis, P.; Witteveen, J. P.; Kooij, E. S.; Lohse, D.; Poelsema, B.; Zandvliet, H. J. W. Structure and Dynamics of Confined Alcohol−Water Mixtures. ACS Nano 2016, 10, 6762−6768. (35) Sotthewes, K.; Bampoulis, P.; Zandvliet, H. J. W.; Lohse, D.; Poelsema, B. Pressure-Induced Melting of Confined Ice. ACS Nano 2017, 11, 12723−12731. (36) Li, Q.; Song, J.; Besenbacher, F.; Dong, M. Two-Dimensional Material Confined Water. Acc. Chem. Res. 2015, 48, 119−127. (37) Sendner, C.; Horinek, D.; Bocquet, L.; Netz, R. R. Interfacial Water at Hydrophobic and Hydrophilic Surfaces: Slip, Viscosity, and Diffusion. Langmuir 2009, 25, 10768−10781. (38) Song, J.; Li, Q.; Wang, X.; Li, J.; Zhang, S.; Kjems, J.; Besenbacher, F.; Dong, M. Evidence of Stranski-Krastanov Growth at the Initial Stage of Atmospheric Water Condensation. Nat. Commun. 2014, 5, 4837. (39) Kumar, H.; Dasgupta, C.; Maiti, P. K. Phase Transition in Monolayer Water Confined in Janus Nanopore. Langmuir 2018, 34, 12199−12205. (40) Raghav, N.; Chakraborty, S.; Maiti, P. K. Molecular Mechanism of Water Permeation in a Helium Impermeable Graphene and Graphene Oxide Membrane. Phys. Chem. Chem. Phys. 2015, 17, 20557−20562. (41) Malani, A.; Ayappa, K. G.; Murad, S. Influence of Hydrophilic Surface Specificity on the Structural Properties of Confined Water. J. Phys. Chem. B 2009, 113, 13825−13839. (42) Krishnan, N. M. A.; Wang, B.; Falzone, G.; Le Pape, Y.; Neithalath, N.; Pilon, L.; Bauchy, M.; Sant, G. Confined Water in Layered Silicates: The Origin of Anomalous Thermal Expansion Behavior in Calcium-Silicate-Hydrates. ACS Appl. Mater. Interfaces 2016, 8, 35621−35627. (43) Kim, B. I.; Boehm, R. D.; Bonander, J. R. Direct Observation of Self-Assembled Chain-Like Water Structures in a Nanoscopic Water Meniscus. J. Chem. Phys. 2013, 139, No. 054701. (44) Feibelman, P. J. Viscosity of Ultrathin Water Films Confined between Aluminol Surfaces of Kaolinite: Ab Initio Simulations. J. Phys. Chem. C 2013, 117, 6088−6095. (45) Ahmad, M.; Gu, W.; Geyer, T.; Helms, V. Adhesive Water Networks Facilitate Binding of Protein Interfaces. Nat. Commun. 2011, 2, 261. (46) Hu, M.; Goicochea, J. V.; Michel, B.; Poulikakos, D. Water Nanoconfinement Induced Thermal Enhancement at Hydrophilic Quartz Interfaces. Nano Lett. 2010, 10, 279−285. (47) Kim, J.-S.; Choi, J. S.; Lee, M. J.; Park, B. H.; Bukhvalov, D.; Son, Y.-W.; Yoon, D.; Cheong, H.; Yun, J.-N.; Jung, Y.; Park, J. Y.; Salmeron, M. Between Scylla and Charybdis: Hydrophobic Graphene-Guided Water Diffusion on Hydrophilic Substrates. Sci. Rep. 2013, 3, 2309. (48) Hu, J.; Xiao, X.-D.; Ogletree, D. F.; Salmeron, M. Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution. Science 1995, 268, 267−269. (49) Teschke, O.; Filho, J. F. V.; de Souza, E. F. Imaging TwoDimensional Ice-Like Structures at Room Temperature. Chem. Phys. Lett. 2010, 485, 133−136. (50) Beaglehole, D.; Christenson, H. K. Vapor Adsorption on Mica and Silicon: Entropy Effects, Layering, and Surface Forces. J. Phys. Chem. 1992, 96, 3395−3403.
(9) Fu, Q.; Bao, X. Surface Chemistry and Catalysis Confined under Two-Dimensional Materials. Chem. Soc. Rev. 2017, 46, 1842−1874. (10) Li, H.; Xiao, J.; Fu, Q.; Bao, X. Confined Catalysis under TwoDimensional Materials. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 5930−5934. (11) Verdaguer, A.; Sacha, G. M.; Bluhm, H.; Salmeron, M. Molecular Structure of Water at Interfaces: Wetting at the Nanometer Scale. Chem. Rev. 2006, 106, 1478−1510. (12) Kasuya, M.; Hino, M.; Yamada, H.; Mizukami, M.; Mori, H.; Kajita, S.; Ohmori, T.; Suzuki, A.; Kurihara, K. Characterization of Water Confined between Silica Surfaces Using the Resonance Shear Measurement. J. Phys. Chem. C 2013, 117, 13540−13546. (13) Li, H.; Zhang, F.; Zhang, Y.; Ye, M.; Zhou, B.; Tang, Y.-Z.; Yang, H.-J.; Xie, M.-Y.; Chen, S.-F.; He, J.-H.; Fang, H.-P.; Hu, J. Peptide Diffusion and Self-Assembly in Ambient Water Nanofilm on Mica Surface. J. Phys. Chem. B 2009, 113, 8795−8799. (14) Ye, M.; Zhang, Y.; Li, H.; Xie, M.; Hu, J. Supramolecular Structures of Amyloid-Related Peptides in an Ambient Water Nanofilm. J. Phys. Chem. B 2010, 114, 15759−15765. (15) Ye, M.; Li, B.; Zhang, Y.; Li, H.; Wang, X.; Hu, J. Confined Water Nanofilm Promoting Nonenzymatic Degradation of DNA Molecules. J. Phys. Chem. B 2011, 115, 2754−2758. (16) Küchler, A.; Yoshimoto, M.; Luginbühl, S.; Mavelli, F.; Walde, P. Enzymatic Reactions in Confined Environments. Nat. Nanotechnol. 2016, 11, 409−420. (17) Major, R. C.; Houston, J. E.; McGrath, M. J.; Siepmann, J. I.; Zhu, X. Y. Viscous Water Meniscus under Nanoconfinement. Phys. Rev. Lett. 2006, 96, 177803. (18) Raviv, U.; Laurat, P.; Klein, J. Fluidity of Water Confined to Subnanometre Films. Nature 2001, 413, 51−54. (19) Zhu, Y.; Granick, S. Viscosity of Interfacial Water. Phys. Rev. Lett. 2001, 87, No. 096104. (20) Sakuma, H.; Otsuki, K.; Kurihara, K. Viscosity and Lubricity of Aqueous NaCl Solution Confined between Mica Surfaces Studied by Shear Resonance Measurement. Phys. Rev. Lett. 2006, 96, No. 046104. (21) Mallamace, F.; Broccio, M.; Corsaro, C.; Faraone, A.; Majolino, D.; Venuti, V.; Liu, L.; Mou, C.-Y.; Chen, S.-H. Evidence of the Existence of the Low-Density Liquid Phase in Supercooled Confined Water. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 424−428. (22) Cao, P.; Xu, K.; Varghese, J. O.; Heath, J. R. Atomic Force Microscopy Characterization of Room-Temperature Adlayers of Small Organic Molecules through Graphene Templating. J. Am. Chem. Soc. 2011, 133, 2334−2337. (23) Chen, S.; Li, H.; Cao, P.; Zeng, X. C. Understanding Liquid− Solid-Like Behavior of Tetrahydrofuran Adlayers at Room Temperature between Graphene and Mica: A Born−Oppenheimer Molecular Dynamics Study. J. Phys. Chem. C 2013, 117, 21894−21900. (24) Xu, K.; Cao, P.; Heath, J. R. Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. Science 2010, 329, 1188−1191. (25) Shim, J.; Lui, C. H.; Ko, T. Y.; Yu, Y.-J.; Kim, P.; Heinz, T. F.; Ryu, S. Water-Gated Charge Doping of Graphene Induced by Mica Substrates. Nano Lett. 2012, 12, 648−654. (26) Lee, H.; Ko, J. H.; Choi, J. S.; Hwang, J. H.; Kim, Y. H.; Salmeron, M.; Park, J. Y. Enhancement of Friction by Water Intercalated between Graphene and Mica. J. Phys. Chem. Lett. 2017, 8, 3482−3487. (27) Fumagalli, L.; Esfandiar, A.; Fabregas, R.; Hu, S.; Ares, P.; Janardanan, A.; Yang, Q.; Radha, B.; Taniguchi, T.; Watanabe, K.; Gomila, G.; Novoselov, K. S.; Giem, A. K. Anomalously Low Dielectric Constant of Confined Water. Science 2018, 360, 1339− 1342. (28) Varghese, J. O.; Agbo, P.; Sutherland, A. M.; Brar, V. W.; Rossman, G. R.; Gray, H. B.; Heath, J. R. The Influence of Water on the Optical Properties of Single-Layer Molybdenum Disulfide. Adv. Mater. 2015, 27, 2734−2740. (29) Komurasaki, H.; Tsukamoto, T.; Yamazaki, K.; Ogino, T. Layered Structures of Interfacial Water and Their Effects on Raman I
DOI: 10.1021/acs.langmuir.8b04232 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (51) Beaglehole, D.; Radlinska, E. Z.; Ninham, B. W.; Christenson, H. K. Inadequacy of Lifshitz Theory for Thin Liquid Films. Phys. Rev. Lett. 1991, 66, 2084−2087. (52) Cantrell, W.; Ewing, G. E. Thin Film Water on Muscovite Mica. J. Phys. Chem. B 2001, 105, 5434−5439. (53) Björneholm, O.; Hansen, M. H.; Hodgson, A.; Liu, L.-M.; Limmer, D. T.; Michaelides, A.; Pedevilla, P.; Rossmeisl, J.; Shen, H.; Tocci, G.; Tyrode, E.; Walz, M. M.; Werner, J.; Bluhm, H. Water at Interfaces. Chem. Rev. 2016, 116, 7698−7726. (54) Arai, T.; Sato, K.; Iida, A.; Tomitori, M. Quasi-Stabilized Hydration Layers on Muscovite Mica under a Thin Water Film Grown from Humid Air. Sci. Rep. 2017, 7, 4054. (55) Wu, S.; Shi, X.; Liu, Y.; Wang, L.; Zhang, J.; Zhao, W.; Wei, P.; Huang, W.; Huang, X.; Li, H. The Influence of Two-Dimensional Organic Adlayer Thickness on the Ultralow Frequency Raman Spectra of Transition Metal Dichalcogenide Nanosheets. Sci. China Mater. 2019, 62, 181−193. (56) Severin, N.; Gienger, J.; Scenev, V.; Lange, P.; Sokolov, I. M.; Rabe, J. P. Nanophase Separation in Monomolecularly Thin Water− Ethanol Films Controlled by Graphene. Nano Lett. 2015, 15, 1171− 1176. (57) Severin, N.; Sokolov, I. M.; Rabe, J. P. Dynamics of Ethanol and Water Mixtures Observed in a Self-Adjusting Molecularly Thin Slit Pore. Langmuir 2014, 30, 3455−3459. (58) Rockland, L. B. Saturated Salt Solutions for Static Control of Relative Humidity between 5° and 40° C. Anal. Chem. 1960, 32, 1375−1376.
J
DOI: 10.1021/acs.langmuir.8b04232 Langmuir XXXX, XXX, XXX−XXX
