Enhanced Adhesivity of Water Molecules Confined in Angstrom-Scale

Nov 14, 2016 - ABSTRACT: Angstrom-scale open spaces formed by two-dimensional (2D) nanosheets possess characteristic molecular sites as a nanosheet ...
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Enhanced Adhesivity of Water Molecules Confined in AngstromScale Open Spaces Formed by Two-Dimensional Nanosheets Kiminori Sato,*,† Koichiro Fujimoto,*,† and Katsuyuki Kawamura‡ †

Department of Environmental Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan Graduate School of Environmental Science, Okayama University, Tsushima, Okayama 700-8530, Japan

‡

ABSTRACT: Angstrom-scale open spaces formed by two-dimensional (2D) nanosheets possess characteristic molecular sites as a nanosheet edge on an interior wall, which are of particular importance in view of molecular adsorption but spontaneously annihilate due to a self-assembly induced by hydration. In the present study, we succeeded in synthesizing a semitransparent silicate mineral homogeneously agglomerated with micrometer-sized particles, in which a large amount of open space with the size of ∼6 Å as a consequence of twonanosheet insertion into interlayer spaces is preserved even after 1 year hydration. A dehydration enthalpy of water molecules for the present sample was found to be ∼46% higher than that of a conventional one, evidencing that an adhesivity of water molecules confined in the 2D nanosheet-forming open spaces is enhanced compared with that in the interlayer spaces of an ordinary adsorption site. with the 2D nanosheets for the saponite minerals.10 One is the local molecular structure, in which a nanosheet is inserted into the interlayer spaces forming the angstrom-scale open space with the size of ∼3 Å. Another is the local structure built by the insertion of two nanosheets into the interlayer space forming the open space with the size of ∼9 Å. It has been turned out that the angstrom-scale open spaces created by the 2D nanosheets possess characteristic molecular sites enhancing an adsorption ability. An example of such molecular sites available on the interior walls of open spaces is a nanosheet edge acting as the specific Cs adsorption site.14,15 One can speculate that a large amount of the 2D nanosheets forming open space are preferably introduced for obtaining functionality in the 2D materials as peculiar molecular adsorption. However, this could not be easily feasible since the 2D nanosheets inserted into the interlayer spaces are gradually released away due to the self-assembly, leading to annihilation of 2D nanosheet-forming open spaces. In this study, the 2D material of saponite silicate mineral homogeneously agglomerated with micrometer-sized particles was synthesized by the gravitational sedimentation method applying Stokes’ law. The 2D nanosheets were found to create angstromscale open spaces, in which H2O molecules are adhesively confined. We first discuss the molecular mechanism that preserves the 2D nanosheets-forming open spaces on the basis of an enlargement and homogeneity of particle sizes. Then, the origin of enhanced adhesivity of H2O molecules confined in the open spaces is explored focusing on the information on

1. INTRODUCTION Along with current concerns for global warming, “water retention” has become well acknowledged as a scientific term that appears in a wide variety of fields as e.g. environmental,1 soil,2 agriculture,3 material,4,5 and cosmetics.6 It is stated in World Day to Combat Desertification 2016 Theme of United Nations that the residue from pruned tress can be used to provide mulching for fields, thus increasing soil water retention and reducing evaporation.7 A water retention polymer could be one of typical water retention materials with high adsorption capacity, which has been utilized for a polymer electrolyte having high proton conductivity4,5 as well as the waterabsorbing ingredients of baby diapers.8 An inorganic layered silicate mineral can be a potential candidate for water retention materials as well,9 since interlayer cations in compensation for a negative layer charge attract polarized H2O molecules. Indeed, an effective improvement of water retention has been reported for a Nafion polymer even at high temperature above 100 °C when organo-montmorillonites are used as fillers.5 Saponite is one of inorganic layered silicate minerals, in which 2D nanosheets with a thickness of a few atomic layers are separated by angstrom-scale interlayer spaces. It is known that the 2D nanosheets, minimum structural unit, spontaneously agglomerate with the aid of H 2 O molecules toward densification.10 This agglomeration process sometimes takes over several months, which is thus called as a long-term selfassembly of 2D nanosheets. The self-assembly has been found to be sensitively influenced by ambient temperature, relative humidity, cation species and state, particle size, and so on.11−13 Our previous studies of positronium (Ps) lifetime spectroscopy together with molecular dynamics (MD) simulation have shown that two kinds of local molecular structures are formed © XXXX American Chemical Society

Received: November 10, 2016 Published: November 14, 2016 A

DOI: 10.1021/acs.jpcc.6b11308 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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elements and annihilated into two photons with a lifetime ∼450 ps. The positron in o-Ps undergoes two-photon annihilation with one of the bound electrons with a lifetime of a few nanoseconds after localization in angstrom-scale pores. The last process is known as o-Ps pick-off annihilation and provides information on the size of spherical open space R through its lifetime τo‑Ps based on the Tao−Eldrup model:19,20

angstrom-scale open spaces, adsorption properties of H2O molecules, and bound state.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. A Na-type saponite powder (54.71% SO2, 5.02% Al2O3, 0.03% Fe2O3, 30.74% MgO, 2.15% Na2O, 0.07% CaO, 0.67% SO3, 6.64% H2O) produced by Kunimine Industries Co. Ltd., Japan, was employed in this study. The average particle size is approximately 20 nm in diameter, and the powder is apparently white. For isolating saponite particles larger than 2 μm in diameter from the powder, the gravitational sedimentation method was conducted, the principle of which is detailed below. In fluid, two main forcesgravity and drag forceare in principle applied for a particle. Immediately after dispersing into fluid, the particle is approximated to be at rest and no drag force is applied, by which the particle begins to be accelerated only by gravity. With increasing acceleration, the contribution of the drag force in the direction opposite to the particle’s motion increases retarding further acceleration. Eventually, gravity and the drag force are balanced each other, yielding the constant velocity, which is known as the terminal velocity under the steady state condition. Assuming a spherical shape of particle, the terminal velocity v of the particle in fluid under the steady state condition is described by Stokes’ law as v=

τo‐Ps

(2)

where R0 = R + ΔR, and ΔR = 0.166 nm is the thickness of homogeneous electron layer in which the positron in o-Ps annihilates. The positron source (22Na), sealed in a thin foil of Kapton, was mounted in a sample−source−sample sandwich. A multiexponential analysis using the POSITRONFIT code22 was applied to the recorded positron lifetime data to deduce o-Ps lifetimes τo‑Ps. The experimental uncertainties were estimated by repeating measurements four times. 2.3. Adsorption Properties of Water Molecules. Adsorption properties of H2O molecules was investigated with respect to the thermogravity (TG) caused by weight loss and differential scanning calorimetry (DSC). The TG data were obtained by TG-DTA system (TG-DTA 2020SA, BRUKER AXS Co. Ltd.) at room temperature with α corundum (α Al2O3) as an internal standard. The 10 mg samples were mounted in aluminum pans and measured after temperature calibration with indium and tin. A nitrogen gas was purged during the heating run. Measurements were performed from 293 to 573 K at a heating rate of 5 K/min. The DSC data were obtained by DSC system (DSC3300SA, BRUKER AXS Co. Ltd.) under the same condition with those of TG measurements. The dehydration enthalpies ΔH of H2O molecules were evaluated by integrating the endothermic peaks. The experimental uncertainties of were estimated by repeating measurements three times.

g (ρs − ρw )d 2 18μ

−1 ⎡ ⎛ 2πR ⎞⎤ R 1 ⎢ ⎥ = 0.5 1 − + sin⎜ ⎟ ⎢⎣ R0 2π ⎝ R 0 ⎠⎥⎦

(1)

where g, ρs, ρw, d, and μ are gravity, density of solid, density of water, particle diameter, and molecular viscosity, respectively. Our former FE-SEM studies revealed that the 2D nanosheets in the nanoparticle samples are agglomerated forming a spherelike shape.11 It is obvious from eq 1 that the particles with smaller size take long to sediment. For example, the terminal velocity v of the particle with 2 μm is calculated to be 1.7 × 10−2 cm min−1 by putting g = 980 cm s−2, ρs = 2.3 g cm−3, ρw = 1.0 g cm−3, d = 2.0 × 10−4 cm, and μ = 1.0 × 10−2 g cm−1 s−1 into eq 1. The present experiments were conducted with the sedimentation distance of 20 cm in the graduated cylinder, which takes 20 h for complete sedimentation. The supernatant solution containing the particles smaller than 2 μm was discarded after 20 h. This process was repeated 5 times so as to enhance the homogeneity of particle size. The residual particles were dried in the thermostatic chamber for a half year and then hydrated for 1 year under the atmospheric condition. The general appearance, surface morphology, phase structure, and bond state were examined by optical microscopy, field-emission type scanning electron microscopy (FE-SEM, SU8010, Hitachi High-Technologies), X-ray diffraction (XRD, Ultima IV, RIGAKU), and Fourier transform infrared (FT-IR) spectroscopy (Nicolet iS5, Thermo Fisher Scientific), respectively. 2.2. Characterization of the Open Spaces. The sizes of angstrom-scale open spaces and their fractions were investigated by Ps lifetime spectroscopy.16−18 A fraction of energetic positrons injected into samples form the bound state with an electron, Ps. Singlet para-Ps (p-Ps) with the spins of the positron and electron antiparallel and triplet ortho-Ps (o-Ps) with parallel spins are formed at a ratio of 1:3. Hence, three states of positronsp-Ps, o-Ps, and free positronsexist in samples. The annihilation of p-Ps results in the emission of two γ-ray photons of 511 keV with a lifetime ∼125 ps. Free positrons are trapped by negatively charged parts such as polar

3. RESULTS AND DISCUSSION Figure 1a shows a photograph image of synthesized sample after the long-term hydration of 1 year. It is obvious that no

Figure 1. (a) Photograph and (b) FE-SEM image of the present film sample after 1 year hydration. Several particles in the FE-SEM image are marked in white.

dissolution occurs for the sample after 1 year hydration. The sample is a film-like state with a thickness of ∼30 μm exhibiting a semitransparent nature in contrast to the as-received powder, which is referred as the film sample thereafter. On the other hand, the as-received powder sample is called the nanoparticle one. The semitransparent nature of the film sample indicates that the diffuse reflection of visible light from the surface of the particle interface is suppressed. It is thus presumed that the saponite particles with the size larger than 2 μm are more B

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sample prepared by isolating the micrometer-sized particles is stable with respect to the mechanical distortion in the direction perpendicular to the basal planes. Ps lifetime spectroscopy yielded two components of o-Ps lifetimes τ3 and τ4 with the relative intensities I3 and I4 for both the film and nanoparticle samples as listed in Table 1. The o-Ps lifetimes τ3 and τ4 are ∼2.6 ns and ∼9.6 ns for the film sample, where the sizes of the open spaces R3 and R4 in radius are evaluated as ∼3.4 Å and 6.4 Å, respectively. The o-Ps lifetimes τ3 ∼ 2.2 ns and τ4 ∼ 10.1 corresponding to the sizes of ∼3.0 and 6.6 Å are obtained for the nanoparticle one, in consistent with our previous work.10 Note that the small and large open spaces for the film sample have similar sizes to those of the nanoparticle one. It is reasonably inferred that two species of open spaces introduced in the film sample are essentially identical to those in the nanoparticle one. The relative intensities I3 and I4 are ∼2% and ∼14% for the film sample, whereas the nanoparticle one exhibits the higher I3 ∼ 15% and lower I4 ∼ 2%. The local molecular structures with open spaces attributable to τ3 and τ4 in the nanoparticle sample have been already figured out based on the results of MD calculations.10 According to the rheological model with the 2D nanosheets established recently,10−12 the nanoparticle sample possess two kinds of local molecular structures, where one and two nanosheets are inserted into interlayer spaces forming open spaces with their sizes of ∼3 and ∼6 Å, respectively. Since the spherical shape of open space is assumed in the present model, the radii of inscribed sphere in the open spaces of the local molecular structures formed by nanosheet insertion are evaluated, which is well seen in the cross-section image simulated by MD calculations.10 The local molecular structures associated with nanosheet insertion could be easily formed as expected from the FE-SEM image of the nanoparticle sample, in which some of nanosheets are partially stacked (see Figure 3). Prior to the self-assembly, the local molecular structure formed by two-nanosheet insertion dominantly exists as evidenced from the higher relative intensity I4 of large open space τ4.10 H2O molecules adsorbed at the Na+ cations in the interlayer spaces act as a lubricant and are the driving force for the rheological motion of nanosheets parallel to the layer direction. One of the two nanosheets inserted into the interlayer spaces is thus released away. Consequently, the local molecular structures with the larger open spaces in the two-nanosheet insertion type are gradually altered to those with the smaller open spaces of one-nanosheet insertion type which become dominant for the well self-assembled nanoparticle sample as shown in Table 1. It is noted here that the relative intensity I4 of large open space is much higher for the film sample than that of nanoparticle one. This evidences that the open spaces in the local molecular structure of two-nanosheet insertion type remain even after the long-term hydration of 1 year. As revealed by the XRD studies coupled with FE-SEM observation, the 2D

homogeneously precipitated all together than those of nanoparticle sample. Homogeneous precipitation of 2 μm sized particles is well seen in a FE-SEM image of Figure 1b, in consistent with the semitransparent nature in the photograph image of Figure 1a. Figure 2a shows XRD patterns observed for the nanoparticle (black) and film (red) samples. The nanoparticle sample

Figure 2. XRD patterns for (a) nanoparticle (black) and film (red) samples together with that (b) powderized again.

exhibits a wide variety of diffraction peaks as (00l) reflections characteristic for the basal planes of 2D layered minerals as well as (0k0) perpendicular to (00l). This is typical for the multiphases composite with each phase, in which the 2D nanosheets are ordered in every direction. In addition to that all the peaks are relatively broad with a significantly high background for the nanoparticle sample, indicating that the 2D nanosheets are agglomerated with a wide distribution in sizes. On the contrary, there appear diffraction peaks exclusively arising from basal reflection as (001) and (004) with a low background for the film sample. They are relatively sharp compared with those of nanoparticle sample, signifying that the 2D nanosheets are stacked in the direction of basal planes inside the 2 μm sized particles selectively isolated here. Presumably, the present synthesis with gravitational sedimentation in fluid facilitates to rotate the 2D nanosheets so that their basal planes can be lay down perpendicular to the direction of gravity. Figure 2b shows XRD pattern observed for the sample, which was powderized again. The diffraction peaks arising from the crystal planes normal to (00l) reappear together with (130) reflection as indicated by arrows upon repowderization, but the multiphases composite structure as shown in Figure 1a is not fully obtained. The difficulty in recovery indicates that the film

Table 1. o-Ps Lifetimes τ3 and τ4 and Their Corresponding Sizes in Radius R3 and R4 Together with Relative Intensities I3 and I4 for the Film and Nanoparticle Samples as Well as Values of Dehydration Enthalpy ΔH sample

τ3 [ns]

R3 [Å]

I3 [%]

τ4 [ns]

R4 [Å]

I4 [%]

ΔH [cal]

film nanoparticle

2.6 ± 0.4 2.2 ± 0.3

3.4 ± 0.2 3.0 ± 0.2

2±1 15 ± 2

9.6 ± 0.5 10.1 ± 0.5

6.4 ± 0.3 6.6 ± 0.3

14 ± 2 2±1

0.8 ± 0.05 0.5 ± 0.05

C

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spaces are very similar between the film and nanoparticle samples. Figure 5 shows DSC curves for the film and nanoparticle samples, in which the endothermic peaks relevant to

Figure 3. FE-SEM image of nanoparticle sample indicating partially stacked nanosheets. Figure 5. DSC curves for the nanoparticle (black) and film (red) samples.

nanosheets are stacked all together in the direction of basal planes inside the micrometer-sized particles selectively isolated in the film sample. In addition to that the XRD experiment for the repowderized sample implies that the isolated particles are highly stable with respect to the mechanical distortion in the direction perpendicular to the basal planes. It is thus expected that the 2D nanosheets are well in-plane stacked in the large particles for the film sample in an excellently stable manner. In such a rigid agglomeration of 2D nanosheets, the self-assembly toward structural densification could be suppressed preserving the open space with the size of ∼6 Å. It is worth investigating an adsorption property of H2O molecules confined in the angstrom-scale open spaces introduced in the present film sample. Figure 4 shows the

dehydration are highlighted. In contrast to the TG results, the endothermic peak for the film sample is deep and significantly shifted to higher temperature, enabling to distinguish the adsorption property of H2O molecules in the open space of two-nanosheet insertion type. The dehydration enthalpies ΔH of H2O molecules evaluated by integrating the endothermic peaks are 0.8 and 0.5 cal for the film and nanoparticle samples, respectively, where the value of ΔH for the film sample is ∼46% higher than the nanoparticle one (see Table 1). As revealed by positron lifetime spectroscopy, more fraction of open spaces in the local molecular structures of twonanosheet insertion type are introduced for the film sample than those of nanoparticle one. An adsorption property in the interlayer spaces could be unchanged for the film sample because the local cation state inside the interlayer spaces remains the same.12 It is thus expected that the higher dehydration enthalpy by ∼46% for the film sample originates from an increased adhesivity of H2O adsorption in the open spaces of two-nanosheet insertion type. Here, we explore the reason why H2O molecules become adhesive in the open space of two-nanosheet insertion type in comparison with that in the interlayer spaces of ordinary adsorption site. The characteristic molecular site available on the interior wall in the above open space is a nanosheet edge resultant from nanosheet insertion, where Si atoms are missing yielding dangling electrons and chemical species associated with them. Such chemical defects could cause the charge transfer to H2O molecules, which has been predicted by ab initio calculation.22 This assumption is experimentally supported by the results of FT-IR spectroscopy for the film and nanoparticle samples shown in Figure 6, in which absorption bands at wavenumbers in the vicinity of 1000 cm−1 is highlighted. This band has been often observed for silicate minerals as the present saponite, being attributable to Si−O stretching vibrations.23 In the film sample, the Si−O stretching vibration band is red-shifted toward to the lower wavenumber along with significant narrowing. The spectral narrowing could be caused by the equalization of bond length arising from homogeneous agglomeration of micrometer-sized particles, which is consistent with the results of XRD and optical microscopy. Of particular interest is the red-shift in the spectrum of the film sample,

Figure 4. Temperature variation of TG for the nanoparticle (black) and film (red) samples.

temperature variation of TG for the film and nanoparticle samples. TG curves exhibit ∼15% decreases in the weight with increasing temperature owing to dehydration for both the film and nanoparticle samples. Our former 1H NMR studies for the nanoparticle sample have shown that the weight gain due to water adsorption seen in TG curves is solely contributed from adsorption at Na+ cations, signifying an occurrence of water adsorption exclusively in the above-mentioned open spaces and interlayer spaces.10 The TG results in Figure 4 thus indicate that the adsorption capacities of H2O molecules totally contributed from the open spaces found here and interlayer D

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molecules totally contributed from the open spaces found here and interlayer spaces are very similar between the film and nanoparticle samples. On the contrary, the DSC analyses revealed that the dehydration enthalpy of H2O molecules for the film sample is ∼46% higher than that of the nanoparticle one, originating from an increased adhesivity of H2O molecules confined in the 2D nanosheet-formed open spaces.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (K.S.). *E-mail [email protected] (K.F.). Notes

Figure 6. FT-IR spectra obtained for the nanoparticle (black) and film (red) samples.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are indebted to K. Ito (AIST) for fruitful discussions. This work was partially supported by a Grant-inAid of the Japanese Ministry of Education, Science, Sports and Culture (Grant Nos. 15K05308 and 16K05394).

indicating that the Si−O bonding gets weakened. We expect that the weakened Si−O bonding arises from less Coulombic attraction due to charge transfer from Si atoms. The charge transfer from Si atoms to H2O molecules causes more polarized H2O structure enhancing physisorption at Na+ cations located in the vicinity of the surface of 2D nanosheets. In general, an excellent water holding material can be achieved not only by a high adsorption capacity of H2O molecules but also a retention ability. Needless to say, H2O molecules confined in open spaces would be easily released with the poor retention ability even though a large amount of open space for the water uptake are available. As evidenced by the present TG experiments, the high adsorption capacity of H2O molecules is maintained upon the isolation of the micrometer-sized particles. On the one hand, DSC revealed that the dehydration enthalpy of H2O molecules ΔH for the film sample is ∼46% higher than that of the nanoparticle one, demonstrating an increased adhesivity of H2O molecules. It is highly beneficial that water retention ability is largely enhanced without deteriorating a water adsorption capacity of material. The present synthetic technique simply focusing on the isolation of micrometer-sized particles would open up a new route for the molecular design of water holding materials with not only a high adsorption capacity but also a high retention ability.

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REFERENCES

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4. CONCLUSIONS Saponite particles larger than ∼2 μm in diameter were isolated by means of gravitational sedimentation method applying Stokes’ law. Optical microscopy and FE-SEM observations showed a film-like state with a thickness of ∼30 μm and a semitransparent nature, signifying that the saponite particles are homogeneously agglomerated all together. XRD experiments provided further information on 2D nanosheets that are stacked in the direction of basal planes inside the micrometer-sized particles selectively isolated. Ps lifetime spectroscopy for the film sample yielded o-Ps lifetimes ∼2.6 and ∼9.6 ns, corresponding to the sizes ∼3 and ∼6 Å, respectively, which are similar to those of the nanoparticle sample. The film sample is thus expected to possess the same species of open spaces as those of the nanoparticle one, i.e., open spaces in the local molecular structures of one- and two-nanosheet insertion types. The local molecular structure of two-nanosheet insertion type is preserved for the film sample due to suppression of selfassembly even after the long-term hydration of 1 year. The TG results indicated that the adsorption capacities of H2O E

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DOI: 10.1021/acs.jpcc.6b11308 J. Phys. Chem. C XXXX, XXX, XXX−XXX