Porous Ice Phases with VI and Distorted VII Structures Constrained in

Oct 22, 2014 - Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, New York 14853, United States. ¶ HiPSEC, Depart...
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Porous Ice Phases with VI and Distorted VII Structures Constrained in Nanoporous Silica Jinlong Zhu,*,†,‡ Zewei Quan,*,§ Yu-Shen Lin,∥,⊥ Ying-Bing Jiang,# Zhongwu Wang,∇ Jianzhong Zhang,‡ Changqing Jin,† Yusheng Zhao,¶ Zhenxian Liu,● C. Jeffrey Brinker,∥ and Hongwu Xu*,§ †

National Lab for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China LANSCE and §EES Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Center for Micro-Engineered Materials, ⊥Department of Internal Medicine, Division of Molecular Medicine, and #TEM Laboratory, The University of New Mexico, Albuquerque, New Mexico 87131, United States ∇ Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, New York 14853, United States ¶ HiPSEC, Department of Physics and Astronomy, The University of Nevada, Las Vegas, Nevada 89154, United States ● Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, United States ‡

S Supporting Information *

ABSTRACT: High-pressure compression of water contained in nanoporous silica allowed fabrication of novel porous ice phases as a function of pressure. The starting liquid nanoporous H2O transformed to ice VI and VII at 1.7 and 2.5 GPa, respectively, which are 0.6 and 0.4 GPa higher than commonly accepted pressures for bulk H2O. The continuous increase of pressure drives the formation of a tetragonally distorted VII structure with the space group I4mm, rather than a cubic Pn3m phase in bulk ice. The enhanced incompressibility of the tetragonal ice is related to the unique nanoporous configuration, and the distortion ratio c/a gradually increases with increasing pressure. The structural changes and enhanced thermodynamic stability may be interpreted by the two-dimensional distribution of silanol groups on the porous silica surfaces and the associated anisotropic interactions with H2O at the interfaces. KEYWORDS: Nanoporous ice, high pressure, DAC, SAXS, WAXS

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nanoscale due to the synthetic and technical difficulties. Currently, nanoscale manipulation of H2O can be reasonably achieved by trapping H2O in nanoporous networks (e.g., zeolites) or nanochannels (carbon nanotubes8,9). Once water is trapped in these nanoscale spaces, a series of in situ characterizations can be performed to explore its interaction with surrounding medium at interfaces, such as the hydrophilic −OH group or the hydrophobic −CH3 group. Characterization of the associated structure and property changes8−14 allow us to understand possible influence and rearrangement of the hydrogen bonds of bulk H2O in various environments. Nanoporous materials including nanotube,8,9,15,16 nanoslit,17 and nanoporous silica10 can be considered as ideal media to trap water or ice inside, thereby producing either one, two, or three-dimensional nanoscale forms of water or ices with various packing symmetries.8−11,15−17 Nanoporous silica possesses large surface areas and constitutes two-dimensional (2D) hexagonal arrays of hollow channels with tunable diameters

ater is the most abundant species on the surface of the Earth and is of vital importance to the terrestrial life. The interaction of water with biological matter can result in the formation of nanoscale water in biological systems. For example, a large fraction of cellular waters are commonly referred to as nanoscale waters, including semiclathrate water,1 layered water,2 and hydrogen-bonded chains,3 with lifetimes over tens of picoseconds in a series of biological processes such as protein folding, pressure-driven unfolding, and a series of procedures developing anisotropic stresses and constrained effect of water. Interfaces between amorphous silica and water generally exist in environmental, chemical, and biochemical systems. Technologically, micro- or even nanodevices are fabricated from amorphous silica, which often involves the surface contact with water for oxide coating.4−7 However, intense efforts have mostly been focused on the investigation of structural complexity of water in bulk forms over a wide range of pressure (P)/temperature (T) conditions. Consequently, more than 15 crystal structures have been revealed in H2O, which are related to either proton-order or disorder state. Although nanoscale H2O has been well recognized of importance in various environments or systems, very little is known about the structures of liquid/solid H2O confined at © 2014 American Chemical Society

Received: August 17, 2014 Revised: October 20, 2014 Published: October 22, 2014 6554

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typically in the range of 2−15 nm.12 Fabrication of high-quality nanoporous silica with control of the pore/channel size has enabled feasible manipulation of H2O into embedded nanoscale channels, providing new opportunities to exploit the H2O− H2O or silica−OH group interactions and possible anisotropic ordering of H2O molecules at the nanoscale.13 To understand the nanoscale confining effect of H2O and construct nanoenabled phase diagram of H2O, we used a pressure-processing approach to fabricate the nanoporous water and ices via use of a nanoporous silica template (amorphous SiO2). Thus, by employing a diamond−anvil cell (DAC) coupled with newly developed synchrotron-based small-angle/wide-angle X-ray scattering (SAXS/WAXS) techniques, we simultaneously monitored the evolution of the porous superlattice array and pressure-induced atomic-scale structural changes of porous ice. The nanoporous silica samples of varied pore sizes (1, 2.5, 2.8, and 6 nm pore diameters, Figure 1 and Figure S1) used in

using a 500-ton, toroidal anvil press (TAP-98) at the beamline of high-pressure-preferred orientation (HIPPO), Los Alamos Neutron Science Center (LANSCE). The time-of-flight neutron data of the powder samples were collected by three detector banks at a fixed Bragg angle of 2θ = ± 90°. Transmission electron microscopy (TEM) was used to characterize the pore size and structure of the starting and recovered nanoporous silica samples. Infrared (IR) spectroscopic experiments on nanoporous silica were performed at the U2A beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The IR spectra were collected in a transmission mode by a Bruker FTIR spectrometer using a nitrogen-cooled midband MCT (MCTA) detector. The recorded frequencies are in the range of 600− 8000 cm−1 with a resolution of 4 cm−1. IR spectra of nanoporous silica samples with different pore sizes indicate that the silanol (Si−OH) species generally exist on the surfaces of the nanoporous silica (Figure S2). Considering the large surface areas of nanoporous silica and therefore the large amount of hydrated bonds formed between the surface Si−OH groups and absorbed water, it is reasonable to obtain the phase diagram and nucleation pressures of H2O in nanoporous system that are distinct from those in bulk H2O. The nanoporous ice VI nucleates in porous silica at the pressure of ∼1.7 GPa (Figure 2), significantly higher than 1.05 GPa observed in bulk ice.23,24 Further increase of pressure does not transform ice VI to cubic ice VII observed in bulk at 2.1 GPa,23,24 but, instead, to a distorted “pseudocubic” structure at a higher pressure of 2.5 GPa (Figure 2). Using Le Bail refinement approach, this “pseudocubic” ice VII was found to adopt a tetragonal structure with the space group I4mm. Consistent increases of transition pressures in ice VI and VII indicate that the two ice phases do not crystallize from H2O confined in nanopores but instead from H2O adsorbed on the surfaces of nanoporous silica (right inset of Figure 3a). Otherwise, the diffraction peaks of ice formed in different size pores should display different magnitudes of peak broadening, which were not observed in our wide angle diffraction patterns (Figure 2). The nanoporous ice VI displays a strong preferred orientation compared to bulk ice (Figure S3), implying that anisotropic interactions occur at the interfaces between surface Si−OH species and adsorbed H2O. Thus, the increase of transition pressure can be used to estimate the strength of the hydrogen interactions, allowing for qualitatively understanding of the influence of nanoscale water in biologically related systems. As shown in Figure 3a, the crystallization of ice VI displays a directional H···H2O alignment. The first layer of H2O molecules bonded with the −OH groups on nanoporous silica surfaces has a 2D configuration. Therefore, such a boundary layer prevents realignment of H2O molecules once while the crystallization of ice VI occurs. The quasi-liquid layer of water1 or the icebinding surface25 induces a strong stress which results in ∼0.6 GPa enhancement of the ice nucleation pressure. This enhanced strength through contribution of nano water in biological systems is estimated on an order of 0.1 GPa,20 large enough to fold/unfold soft materials like protein.1,25−27 This further implies that the binding between H2O and hydrophilic surface with −OH groups becomes stronger. Accordingly, the enhanced activation barrier prevents effective diffusion of water on the surface, leading the ion diffusion rate 5 times slower than that in bulk water.2

Figure 1. SAXS patterns of nanoporous silica with a two-dimensional hexagonal superlattice, and inserts are several typical TEM images of nanoporous silica samples that have pore sizes of 1, 2.5, 2.8, and 6 nm, respectively. All of the nanoamorphous silica samples have hydrophilic −OH groups attached on the surfaces.

this study were synthesized according to previously developed approaches.14,18,19 The option of these four particular pore sizes is based on their close relation to the observed nanoscale waters. For instance, diffusion of water within ∼1 nm at hydrophilic lipid vesicle surfaces is much slower than the translational diffusion of bulk water;2 the distance between four-helix bundles is estimated on an order of 2−3 nm, and inside semiclathrate water is formed.1 At hydrophilic or highly charged surfaces, water molecules can aggregate and thus form a weak hydrogen-bonded zone with a thickness of ∼6 nm, in which both cations and anions display selection rules during biological processes.20 Using a symmetric DAC, a mixture of nanoporous silica and H2O was loaded into a preindented (∼60 μm in thickness) T301 steel gasket chamber with 200 μm in diameter. A small ruby chip was placed on the top of the samples as a pressure marker. Upon the increase of pressure, synchrotron-based SAXS and WAXS images were simultaneously collected from the same volume of the samples at the B1 beamline of CHESS.21,22 The incident X-rays collimated at λ = 0.485946 Å was used to illuminate the samples; the X-ray scattering signals were recorded by a large area MAR345 detector. Neutron diffraction under pressure was performed by 6555

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Figure 2. In situ WAXS patterns of porous ices constrained in nanoporous silica with various pore sizes under compression. Ice VI started to occur at ∼1.7 GPa and then transformed to a distorted ice VII at ∼2.5 GPa.

pressure. Figure 3b plots the Le Bail refined c/a ratios of tetragonal distorted ice VII as a function of pressure; the c/a ratio increases from 1.003 at 2.47 GPa to 1.012 at 4.7 GPa to 1.047 at 14.1 GPa. Such an increase of c/a ratio corresponds to an increased magnitude of the structural distortion of nanoporous ice VII. We hypothesize that the hydrogen binding interactions between H2O molecules and Si−OH groups at interfaces result in this behavior, as demonstrated by the schematic show in the inset of Figure 3a. The 2D distribution of Si−OH groups at silica nanopore surfaces enables anisotropic interactions of H2O molecules with Si−OH groups, thus causing preferred orientation of hydrogen-mediated binding. Meanwhile, the interactions between H2O molecules and Si−OH groups could also allow additional binding configurations (Figure 3a, inset), leading to additional structural anisotropy. In summary, we have successfully synthesized nanoporous ice phases that nucleate at higher pressures and/or in different structures. In particular, the tetragonally distorted ice VII phase possesses a new structure that has not been observed in bulk ice. In situ high-pressure SAXS and WAXS studies indicate that

Fitting the volume−pressure data of porous ice VI yielded a bulk modulus of 14.6(15) GPa (K′ = 4), similar to that in bulk ice VI. On the other hand, the bulk modulus of the tetragonally distorted VII ice is determined to be 24.7 (9) GPa, which is higher than all the reported bulk moduli of ice VII ranging from 12.5 (3) to 24.0 (9) GPa.24 In order to rule out the claim possibly made by accident or by a series of random works, this study employed four samples with different pore sizes to make investigations systematically and comparably. The bulk moduli of different sized samples are quite consistent, rather than a random distribution, implying a universal size-tuned increase of the bulk modulus. This elevated bulk modulus is most likely related to the H2O/Si−OH interactions that control the formation of this nanoporous ice phase. Bulk ice VII has been extensively studied and considered as a proton disordered cubic structure with the space group Pn− 3m,28 but how the typical configuration inside the structure can be correlated to the proton order remains elusive.29−31 As stated above, the nanoporous VII ice phase is stronger than its bulk counterpart. Structurally, nanoporous ice VII displays an apparent tetragonal distortion that enhances upon increase of 6556

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AUTHOR INFORMATION

Corresponding Authors

*Jinlong Zhu: [email protected]. *Hongwu Xu: [email protected]. *Zewei Quan: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge the support from NSF & MOST of China through research projects at Institute of Physics, Chinese Academy of Sciences. This work is also supported by the laboratory-directed research and development (LDRD) program of Los Alamos National Laboratory, which is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396. The neutron experiment has benefited from the use of the Lujan Neutron Scattering Center at Los Alamos Neutron Science Center, which is funded by the U.S. Department of Energy’s Office of Basic Energy Sciences. CHESS is supported by the NSF award DMR-0936384. U2A is supported by COMPRES under NSF Cooperative Agreement EAR 11-57758 and DOE-NNSA (DE-FC-52-O8NA28554, CDAC). Use of the NSLS was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences. C.J.B. acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Y.-S.L. acknowledges support from the Air Force Office of Scientific Research under grant #FA9550-14-1-0066 and the U.S. Department of Energy, Office of Science, Catalysis Science program under grant DE-FG02-02ER15368.

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Figure 3. (a) Volumetric changes (Le Bail method) of ice VI and distorted ice VII constrained in porous silica as a function of pressure at room temperature. Insets show the structures of ice VI (left), tetragonal distorted (I4mm) ice VII (middle), and the bonding interaction at the interface of porous silica and H2O; the red balls represent oxygen, yellow represent silicon, light gray represent hydrogen of water, and the blue balls represent the hydrogen of the Si−OH group on silica surface. (b) The dependence of c/a ratio of distorted ice VII on pressure. The data fittings by equation of state at room temperature give the bulk modulus and cell parameters: ice VI, K0 = 14.6(15) GPa and V0 = 234(5) Å3; distorted ice VII, K0 = 24.7(9) GPa, V0 = 40.7(5) Å3. K0′ = 4 was fixed for both ice VI and VII due to lack of enough data points and a narrow pressure range; upper left inset plots the (110/101) peaks of distorted ice VII at 14.1 GPa.

these ice phases crystallize from H2O on hydrophilic porous silica surfaces. The hydrogen binding interactions between H2O molecules and surface-absorbed Si−OH groups are likely responsible for the higher transition pressures and crystallization of the new distorted ice VII.

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ASSOCIATED CONTENT

S Supporting Information *

Details of sample synthesis and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. 6557

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