Ice-VII-like Structure Observed By XRD in Water Confined in Silica Nanoholes
Authors: Vereecke Guy , Kenis Karine , Wostyn Kurt , Sanchez Efrain Altamirano 
Published: July 2022

Water structuring has been proposed to occur in water confined in nanochannels and nanoholes based on shifts of the OH stretching peak in Attenuated Total Reflection – Fourier Transformed Infra-Red (ATR-FTIR) measurements (Vereecke, Micro. Eng., 2021 and 2022). Nanoconfined water was characterized by the presence of H-bonds with a stronger vibration frequency compared to ice. Using CO2 as a probe molecule in ATR-FTIR measurements on nanoholes, it was also shown that nanoconfined water was characterized by a slower diffusivity and a lower permittivity compared to bulk water (Vereecke, Micro. Eng., 2021), in agreement with previous studies performed using different techniques (Mawatari, Anal. Chem., 2014; Morikawa, Anal. Chem., 2015). Also, the decreased rate of a model surface reaction performed in nanotrenches of decreasing width correlated well with water structuring characterized by IR spectroscopy (Vereecke, Solid State Phenom., 2018). In the present study, X-Ray diffraction (XRD) measurements were used to further characterize the structuring of water confined in nanoholes.
XRD measurements were performed on a X’pert Pro from Malvern Panalytical equipped with a PixCel detector. Measurements were performed on a parametric space defined by an incident XR angle w varying from 3 to 23°, and a spectra angle 2q varying from 12 to 42°. Measurements of 101 spectra with Dw = 0.2 took 2 days 7 hours. ATR-FTIR measurements were performed in a flow cell on crystals made from wafers as described in Vereecke (Micro. Eng., 2021). The cell was equipped with a bubbler filled with water to control the relative humidity (RH) of the N2 flow in the cell. Samples consisted in nanoholes in an oxide matrix with a diameter of about 20 nm and a depth of about 300 nm, manufactured on Si wafers as described in Vereecke (Micro. Eng., 2021). The last step in making the nanoholes structure consisted in a 30 min bake at 400 °C to remove any tetra-methyl ammonium hydroxide (TMAH) residue present in the holes from etching. The contact angle (CA) of pristine nanoholes was less than 5°. It was increased by silanization treatments. One treatment consisted in the classical HMDS priming photoresist pre-treatment, which gives a CA of about 70° on a flat Si substrate, while the second consisted in the SMT treatment developed by SCREEN, which gives a CA of about 80° on a flat Si substrate. These treatments were performed immediately after drying the sample to prevent any condensation before silanization.
Long XRD measurements were made possible by the anomalous capillary condensation occurring in the 20 nm nanoholes. In turn, condensation was evidenced by ATR-FTIR measurements by monitoring the OH stretching peak. Condensation occurred in nanoholes submitted to a 200 °C bake in about 30 min when submitted to the clean-room air with a RH of about 40%. On the other hand, no condensation was observed in nanoholes under a N2 flow with a RH of about 30%, indicating that condensation was not an artefact, caused for example by the presence of impurities in the nanoholes, but was truly the result of capillary condensation.
XRD measurements showed the presence of a small peak varying as a function of w, from about 2q = 28° at w = 15° to 2q = 33° at w = 18°, which was not present in the Si and oxide references. This peak was close to the 27° peak of ice-VII that has been identified by Raman spectroscopy measurements in water condensed in the nanomeniscus of nanoparticles (Shin, Nature Commun., 2019). The peak was not changed by ageing of the sample from 2 days to 2 months. It was also observed in samples that had been submitted to silanization treatments, bringing the CA on flat substrates from about 5° for the pristine sample, to 70° and 80° for the sample submitted to the HMDS priming and the SMT clean, respectively. This peak was evidence for structuring of water confined in nanoholes. Shifting of the peak as a function of w indicated that water structuring in nanoholes was comparable to a monocrystalline structure. However, the structure of nanoconfined water differed from that of ice-VII. Indeed, the presence of a broad peak at about 22°, which was not observed in the oxide reference, indicated the simultaneous presence of bulk H2O in the nanoholes. These observations added to our understanding of structured water in nanoholes that had been proposed previously based on ATR-FTIR measurements.