Freezing Transition of Interfacial Water at Room Temperature under Electric Fields

The freezing of liquid water into ice was studied inside a gap of nanometer spacing under the control of electric fields and gap distance. The interfacial water underwent a sudden, reversible phase transition to ice in electric fields of ${10}^6\mathrm{V}{\mathrm{m}}^{-1}$ at room temperature. The critical field strength for the freezing transition was much weaker than that theoretically predicted for alignment of water dipoles and crystallization into polar cubic ice ($>{10}^9\mathrm{V}{\mathrm{m}}^{-1}$). This new type of freezing mechanism, occurring in weak electric fields and at room temperature, may have immediate implications for ice formation in diverse natural environments.

Figure 1

(a) Schematic of the electrochemical STM setup and the distance-modulation tunneling spectroscopy. The Au(111) surface of WE is flame annealed, and the gold tip is electrochemically etched and coated with a varnish material except at its apex area of $1–10\mu {\mathrm{m}}^2$. A bipotentiostat controls the electrode potentials within the range of values that avoid specific adsorption of electrolytes at the WE and tip surfaces. As we move the oscillating tip slowly toward the WE surface, we simultaneously measure $I_{\mathrm{t}}$ and $d{I}_{\mathrm{t}}/dz$ by a current amplifier, the latter signal being extracted via phase-sensitive detection. $V_{\mathrm{t}}$ is measured by a high input-impedance ($>1\mathrm{T}\Omega$) voltage follower (not shown). The labels $z$ and $s$ denote the tip apex position and the gap distance, respectively. The normal to Au(111) surface is defined as the $z$ direction. The approach speed of tip can be varied over the range of $0.5–10\mathrm{nm}{\mathrm{s}}^{-1}$, and the oscillation frequency, $5\times 10–1\times {10}^4\mathrm{Hz}$, which ensures that the speeds of the linear and oscillatory motions overlap in the limiting conditions. (b) A sketch of water molecules that form an ice structure of within the STM gap. The oxygen and hydrogen atoms are represented by white and black spheres, respectively. The Au(111) surface is negatively biased.

Figure 2

Tunneling conductance (upper panels), differential tunneling currents (middle panels), and $ds/dz$ (lower panels) measured as a function of gap distance in a 1 mM ${\mathrm{NaClO}}_4$ solution. $V_{\mathrm{b}}$ is $-8\mathrm{mV}$ in (a) and $-100\mathrm{mV}$ in (b). The arrows in the upper panels indicate the bending of conductance curves due to tip-surface contact. In the middle panels, $d{I}_{\mathrm{t}}/dz$ curves are shown with black triangles, and $d{I}_{\mathrm{t}}/ds$ curves with open circles. The curves shown are the average of several repeated measurements, the statistical fluctuations being indicated by the error bars. The tip approach speed is $5\mathrm{nm}{\mathrm{s}}^{-1}$ and the oscillation frequency, $5\times {10}^3\mathrm{Hz}$.

Figure 3

The critical gap distance for water freezing as a function of $V_{\mathrm{t}}$ (in logarithmic scale) measured in 1 mM ${\mathrm{NaClO}}_4$ solution (●) and pure water (○). The zero value of critical distance indicates the absence of freezing transition. The dotted lines denote the sharp transitions at threshold voltages. $V_{\mathrm{t}}$ is substantially lower than the external bias voltage $V_{\mathrm{b}}$ (see the text).