Observation of extreme phase transition temperatures of water confined inside isolated carbon nanotubes

Journal name:
Nature Nanotechnology
Year published:
Published online


Fluid phase transitions inside single, isolated carbon nanotubes are predicted to deviate substantially from classical thermodynamics. This behaviour enables the study of ice nanotubes and the exploration of their potential applications. Here we report measurements of the phase boundaries of water confined within six isolated carbon nanotubes of different diameters (1.05, 1.06, 1.15, 1.24, 1.44 and 1.52 nm) using Raman spectroscopy. The results reveal an exquisite sensitivity to diameter and substantially larger temperature elevations of the freezing transition (by as much as 100 °C) than have been theoretically predicted. Dynamic water filling and reversible freezing transitions were marked by 2–5 cm−1 shifts in the radial breathing mode frequency, revealing reversible melting bracketed to 105–151 °C and 87–117 °C for 1.05 and 1.06 nm single-walled carbon nanotubes, respectively. Near-ambient phase changes were observed for 1.44 and 1.52 nm nanotubes, bracketed between 15–49 °C and 3–30 °C, respectively, whereas the depression of the freezing point was observed for the 1.15 nm nanotube between −35 and 10 °C. We also find that the interior aqueous phase reversibly decreases the axial thermal conductivity of the nanotube by as much as 500%, allowing digital control of the heat flux.

At a glance


  1. Evidence of filling and phase transition of water inside CNTs.
    Figure 1: Evidence of filling and phase transition of water inside CNTs.

    a, Fabrication of the platform, which comprises isolated CNTs connected to two reservoirs. (i) A catalyst solution consisting of iron nanoparticles (red) is deposited along an edge of the Si substrate. (ii) Centimere-long, horizontally aligned CNTs are grown using CVD with methane. (iii) A PDMS protective mask is placed on the middle section of the CNTs. (iv) Air plasma removes the uncovered parts of the CNTs, opening up the ends. (v) The PDMS mask is removed. (vi) Two PDMS reservoirs are adhered to the surface. (vii) The reservoirs are filled with water and microRaman spectroscopy is performed on the CNT. b, Temporal study of the RBM frequency. (i,ii) A time-map showing the evolution of RBM frequency and intensities for the 1.15nm DWNT. Dotted lines indicate the time points at which water is added to the reservoirs. (iii,iv) Fitted peak positions for the outer and inner RBMs, respectively. 1, reservoirs empty; 2, water added to right reservoir; 3, water added to left reservoir. c, A visual MD model of the CNTs studied here. The 1.06nm SWNT was bundled with a 1.55nm SWNT. d, A contour plot of the representative RBM frequency (based on the signal-to-noise ratio) versus temperature for the six tubes studied here. The white line represents the transition from the dry to the water-filled state, whereas the black line signifies the freezing transition of the confined water.

  2. Diameter-dependent vapour–liquid and liquid–solid phase transitions of confined water inside CNTs.
    Figure 2: Diameter-dependent vapour–liquid and liquid–solid phase transitions of confined water inside CNTs.

    a, Histograms of the RBM peak positions for the six CNTs. Gaussian peaks are fitted to the histogram data, showing up to three peaks that correspond to the distinct states of the CNTs. b, RBM peak position as a function of tube temperature. A combination of method 1 and method 2 was used to achieve the temperature. Blue left-pointing arrows indicate cooling cycles; red right-pointing arrows indicate heating cycles; the lack of arrows indicates a neutral, starting state. In the case of an RBM peak with a shoulder, we deconvoluted the peaks and the resulting data points are plotted using circular markers with a size that is proportional to the peak area. Single-peak RBMs are represented by the diamond markers. The fitting for deconvoluted data points included weights proportional to the peak area. The shaded brackets correspond to the three states of the CNTs (cyan,  solid water; green, liquid water and pink, dry CNTs). The width of each bracket is the FWHM of the peaks fitted to the RBM histogram in a. The black curves are finite-width Heaviside fits (confidence interval in Supplementary Table 3) and comparative linear fits are shown in Supplementary Fig. 10 and Supplementary Table 4. The RBM error bars originate from the error in fitting the RBM peak with a Lorentzian curve. c, Illustration of the three CNT states.

  3. Reversibility of the water phases inside the CNT on heating and cooling, and agreement in the phase transition data obtained by the two heating methods.
    Figure 3: Reversibility of the water phases inside the CNT on heating and cooling, and agreement in the phase transition data obtained by the two heating methods.

    a, RBM frequency of the CNTs as a function of heating and cooling oscillations. (i) The temperature of the 1.15 nm CNT was varied by method 1, changing the stage temperature. (ii) The temperature of the 1.06 nm CNT was varied by method 2, changing laser intensity between the low power 0.8 mW (D1) and the high power 8 mW (D0) modes. The error bars for the peak positions are the errors in fitting them with a Lorentzian curve. b, (i) Calibration of the G peak position with the stage temperature. (ii) Change in the G peak position as a function of the laser power. (iii) Agreement between temperature calculated by equation (1) and that calculated using the G peak position calibration. The error bars in temperature are errors propagated from errors in χ and β (Supplementary Tables 6 and 7). (iv) A good agreement is found within the phase transition data generated by the two distinct heating methods for a 1.44 nm CNT. Here data highlighted by the black markers refer to method 1 (stage heating) and the red markers refer to method 2 (laser heating). The error bars originate from the errors in fitting the RBM peak positions.

  4. Comparison of the diameter-dependent phase transition temperatures and reduction in the axial conductivity of the CNTs on water filling.
    Figure 4: Comparison of the diameter-dependent phase transition temperatures and reduction in the axial conductivity of the CNTs on water filling.

    a, A global plot for the freezing point depressions of water confined in CNTs. Red markers indicate experimental data from this work. The error bars in these data originate from the uncertainty in Heaviside fittings in Fig. 2b. The dashed red vertical line represents the 1.24 nm SWNT for which no freezing transition was observed. The dashed magenta horizontal line is the lower temperature bound for the experiment (−80 °C). Cyan markers indicate experimental freezing point depressions for water inside larger MWNTs47. The fitted cyan line is the expected trend for freezing point depression as per the Gibbs–Thomson effect. Grey markers indicate the results from the MD simulations on freezing transition of water inside SWNT12. The green and blue markers are experimental (XRD and NMR) measurements on polydisperse SWNT powders and suspensions15, 21. b, A schematic for laser heating CNTs on a silicon substrate. (i) A laser is focused on a spot along the length of the tube. This position is defined to be y = 0 in the model. (ii) The Si substrate in the experiment is placed on top of a silver heating block. The temperature of the Si substrate, TSi, is assumed to be the same as that of the stage, Tstage. (iii) The energy balance for a differential element of the CNT. Heat from the laser beam, qlaser, is dissipated to the colder regions of the CNT, and to the underlying silicon substrate. c, Local temperature of the 1.24 nm CNT versus the laser power in the filled and dry states. d, Comparison of β/χ for CNTs in the filled versus dry states. The data used for the plot are provided as Supplementary Tables 6 and 7. The error bars in β/χ originate from the errors in estimating χ and β (Supplementary Tables 6 and 7). e, A plot of β/χ versus ka showing that on filling, CNTs undergo decrease in ka. The black and the pink curves are obtained from the laser heating model (Supplementary equation (16)) for tubes of 1.15 and 1.63 nm in diameter, respectively. The black, blue, red, green and cyan symbols represent 1.15, 1.24, 1.44, 1.52 and 1.63 nm diameter tubes, respectively. Open symbols represent tubes in the dry state, whereas solid symbols represent tubes in the filled state.


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Author information

  1. These authors contributed equally to this work

    • Kumar Varoon Agrawal &
    • Steven Shimizu


  1. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • Kumar Varoon Agrawal,
    • Steven Shimizu,
    • Lee W. Drahushuk,
    • Daniel Kilcoyne &
    • Michael S. Strano


K.V.A, S.S. and M.S.S. conceived and designed the experiments. K.V.A., S.S., L.W.D. and D.K. performed the experiments, analyzed the data, and contributed materials/analysis tools. K.V.A. and M.S.S. wrote the paper. All authors commented on the manuscript.

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