Mechanisms for hydrophilic/hydrophobic wetting transitions on cellulose cotton fibers coated using Al2O3 atomic layer deposition
(Color online) Water contact angle on cotton fiber mats after (a) Al2O3 ALD and (b) ZnO ALD at 60 and 90 °C. In both cases, hydrophilic cotton fibers change to hydrophobic after a few number of ALD cycles, and come back to hydrophilic again after a certain number of ALD cycles with no significant dependence on the deposition temperature and different kinds of metal oxide film.
(Color online) Pictures of a water drop on (a) cotton ball and (b) cotton fiber mat after changed to hydrophobic with three cycles of Al2O3 ALD at 60 °C. (c) The magnified image that shows the interface between cotton fiber mat and water drop. (d) To test stability of hydrophobic cotton, cotton ball with three cycles of Al2O3 ALD was placed in water with bare cotton ball and it has been floated on water surface more than a month, which shows that hydrophobicity induced by ALD is a stable property.
(Color online) Water contact angle (WCA) on cotton fiber mats after (a) a repeated sequence of three Al2O3 ALD cycles 60 °C followed by air exposure and contact angle measurement (sequence labeled “A”); and (b) after a different procedure involving sequence “A” and sequence “Z,” where “Z” corresponds to 3 ZnO ALD cycles followed by contact angle measurement in air. In both cases, the surface remains hydrophobic even after more than ten repetitions (i.e., after more than 30 ALD cycles). Samples prepared using 30 cycles of ALD coating without breaks for air exposure produced a hydrophilic finish.
(Color online) XPS spectra of bare cotton fiber mat (black line), cotton fiber mats after three cycles (red line), and 50 cycles (blue line) of Al2O3 ALD at 60 °C. (a) The predominant peaks at 286.2 and 287.8 eV from bare cotton fiber mat correspond to O-C-O and C-O bonds, respectively, but the intensity of those peaks is decreased with increasing ALD cycles. (b) Al-O-C peak is shown on the cotton fiber mat after three cycles of ALD, whereas Al-O peak becomes dominant with 50 cycles of ALD. (c) Al-O-C peak on cotton fiber mat after three cycles and Al-O peak after 50 cycles of Al2O3 ALD are consistent with the peaks shown in panel (b).
(Color online) In situ FTIR spectra collected after 1st and 10th TMA and H2O dose at 60 °C on cotton fiber mat with the absorption spectrum of the untreated cotton (bottom) and differential spectra after TMA and water exposure cycles. Peak changes at 1210 and 1460 cm−1 indicated by dot lines correspond to symmetric and asymmetric CH3 deformations. These deformation modes are not visible during the first TMA exposures, but are visible after the 10th TMA dose. Furthermore, the broad peak change between 3080 and 3650 cm−1 shows a sequential decrease and increase in the bonded OH corresponding to TMA and H2O doses, respectively.
FESEM images of (a)–(c): bare cotton ball fibers; (d)–(f): cotton fibers after three cycles of Al2O3 ALD; and (g)–(i): fibers after 50 cycles of Al2O3 ALD at 60 °C. Panels (a), (d), and (g) [scale bar = 5 mm]: each cotton fiber consists of a lot of fibrils. Panels (b), (e), and (h) [scale bar = 1 mm]: the surfaces of cotton fibers look covered by Al2O3 film when ALD cycles increases. Some cracks on the surface of cotton fiber ball after 50 cycle of ALD are induced by the stiffness of Al2O3 film. Panels (c), (f), and (i) [scale bar = 200 mm]: higher resolution images show rougher surface after three cycles of Al2O3 ALD compared with other cotton balls.
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