Reversible wettability control of silicon nanowire surfaces: From superhydrophilicity to superhydrophobicity

Background

Superhydrophobic surfaces with a large CA hysteresis pin water droplets instead of letting them roll off, which is useful for liquid transport and biochemical separation. Earlier water-adhesive superhydrophobic surfaces required complex processes that combined micro- or nanostructuring with chemical functionalization to lower the surface energy, and their wetting could not be switched from superhydrophilic to superhydrophobic or reversed.

Design and approach

Si NW arrays were etched into p-type Si wafers by metal assisted chemical etching (MACE) in 30 mM AgNO3 and 4.9 M HF at 70 °C, with the etching time (te) setting the wire length. The surface was then modified by RTA at 1000 °C for 10 cycles at 13.3 Pa; each cycle consisted of a 65 s ramp under N2, 30 s of annealing under O2 and rapid cooling under vacuum. The proposed mechanism is that annealing in oxygen converts hydrophilic surface silanol groups (–Si–O–H) into hydrophobic siloxane groups (–Si–O–Si–) by releasing H2O from adjacent silanol pairs, and that silanol groups re-form when the wires are stored in air.

How it was evaluated

Wire morphology was examined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM), and surface groups by Fourier transform infrared (FTIR) spectroscopy. Static, advancing and receding water CAs were measured with droplets of about 5 μL at five locations per sample and averaged, as a function of te and of storage time in air. The same RTA was applied to flat Si, RTA was also run under vacuum, nitrogen, air and hydrogen, and the oxygen-RTA surfaces were compared with flat Si and Si NW surfaces terminated with H, with ~30% F and ~70% H, or with dodecyltrichlorosilane (DTS).

Key results

At te = 25 min, the wires were vertically aligned with uniform lengths of about 14 μm and diameters of 100 to 150 nm, and the average length increased from 0.94 to 14.8 μm as te increased from 5 to 35 min. After oxygen RTA, the CA hysteresis exceeded 21° for all samples; for te ≥ 20 min the static CA exceeded 150° and the hysteresis exceeded 48°. Droplets did not slide when the substrate was tilted vertically or turned upside down, and no significant morphological change of the wires was observed after RTA.

HR-TEM showed a 2.4 ± 0.4 nm native oxide on as-fabricated wires and a thermally grown SiOx layer of ≥~9 nm with a rougher outer surface after cycled RTA. FTIR showed siloxane-related peaks at ~888 and 908 cm⁻¹ after RTA, which disappeared after air storage, while the silanol band at ~3000 to 3500 cm⁻¹ decreased after RTA and was restored after 1 h in air. RTA under vacuum, air or nitrogen raised the CA only from nearly 0 to ~10°, and the surface returned to ~0° within a few minutes; RTA under hydrogen damaged the wire structure.

Stored in air, the oxygen-RTA surface returned fully to superhydrophilic after 50 min, and the change was faster beneath a droplet resting on the surface than under ambient storage. The conversion between superhydrophilic and superhydrophobic was repeated 4 times without significant change in the CAs of either state, although the outer surface of the wires roughened with repeated RTA; the authors concluded that siloxane formation, rather than roughness, mainly governed the wettability change.

On flat Si, the CA was 45 ± 6.4° on the bare substrate and rose only to 61° after the same RTA. Si NW surfaces treated with H, F/H or DTS were also superhydrophobic but repelled water and showed roll-off behaviour consistent with the Cassie state, whereas the oxygen-RTA wires pinned droplets in the Wenzel state. The authors attributed this adhesion to a small fraction of silanol groups re-formed among the dominant siloxane groups.

Open questions

The authors described the reversibility within their experimental range of four cycles. Because siloxane groups are energetically stable only at high temperature, the superhydrophobic state did not persist in air and reverted to superhydrophilic within 50 min.

How it connects to other research

The related Science paper on making water run uphill moves droplets with a fixed gradient in surface free energy; here the wettability of a single Si NW surface was switched between two states by annealing in oxygen and storage in air, and droplets were pinned rather than transported.