Large-scale efficient water harvesting using bioinspired micro-patterned copper oxide nanoneedle surfaces and guided droplet transport

Abstract

As the Earth's atmosphere contains an abundant amount of water as vapors, a device which can capture a fraction of this water could be a cost-effective and practical way of solving the water crisis. There are many biological surfaces found in nature which display unique wettability due to the presence of hierarchical micro-nanostructures and play a major role in water deposition. Inspired by these biological microstructures, we present a large scale, facile and cost-effective method to fabricate water-harvesting functional surfaces consisting of high-density copper oxide nanoneedles. A controlled chemical oxidation approach on copper surfaces was employed to fabricate nanoneedles with controlled morphology, assisted by bisulfate ion adsorption on the surface. The fabricated surfaces with nanoneedles displayed high wettability and excellent fog harvesting capability. Furthermore, when the fabricated nanoneedles were subjected to hydrophobic coating, these were able to rapidly generate and shed coalesced droplets leading to further increase in fog harvesting efficiency. Overall, ∼99% and ∼150% increase in fog harvesting efficiency was achieved with non-coated and hydrophobic layer coated copper oxide nanoneedle surfaces respectively when compared to the control surfaces. As the transport of the harvested water is very important in any fog collection system, hydrophilic channels inspired by leaf veins were made on the surfaces via a milling technique which allowed an effective and sustainable way to transport the captured water and further enhanced the water collection efficiency by ∼9%. The system presented in this study can provide valuable insights towards the design and fabrication of fog harvesting systems, adaptable to arid or semi-arid environmental conditions.

Fig. 1. (a) Picture showing the design, dimensions and channel orientation. (b) and (c) are the unprocessed and processed sample surfaces. The scalebars in (b) and (c) are 30 mm. (d) Schematic of the fog harvesting setup for water collection.

Fig. 2. Schematic of large-scale fabrication of patterned high-density CuO nanoneedle structures.

Fig. 3. SEM images of the uncoated bare Cu (a), uncoated scored Cu (b) and uncoated CuO nanoneedle surfaces (c) at different resolutions (1 and 2). (3) Corresponding EDS spectra of the surfaces. The CuO nanoneedle samples here were 2 days old and kept under ambient conditions.

Fig. 4. (a) XPS survey spectra (black) and selected low-intensity core level spectra which are not clearly visible in the survey scan (red). (i) Bare Cu, (ii) CuO nanoneedles and (iii) silanized CuO nanoneedles. (b) XAES Cu L₃M₄₅M₄₅ spectra of (i) bare Cu, Cu⁰: 10.9%, Cu⁺: 89.1%, (ii) CuO nanoneedles, Cu²⁺: 100% and (iii) silanized CuO nanoneedles, Cu²⁺: 100%. Measured data are shown as dots. Fitted experimental reference line shapes in the greyscale fill are shown for bare Cu surface and oxidized surfaces. (c) High-resolution XPS spectra of O 1s, F 1s, Si 2p, and Cu 2p transitions. The exposure time for uncoated CuO nanoneedles used in XPS analysis was 7 days from the fabrication.

Fig. 5. (a) Images of a water droplet (1 μL) spreading on hydrophilic CuO nanoneedle surface and (b) schematic diagram of the spreading of a water droplet. The scale bar in (a) is 2 mm.

Fig. 6. Contact angle θ (deg) images of (1) uncoated and (2) hydrophobic coated surfaces composed of (a) bare Cu surface, (b) scored Cu surface and (c) CuO nanoneedle surfaces. The scale bars in the images are 1 mm and the volume of water is ∼6 μL.

Fig. 7. Snapshots of droplet formation and transport on different surfaces: (a) uncoated bare Cu, (b) uncoated scored Cu, (c) uncoated CuO nanoneedles, (d) coated bare Cu, (e) coated scored Cu and (f) coated CuO nanoneedles at time intervals 0, 150, 300, 450 and 600 s. Scale bars are 1 mm.

Fig. 8. (a) Fog harvesting dynamics as volume of water collected per square meter, V (mL m⁻²), in time, t (min), over a surface area of 22 500 mm² for all surfaces. (b) Average volume and onset time of the first single droplet detaching from the surfaces.

Fig. 9. Schematic graphic showing the fog collection mechanism on (a) uncoated bare Cu, (b) uncoated scored Cu, (c) uncoated CuO nanoneedles, (d) coated bare Cu, (e) coated scored Cu and (f) coated CuO nanoneedles.

Fig. 10. Comparison of the rates of water collection for surfaces with and without channels.

Fig. 11. Guided transport of water via hydrophilic milled channels in (a) CuO nanoneedles (uncoated) and (b) CuO nanoneedles (coated).

Fig. 12. Two-dimensional model of a drop interacting with an infinite length channel in the case where the channels are hydrophilic and rest of the surface is hydrophobic.