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Open AccessArticle

Facile Fabrication of Multifunctional ZnO Urchins on Surfaces

1
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
2
Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India
3
Bristol Centre for Functional Nanomaterials (BCFN), HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
4
Department of Biochemistry, Indian Institute of Science, Bangalore 560012, India
5
XMas, The UK CRG Beamline at the ESRF, The European Synchrotron, 71, avenue des Martyrs, CS 40220, 38043 Grenoble CEDEX 9, France
6
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
7
Bristol Dental School, University of Bristol, Bristol BS1 2LY, UK
*
Author to whom correspondence should be addressed.
Colloids Interfaces 2018, 2(4), 74; https://doi.org/10.3390/colloids2040074
Received: 19 November 2018 / Revised: 10 December 2018 / Accepted: 10 December 2018 / Published: 14 December 2018
(This article belongs to the Special Issue Wetting on Micro/Nano-Scale: From Fundamentals to Application)
Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed, μ m-long, tapered nanoneedles radiating from the urchin center. The ZnO urchin structures were successfully formed on several different substrates with high surface density and coverage, including silicon (Si), glass, polydimethylsiloxane (PDMS), and copper (Cu) sheets, as well as Si seeded with ZnO nanocrystals. Time-resolved SEM revealed growth kinetics of the ZnO nanostructures on Si, capturing the emergence of “infant” urchins at the early growth stage and subsequent progressive increases in the urchin nanoneedle length and density, whilst the spiky nanoneedle morphology was retained throughout the growth. ε -Zn(OH)2 orthorhombic crystals were also observed alongside the urchins. The crystal structures of the nanostructures at different growth times were confirmed by synchrotron X-ray diffraction measurements. On seeded Si substrates, a two-stage growth mechanism was identified, with a primary growth step of vertically aligned ZnO nanoneedle arrays preceding the secondary growth of the urchins atop the nanoneedle array. The antibacterial, anti-reflective, and wetting functionality of the ZnO urchins—with spiky nanoneedles and at high surface density—on Si substrates was demonstrated. First, bacteria colonization was found to be suppressed on the surface after 24 h incubation in gram-negative Escherichia coli (E. coli) culture, in contrast to control substrates (bare Si and Si sputtered with a 20 nm ZnO thin film). Secondly, the ZnO urchin surface, exhibiting superhydrophilic property with a water contact angle ~   0 ° , could be rendered superhydrophobic with a simple silanization step, characterized by an apparent water contact angle θ of 159° ± 1.4° and contact angle hysteresis Δ θ < 7°. The dynamic superhydrophobicity of the surface was demonstrated by the bouncing-off of a falling 10 μ L water droplet, with a contact time of 15.3 milliseconds (ms), captured using a high-speed camera. Thirdly, it was shown that the presence of dense spiky ZnO nanoneedles and urchins on the seeded Si substrate exhibited a reflectance R < 1% over the wavelength range λ = 200–800 nm. The ZnO urchins with a unique morphology fabricated via a simple route at room temperature, and readily implementable on different substrates, may be further exploited for multifunctional surfaces and product formulations. View Full-Text
Keywords: ZnO urchins; nanostructured surfaces; E. coli; superhydrophilic; superhydrophobic; anti-reflective surfaces ZnO urchins; nanostructured surfaces; E. coli; superhydrophilic; superhydrophobic; anti-reflective surfaces
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MDPI and ACS Style

Tripathy, A.; Wąsik, P.; Sreedharan, S.; Nandi, D.; Bikondoa, O.; Su, B.; Sen, P.; Briscoe, W.H. Facile Fabrication of Multifunctional ZnO Urchins on Surfaces. Colloids Interfaces 2018, 2, 74.

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