Superhydrophobic Surfaces and Surface Modification via Vapour Deposition Techniques

Superhydrophobicity, a common property within the field of protective coatings, was first observed in Lotus leaves where water droplets roll off the surface, taking dirt particles with them, rather than wet it.¹^,²<br/><br/>To replicate this process on a synthetic scale, both micro-/nano-scale roughness and a low surface energy reagent are required.³^,⁴ The latter employs the use of toxic fluorinated polymers (which also contributes to the robustness and high transparencies of the films) and requires high depositional temperatures.^5,6 Superhydrophobic materials have potential applications in oil-water separation and photovoltaics. However the widespread application of superhydrophobic coatings has been hindered due to their typically poor durability or robustness.⁷<br/><br/>A relatively unexplored technique combines the use of aerosol-assisted chemical vapour deposition (AACVD) to deposit superhydrophobic polymer films along with the resultant surface modification via AACVD of metal oxides (e.g. TiO₂ and CeO₂). AACVD is a scalable technique as it involves spraying a precursor on a heated substrate. Superhydrophobic coatings are known for their self-cleaning properties but depositing thin layers of metal oxides on top has allowed us to fabricate superhydrophobic metal oxide films, enhancing the photocatalytic properties of the superhydrophobic layer and the self-cleaning properties of the TiO₂ film.<br/><br/>Another surface modification technique is atomic layer deposition (ALD). ALD, a branch of chemical vapour deposition (CVD), is a technique used to deposit atomic layers of complex and layered thin films. By operating at such a scale, thickness control, conformality of the films, low temperature depositions and even the ability to scale-up are possible.⁸<br/><br/>This research seeks to fabricate a superhydrophobic photocatalytic self-cleaning coating, first by producing a superhydrophobic film from simple and non-toxic compounds via AACVD. This depositional technique also provides the highly textured morphology required. Thin films with a rough morphology, formed through island growth of aggregates have been produced, displaying static water contact angles &gt; 160°, transparencies &gt; 40% and maintained superhydrophobicity after 300 tape peel cycles.⁹ Thereafter, the surfaces’ properties are modified via the AACVD of TiO₂. Thus far, the deposited films are hydrophobic/superhydrophobic, exhibit photocatalytic activity and tolerate pencil hardness’ of up to “6H”.<br/><br/>This novel route could be used to produce ‘easy-to-clean’ coatings for slip-resistant flooring as well as coatings on solar cells.<br/><br/>References<br/>1 W. Barthlott and C. Neinhuis, <i>Planta</i>, 1997, <b>202</b>, 1–8.<br/>2 J. Jeevahan, M. Chandrasekaran, G. Britto Joseph, R. B. Durairaj and G. Mageshwaran, <i>J. Coatings Technol. Res.</i>, 2018, <b>15</b>, 231–250.<br/>3 J. Huo, C. I. De Leon Reyes, J. J. Kalmoni, S. Park, G. B. Hwang, S. Sathasivam and C. J. Carmalt, <i>ACS Appl. Nano Mater.</i>, 2023, <b>6</b>, 16383–16391.<br/>4 X. J. Guo, C. H. Xue, S. Sathasivam, K. Page, G. He, J. Guo, P. Promdet, F. L. Heale, C. J. Carmalt and I. P. Parkin, <i>J. Mater. Chem. A</i>, 2019, <b>7</b>, 17604–17612.<br/>5 J. Y. Huang, S. H. Li, M. Z. Ge, L. N. Wang, T. L. Xing, G. Q. Chen, X. F. Liu, S. S. Al-Deyab, K. Q. Zhang, T. Chen and Y. K. Lai, <i>J. Mater. Chem. A</i>, 2015, <b>3</b>, 2825–2832.<br/>6 V. H. Dalvi and P. J. Rossky, <i>Proc. Natl. Acad. Sci. U. S. A.</i>, 2010, <b>107</b>, 13603–13607.<br/>7 F. Chen, Y. Wang, Y. Tian, D. Zhang, J. Song, C. R. Crick, C. J. Carmalt, I. P. Parkin and Y. Lu, <i>Chem. Soc. Rev.</i>, 2022, <b>51</b>, 8476–8583.<br/>8 R. W. Johnson, A. Hultqvist and S. F. Bent, <i>Mater. Today</i>, 2014, <b>17</b>, 236–246.<br/>9 J. J. Kalmoni, F. L. Heale, C. S. Blackman, I. P. Parkin and C. J. Carmalt, <i>Langmuir</i>, 2023, <b>39</b>, 7731–7740.