Multifunctional Self-Cleaning Structural colors based on TiO2 nanostructures

Colors play vital role in textiles, automobiles, and decoration sectors. The general source of colors are dyes and pigments but are not safe for human health and at the same time suffer with stability issues, as it degrades with thermal energy, humidity, and UV exposure etc. Nature itself finds the solution and generates colors by means of structures called structural colors. Many fascinating artworks can be found in nature, such as peacock feather, butterfly wings, and gem opals that generate colors because of microstructures on them.

To generate the colors through structures, you need to apply optical phenomena such as interference, diffraction, scattering etc. through the microstructure fabrication on a surface. Since the origin of the colors in structural colors is the structure and not the chemical nature of the material, structural colors do not degrade over time due to humidity, UV exposure or any such phenamena that can affect the chemical nature of the structure and hence structural colors are also called permanent colors.

We have generated structural colours artificially by applying interference through the TiO₂ nanorods and thin films on Ti deposited hard and flexible surfaces utilizing a method known as glancing angle deposition (GLAD). GLAD is a physical vapour vacuum deposition technique in which the substrate can be rotated at an oblique angle with respect to the vapour flux direction and with this, many complex nanostructures can be grown.  As the interference phenomena will depend on the optical path difference between the waves reflected from the interfaces Air/TiO₂ and TiO₂/Ti (see the Figure 1), in our particular case, the control of the optical path difference can guide us in tuning the colors.

Figure 1. Schematic for the interference of electromagnetic waves being reflected from the air/TiO₂ interface and TiO₂/Ti interface. (n_i :i from 1 to 4, represent refractive indices for corresponding media). Adopted from Reference 1.©OPTICA

We were able to control the optical path difference by varying thickness and refractive index of TiO₂ layer and generated structural colours covering full visible spectrum. Besides, we found that Ti is not the only metal, which can help in generating structural colours with TiO₂ nanostructure but Ni, Co, Al and Cu can also do the same job, as the role of the Ti was to just provide the high reflectance of electromagnetic waves in visible radiation region. We utilized both hard and flexible surfaces to showcase the generation of structural colors.

Although structural colours are stable, the environmental pollutants sit on the surface of the structural colours and redefine the color. So, to maintain the color, regular cleaning of the surface is needed, which is cumbersome and costly. We have solved this problem by making the surface of the colours superhydrophilic, a self-cleaning surface. All structural colours produced by GLAD were found to be hydrophobic and could be transformed into a reversible super-hydrophilic state, a self-cleaning state by UV irradiation. A permanent self-cleaning state is obtained with controlled annealing at elevated temperatures. Annealing at controlled temperatures could also tune the colors and opaqueness of the colors on glass surfaces.

We found that these structural colors are responsive to the external gases like volatile organic compounds (VOCs). We demonstrated ethanol vapour induced colour transformation, which shows the ability of the structural colours to work as an optical sensor. Besides, these structural colors also showed its suitability for it’s the utilization of optical information encryption.

Thus, a multifunctional self-cleaning structural colors are generated with TiO₂ nanostructures.

Figure 2. Structural colours produced by varying thickness of TiO₂ layer on Ti sputtered  glass  and flexible PET surfaces employing GLAD, (b) permanent change in colour, transparency and wettability state due to annealing at elevated temperatures, (c) UV irradiation induced change in wettability state of the colour which can be recovered by dark storage. © Gaurav Shukla

The research was carried out at Centre for Nano and Soft Matter Sciences (CeNS), Bangalore, India.

Source article:

1. Shukla, Gaurav, and Angappane Subramanian. “Self-cleaning structural colors by TiO₂/Ti nanostructures.” Applied Optics 59, no. 33 (2020): 10483-10492.