Anodic TiO2 Films for Water Purification in Solar Evaporators
Authors: Shinnur Manjunath Veeranna , Buccella Matteo , Pedeferri MariaPia , Lughi Vanni , Meroni Daniela , Diamanti Maria Vittoria 
Published: December 2024

The interest in titanium and its oxides keeps growing on account of their peculiar engineered properties, which find applications in several fields, from architecture to bioengineering, from automotive to photovoltaic cells and photocatalytic devices. There are several methods that allow to grow titanium oxides, among which anodic oxidation has the nice advantage of growing TiO2 nanostructures directly immobilized on a substrate, avoiding the issue of nanostructure recovery from the medium [1]. Yet, these systems also present a drawback, i.e., the immobilization on a metallic, non-transparente and non-permeable substrate. For this reason, different methods have been developed to detach the nanotubes layer and use it as a self-standing membrane [2].
In this work, we address this specific challenge and present the obtaining and characterization of self-standing TiO2 nanotubes membranes. The application envisioned requires their mandatory detachment, as the membranes will be used inside a solar evaporator device to improve the quality of evaporating water, by removing also volatile compounds that may evaporate together with water, reducing the purification extent.
Membranes are produced by anodizing commercially pure titanium sheets in ethylene glycol solutions containing different amounts of water, ammonium fluoride and lactic acid. Anodizing time was varied between 30 min and 90 min, and voltage was varied between 30 V and 60 V. Double anodizing was performed to ensure better nanotubes uniformity; then annealing was performed at 500°C for 2 h to allow for oxide crystallization to anatase form. Afterwards, a third, brief anodizing step (10 min) was needed to facilitate nanotubes detachment, which then was carried out chemically, by immersion in HCl. An example of SEM image of detached membrane is shown in Figure 1, indicating that membranes can indeed be obtained and have sufficient mechanical stability to allow for handling and testing. SEM and XRD results indicate a thickness ranging from few micrometers to tens of micrometers, and their crystal structure is mainly anatase, although small quantities of rutile may form in the base of the oxide.
Preliminary photocatalysis tests were conducted on the degradation of organic dyes, showing promising photoactivity. Several decoration methods were also considered, with different aims: silver nanoparticles for antifouling, and quantum dots for improved photoactivity. Silver nanoparticles did not enhance photoactivity, and in some cases decreased it slightly; yet, their scope was different, i.e., exploiting the antibacterial and antifouling characteristics of silver on the membrane layer, therefore the formulation leading to unaltered photocatalytic activity was selected for the prosecution of this work, which envisions antibacterial and antifouling testing procedures.

Acknowledgements: We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 1409 published on 14.9.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title COPE - COmposite nanomaterials coupling Photothermal Evaporation and photocatalysis for durable water purification systems – CUP G53D23006660001 - Grant Assignment Decree No. 1384 adopted on 01.09.2023 by the Italian Ministry of Ministry of University and Research (MUR).

References

Lee, K.; Mazare, A.; Schmuki, P. (2014) One-dimensional titanium dioxide nanomaterials: Nanotubes. Chem. Rev., 114, 9385–9454.

So, S.; Hwang, I.; Riboni, F.; Yoo, J.; Schmuki, P. (2016) Robust free standing flow-through TiO2 nanotube membranes of pure anatase. Electrochem. Commun., 71, 73–78.

Figure
1: Morphology of nanotubular membrane.

Figure 1