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The team, made up of researchers from the Institute of Optics (IO-CSIC), the Complutense University of Madrid, the Carlos III University of Madrid and the KU Leuven university, has developed a new two-step method to produce such ZnO surfaces. The process involves irradiation of zinc metal foils with high repetition rate femosecond laser pulses followed by low temperature (380°C) heat treatment in argon atmosphere. This method results in LIPSS (Laser Induced Periodic Structures) structures coated with ZnO nanowires and columns covered by a relatively thick ZnO layer, which substantially increases the surface area of ​​the material in contact with the contaminant and thus its effectiveness.

The mechanism of ZnO growth during heat treatment is determined by the preconditions of laser processing. The team has studied the different growth mechanisms with the different thicknesses of ZnO induced by the first phase of laser irradiation: nanowire growth by a vapor-liquid-solid mechanism for very thin initial oxide layers, formation of ZnO agglomerates by a vapor-solid mechanism for intermediate oxide thicknesses and complete inhibition of the growth of ZnO nanostructures if the initial oxide layer induced by laser processing is too thick.

In photocatalysis, one of its most important applications, ZnO is used to degrade persistent organic pollutants (POPs) in water. These pollutants are resistant to environmental degradation and cannot be completely removed through current water treatment processes.
To remove these contaminants, the ZnO structure absorbs photons that are used by electrons to gain high energies, allowing them to oxidize organic contaminant molecules into carbon dioxide, water, and other inorganic byproducts.

The two-step process for producing ZnO surfaces

To produce micro- and nanostructured ZnO surfaces, the researchers used a two-step process. First, they irradiated metallic zinc sheets with femtosecond laser pulses at high repetition rates (100-500 kHz) and fast scanning speeds (cm/s) in air at atmospheric pressure. This laser structuring process has two main purposes: to modify the surface morphology to increase its specific surface area and to oxidize the surface to facilitate the subsequent growth of ZnO micro- and nanostructures during heat treatment. After laser irradiation, the sheets were heat treated in an argon flow at 380°C for 5 hours. This thermal treatment was performed to promote spatially selective growth of ZnO micro- and nanostructures. After the growth of the ZnO, the team analyzed the shape, composition, crystal structure, and luminescent properties of the material at different stages of the process. This analysis allowed them to determine the role of the different processing parameters (laser irradiation and thermal treatment) on the properties of the resulting ZnO.

Article: R. Ariza, A. Urbieta, A. Ferreiro, M.E. Rabanal, P. Fernández, J. Solis. “Selective growth of ZnO micro- and nano-structures on fs-laser processed metallic Zn substrates for large area applications”. Journal of Alloys and Compounds. Volume 1010, 5 January 2025, 177797

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