Hierarchical Photocatalytic Surfaces: From 1D to 3D Photoactive TiO2 Nanotubes Grown By Plasma Assisted Deposition Techniques

Abstract

Photoactivity of TiO2 is of extensive recognition in applications like photocatalytic removal of pollutants, self-cleaning coatings, photoelectrochemical and photovoltaic cells and optical filters [1, 2]. Photocatalytic activity is affected by both the TiO2 structure and microstructure. For example, parameters as crystal sizes, texturization, nano and microscale arrangement, and interface conditioning and engineering [3] have been demonstrated to determine the formation and migration to the surface of electron-hole pairs responsible for the photoactivity response [3]. Thus, A successful approach to enhance the specific surface area and light harvesting is the use of highly porous structures such as nanoparticles, nanotubes, nanowires, or 3D nanoarchitectures. In this latter case, the hierarchical distribution of the semiconducting oxide is expected to prompt light scattering effects which might allow high efficiencies in photocatalytic pollutant removal for finely controlled nanoscale motives.In this communication, we propose the development of photoactive highly porous TiO2 nanostructures, forming 1D nanotubes and 3D hierarchical systems as nanotrees and multistack of nanotrees. These nanoarchitectures are fabricated by the combination of plasma enhanced chemical vapour deposition of crystalline TiO2 layers with the use of small-molecule organic nanowires as 1D and 3D soft-templates. This same synthetic approach has been exploited recently in the development of dye-sensitized solar cells [4], piezoelectric nanogenerators [5], and icephobic, freezing-delay, surfaces [6].The study of nanostructure shape, density, and entanglement as well as the combination of crystalline phases in the same nanostructure are revealed as key factors controlling the photocatalytic performance of the system. A thorough characterization analysis is presented including XRD, XPS, Raman, SEM, and TEM providing crucial information on the chemical composition, structure, and microstructure. Moreover, wetting characterization has been also performed to relate the surface tension to the photoactivity of TiO2 nanostructures. Photoactivity has been studied in a home-made system able to be illuminated with UV irradiation and the “in-situ” optical monitoring of different dye concentrations, like methyl orange dye, methylene blue, or rhodamine 6G. Thus, photocatalysis behavior has been evaluated and associated with the nanostructured configuration.[1] M. Grätzel, Nature 2001, 414, 338[2] F. Parrino, A. Ramakrishnan, H. Kisch, Angew. Chem. 2008, 120, 7215[3] P. Romero-Gomez, A. Borras, A. Barranco. J.P. Espinos, A.R. Gonzalez-Elipe ChemPhysChem 2010, 12 (1) 191[4] A.N. Filippin, M. Macías-Montero, A. Borras, et al. Scientific Reports 2017, 7, 9621[5] A.N. Filippin, J.R. Sanchez-Valencia, A. Borras, et al. Nano Energy 2019, 58, 476[6] M. Alcaire, C. Lopez-Santos, A. Borras, et al. Langmuir 2019, 35(51) 16876 Acknowledgements. We thank the AEI-MICINN (PID2019-110430GB-C21 and PID2019-109603RA-I0), the Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (PAIDI-2020 through projects US-1263142, ref. AT17-6079, P18-RT-3480), and the EU through cohesion fund and FEDER 2014–2020 programs for financial support. CLS and JS-V thank the University of Seville through the VI PPIT-US and (JS-V) the Ramon y Cajal Spanish National programs. The project leadings to this article have received funding from the EU H2020 program under the grant agreements 851929 (ERC Starting Grant 3DScavengers) and 899352 (FETOPEN-01-2018-2019-2020 - SOUNDofICE).