Nanomaterials 2021, 11, 2624. https://doi.org/10.3390/nano11102624 www.mdpi.com/journal/nanomaterials Article Layer-by-Layer-Stabilized Plasmonic Gold-Silver Nanoparticles on TiO2: Towards Stable Solar Active Photocatalysts Fons Dingenen 1,2 , Natan Blommaerts 1,2 , Myrthe Van Hal 1,2 , Rituraj Borah 1,2 , Daniel Arenas-Esteban 2,3 , Silvia Lenaerts 1,2 , Sara Bals 2,3 and Sammy W. Verbruggen 1,2, * 1 Sustainable Energy, Air & Water Technology (DuEL), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium; fons.dingenen@uantwerpen.be (F.D.); natan.blommaerts@uantwerpen.be (N.B.); myrthe.vanhal@uantwerpen.be (M.V.H.); rituraj.borah@uantwerpen.be (R.B.); silvia.lenaerts@uantwerpen.be (S.L.) 2 NANOlab Center of Excellence, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium; daniel.arenasesteban@uantwerpen.be (D.A.-E.); sara.bals@uantwerpen.be (S.B.) 3 Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium * Correspondence: sammy.verbruggen@uantwerpen.be Abstract: To broaden the activity window of TiO2, a broadband plasmonic photocatalyst has been designed and optimized. This plasmonic ‘rainbow’ photocatalyst consists of TiO2 modified with gold–silver composite nanoparticles of various sizes and compositions, thus inducing a broadband interaction with polychromatic solar light. However, these nanoparticles are inherently unstable, especially due to the use of silver. Hence, in this study the application of the layer-by-layer technique is introduced to create a protective polymer shell around the metal cores with a very high degree of control. Various TiO2 species (pure anatase, PC500, and P25) were loaded with different plasmonic metal loadings (0–2 wt %) in order to identify the most solar active composite materials. The prepared plasmonic photocatalysts were tested towards stearic acid degradation under simulated sunlight. From all materials tested, P25 + 2 wt % of plasmonic ‘rainbow’ nanoparticles proved to be the most promising (56% more efficient compared to pristine P25) and was also identified as the most cost-effective. Further, 2 wt % of layer-by-layer-stabilized ‘rainbow’ nanoparticles were loaded on P25. These layer-by-layer-stabilized metals showed superior stability under a heated oxidative atmosphere, as well as in a salt solution. Finally, the activity of the composite was almost completely retained after 1 month of aging, while the nonstabilized equivalent lost 34% of its initial activity. This work shows for the first time the synergetic application of a plasmonic ‘rainbow’ concept and the layer-by-layer stabilization technique, resulting in a promising solar active, and long-term stable photocatalyst. Keywords: TiO2; photocatalysis; surface plasmon resonance; layer-by-layer stabilization; core-shell; solar 1. Introduction Already in the first photocatalysis studies, Fujishima and Honda (1972) pointed out the potential of TiO2 [1]. Its ability to produce reactive charge carriers (both conduction band electrons (e - CB) and valence band holes (h + VB)) under appropriate illumination enabled its use in several application fields, e.g., water splitting [2–5] and environmental remediation [6]. Additional advantages of TiO2 include its chemical stability, low cost and suitable band edge positions [7]. In contrast, the large band gap (ca. 3.2 eV) remains a major drawback. This limits the activity window to ultraviolet (UV) light which accounts Citation: Dingenen, F.; Blommaerts, N.; Van Hal, M.; Borah, R.; Arenas-Esteban, D.; Lenaerts, S.; Bals, S.; Verbruggen, S.W. Layer-by-Layer-Stabilized Plasmonic Gold-Silver Nanoparticles on TiO2: Towards Stable Solar Active Photocatalysts. Nanomaterials 2021, 11, 2624. https://doi.org/10.3390/ nano11102624 Academic Editor: Onofrio M. Maragò Received: 31 August 2021 Accepted: 1 October 2021 Published: 6 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://crea- tivecommons.org/licenses/by/4.0/).