metals Article The Inﬂuence of the Ratio of Au and Pt Nanoparticles in Ternary Composites with TiO 2 Boglárka Hampel 1,2 , Lucian Baia 2,3 , Klara Hernadi 1,4, * and Zsolt Pap 1,2,3, *   Citation: Hampel, B.; Baia, L.; Hernadi, K.; Pap, Z. The Inﬂuence of the Ratio of Au and Pt Nanoparticles in Ternary Composites with TiO 2 . Metals 2021, 11, 628. https://doi.org/10.3390/ met11040628 Academic Editors: Jan Vrestal, Leonid M. Kustov and Isabel Duarte Received: 23 February 2021 Accepted: 2 April 2021 Published: 13 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla sqr. 1, HU-6720 Szeged, Hungary; hampelboglarka@chem.u-szeged.hu 2 Nanostructured Materials and Bio-Nano-Interfaces Center, Institute for Interdisciplinary Research on Bio-Nano-Sciences, Babes Bolyai University, Treboniu Laurian str. 42, RO-400271 Cluj-Napoca, Romania; lucian.baia@phys.ubbcluj.ro 3 Faculty of Physics, Babes Bolyai University, Mihail Kogalniceanu str. 1, RO-400084 Cluj-Napoca, Romania 4 Institute of Physical Metallurgy, Metal Forming and Nanotechnology, University of Miskolc, HU-3515 Miskolc-Egyetemváros, Hungary * Correspondence: hernadi@chem.u-szeged.hu (K.H.); pzsolt@chem.u-szeged.hu (Z.P.) Abstract: In the present work, ternary composites were synthesized from commercial titanias (Evonik Aeroxide P25, Aldrich anatase—AA and Aldrich rutile—AR) and two noble metals (gold and platinum). This research focuses on the composition of the photocatalysts, especially on the noble metals. The ratio between the noble metals varies from 0.25 to 0.75% in each composite for each noble metal. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and diffuse reﬂectance spectroscopy (DRS) measurements were carried out to investigate the structural and optical prop- erties. From the TEM, it can be observed that the particle sizes of the noble metals were between 1 and 4 nm, while the (anatase and rutile) crystals of P25 were 20–40 nm. The XRD showed that the semiconductors’ composition remained unchanged during/after the deposition of noble metal nanoparticles. By the DRS measurements, using the Kubelka–Munk equation, it can be concluded that the deposition of the noble metal nanoparticles resulted the decrease of the bandgap energies of the titanias. The photocatalytic activity was investigated under the irradiation of UV light. Oxalic acid and salicylic acid were used as model pollutants. The hydrogen production capacity was investigated as well, where the sacriﬁcial agent was oxalic acid, and UV irradiation was used. Keywords: photocatalysis; commercial titania; noble metal nanoparticles; composites; UV light 1. Introduction Materials at nanometer scale show different chemical and physical properties than their macroscopic forms, which are advantageous for many current research ﬁelds. Nan- otechnology is the study and application of these materials, which can be used in many areas, such as in medicine [1], electronics [2], fuel cells [3], cosmetics and textile industry [4], food industry [5], sensors [6], environmental applications [7], etc. Among many environmental applications, wastewater treatment is a very important process, where photocatalysis emerges as a viable alternative [8]. Among the nanoscaled semiconductor materials (Cu 2 O[9], WO 3 [10], ZnO [11], BiOX [12], etc.) titania photo- catalysts are the most studied ones [13]. The efﬁciency of these semiconductors can be enhanced by creating composites [14]. These nanostructures, containing semiconductors and metal nanoparticles, were synthesized and used so far. It was found that the composi- tion of the composites had a major effect on the overall photocatalytic efﬁciency [15]. In photocatalysis, composite systems are widely used because of their many advanta- geous properties. For example, metal nanoparticles deposited on the surface of a semicon- ductor can accelerate the electron–hole separation [16]; also, charge distribution greatly inﬂuences the photocatalytic activity [17]. For instance, the formation of composites from Metals 2021, 11, 628. https://doi.org/10.3390/met11040628 https://www.mdpi.com/journal/metals