Citation: Gyulavári, T.; Dusnoki, D.; Márta, V.; Yadav, M.; Abedi, M.; Sápi, A.; Kukovecz, Á.; Kónya, Z.; Pap, Z. Dependence of Photocatalytic Activity on the Morphology of Strontium Titanates. Catalysts 2022, 12, 523. https://doi.org/10.3390/ catal12050523 Academic Editor: Roberto Fiorenza Received: 18 March 2022 Accepted: 27 April 2022 Published: 7 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). catalysts Article Dependence of Photocatalytic Activity on the Morphology of Strontium Titanates Tamás Gyulavári 1, * , Daniella Dusnoki 1 , Viktória Márta 1 , Mohit Yadav 1 , Mahsa Abedi 1 , AndrásSápi 1 , Ákos Kukovecz 1 , ZoltánKónya 1 and Zsolt Pap 1,2,3, * 1 Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla Sqr. 1, H-6720 Szeged, Hungary; d.daniella0226@gmail.com (D.D.); martaviktoria95@gmail.com (V.M.); yadavmohit27@gmail.com (M.Y.); mahsa.sh.abedi@gmail.com (M.A.); sapia@chem.u-szeged.hu (A.S.); kakos@chem.u-szeged.hu (Á.K.); konya@chem.u-szeged.hu (Z.K.) 2 Nanostructured Materials and Bio-Nano-Interfaces Center, Interdisciplinary Research Institute on Bio-Nano-Sciences, Babes-Bolyai University, Treboniu Laurian 42, RO-400271 Cluj-Napoca, Romania 3 Institute of Research-Development-Innovation in Applied Natural Sciences, Babes-Bolyai University, Fântânele Str. 30, RO-400294 Cluj-Napoca, Romania * Correspondence: gyulavarit@chem.u-szeged.hu (T.G.); pzsolt@chem.u-szeged.hu (Z.P.); Tel.: +36-62-544-626 (T.G.); +36-62-543-795 (Z.P.) Abstract: Strontium titanates were prepared with different morphologies by varying the ratio of solvents used during the synthesis. The effects of morphology and solvent (ethylene glycol to water) ratio were investigated both on the structure and photocatalytic activity of the samples. Structural properties were determined by X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and nitrogen adsorption measurements. The photocatalytic activity of the samples was evaluated by the photocatalytic oxidation of phenol and by the photocatalytic reduction of carbon dioxide. The ratio of solvents notably influenced the morphology, strontium carbonate content, primary crystallite size, and specific surface area of the samples. Samples prepared at low ethylene glycol to water ratios were spherical, while the ones prepared at high ethylene glycol to water ratios could be characterized predominantly by lamellar morphology. The former samples were found to have the highest efficiency for phenol degradation, while the sample with the most well-defined lamellar morphology proved to be the best for CO 2 reduction. Keywords: strontium titanate; morphology; water treatment; phenol; CO 2 reduction; photocatalysis 1. Introduction For photocatalytic applications, strontium titanate (SrTiO 3 ) can have numerous bene- fits in comparison with conventional binary oxides such as TiO 2 . The conduction band edge of the former is approximately 200 mV more negative than that of the latter [1], making it more suitable for photocatalytic reduction. Strontium ions in SrTiO 3 can facilitate the formation of superoxide radicals and inhibit the recombination of photogenerated charge carriers which is conducive for photocatalytic oxidation [2]. Many attempts have been made to increase the photocatalytic activity of SrTiO 3 photocatalysts via methods such as doping [3–6], metal deposition [7–10], and morphological modifications [9–12]. Morphological modifications include changing properties such as the size, specific surface area (SSA), shape, and porosity of photocatalysts [1], all of which can influence pho- tocatalytic activity. For example, small primary crystallite sizes usually result in large SSAs, more accessible crystallographic planes, and reduced recombination of charge carriers in the bulk [13]. Certain crystallographic planes within the same material can be photocatalyt- ically more active than others. For instance, Chen et al. found that for BiVO 4 , the sample possessing the highest relative intensity ratio of the (040) and (−121) crystallographic planes proved to be the best for the degradation of methylene blue [14]. Kedves et al. found that Catalysts 2022, 12, 523. https://doi.org/10.3390/catal12050523 https://www.mdpi.com/journal/catalysts