nanomaterials Article Effect of Toluene Addition in an Electric Arc on Morphology, Surface Modification, and Oxidation Behavior of Carbon Nanohorns and Their Sedimentation in Water Kseniya I. Baskakova, Olga V. Sedelnikova *, Evgeniy A. Maksimovskiy, Igor P. Asanov , Aida T. Arymbaeva, Lyubov G. Bulusheva and Alexander V. Okotrub *   Citation: Baskakova, K.I.; Sedelnikova, O.V.; Maksimovskiy, E.A.; Asanov, I.P.; Arymbaeva, A.T.; Bulusheva, L.G.; Okotrub, A.V. Effect of Toluene Addition in an Electric Arc on Morphology, Surface Modification, and Oxidation Behavior of Carbon Nanohorns and Their Sedimentation in Water. Nanomaterials 2021, 11, 992. https://doi.org/10.3390/ nano11040992 Academic Editors: Alberto Bianco and Alessandro Barge Received: 27 February 2021 Accepted: 9 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/). Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk, Russia; baskakova@niic.nsc.ru (K.I.B.); eugene@niic.nsc.ru (E.A.M.); asan@niic.nsc.ru (I.P.A.); arymbaeva@niic.nsc.ru (A.T.A.); bul@niic.nsc.ru (L.G.B.) * Correspondence: o.sedelnikova@gmail.com (O.V.S.); spectrum@niic.nsc.ru (A.V.O.) Abstract: Carbon nanohorns (CNHs) are attractive for various applications, where a high specific surface area and long dispersion stability in water are important. In the present work, we study these parameters of CNHs prepared by arc evaporation of graphite depending on the conditions of the synthesis and subsequent oxidation in air. It is shown that the addition of toluene in the reactor during the arcing allows obtaining CNHs functionalized with −CH x groups. Heating of CNHs in air at 400 ◦ C leads to substitution of −CH x groups for oxygen-containing groups. Moreover, the CNH endcaps are opened at 500 ◦ C, and as a result, the specific surface area of CNHs increases 4 times. Aqueous suspensions with a concentration of oxidized CNHs of 100 μg/mL are stable for 8 months. Keywords: carbon nanohorns; oxidation; specific surface area; aqueous suspension; optical absorption 1. Introduction Carbon nanohorns (CNHs) are short tubular structures with conical endcaps, where the wall-constituting carbon atoms are in the sp 2 -hybridized states. As a result, they are rigid, chemically inert, and electrically conductive. During the synthesis, the CNHs are combined into spherical particles, which can form large van der Waals associates [1–5]. High pore volume [6–8] and specific surface area (SSA) [9,10] make CNHs very promising for nanofluids [11,12], catalysis [13,14], medicine [15–18], electrochemistry [9,19], elec- tromagnetic [2,5,20] and sensing applications [6,21,22]. In this regard, the improvement of availability of their pores for guest species, the increase of the SSA value, and the enhancement of the stability of their suspensions become very important. Similarly for carbon nanotubes, oxidation is the most studied method of chemical modification of CNHs [3,15,23,24]. The treatment of CNHs with acids [15,16,25,26] and heating in a presence of oxygen [9,12,22,27–29] could etch endcaps of nanohorns or/and open nanowindows in their walls resulting in a significant increase of the SSA value (ca. 2.6 and 4 times, respectively [30]). At the same time, the surface modification of CNHs with different oxygen-containing moieties, such as –OH, −COOH, −C=O, −C−O−C− groups [1,3,15,31], is a basic strategy to improve the stability of aqueous suspensions of CNHs. Therefore, both the thermal and acid oxidation of nanohorns allow increasing the SSA and dispersibility of CNHs in solutions simultaneously. The development of oxygen-containing groups at the CNH surface may occur as a result of the transformation of already attached functional groups. For example, hydrogen atoms in methyl groups can be replaced by O and OH radicals to form CO and COOH groups on the surface of carbon nanotubes [32]. Since the nanotubes and nanohorns have similar structures, this approach can also be suitable for CNHs. In the present work, we show that −CH x groups present on the CNH surface promote the addition of oxygen-containing groups responsible for the stability of aqueous suspen- Nanomaterials 2021, 11, 992. https://doi.org/10.3390/nano11040992 https://www.mdpi.com/journal/nanomaterials